Kobweb is an opinionated Kotlin framework for creating websites and web apps, built on top of Compose HTML and inspired by Next.js and Chakra UI.

@Page
@Composable
fun HomePage() {
  Column(Modifier.fillMaxWidth(), horizontalAlignment = Alignment.CenterHorizontally) {
    Row(Modifier.align(Alignment.End)) {
      var colorMode by ColorMode.currentState
      Button(
        onClick = { colorMode = colorMode.opposite },
        Modifier.borderRadius(50.percent).padding(0.px)
      ) {
        // Includes support for Font Awesome icons
        if (colorMode.isLight) FaSun() else FaMoon()
      }
    }
    H1 {
      Text("Welcome to Kobweb!")
    }
    Row(Modifier.flexWrap(FlexWrap.Wrap)) {
      SpanText("Create rich, dynamic web apps with ease, leveraging ")
      Link("https://kotlinlang.org/", "Kotlin")
      SpanText(" and ")
      Link("https://github.com/JetBrains/compose-multiplatform#compose-html/", "Compose HTML")
    }
  }
}

Kobweb is still only publishing pre-release versions, but it's been usable for a while now, and it is getting close to stabilization. Please consider starring the project to indicate interest, so we know we're creating something the community wants. How ready is it? ▼

Our goal is to provide:

• an intuitive structure for organizing your Kotlin website or web app
• automatic handling of routing between pages
• a collection of useful batteries included widgets built on top of Compose HTML
• an environment built from the ground up around live reloading
• static site exports for improved SEO and potentially cheaper server setups
• support for responsive (i.e. mobile and desktop) design
• shared, rich types between client and server
• out-of-the-box Markdown support
• a way to easily define server API routes and persistent API streams
• an open source foundation that the community can extend
• and much, much more!

Here's a demo where we create a Compose HTML project from scratch with Markdown support and live reloading, in under 10 seconds:

The first step is to get the Kobweb binary. You can install it, download it, and/or build it, so we'll include instructions for all these approaches.

Major thanks to aalmiray and helpermethod to helping me get these installation options working. Check out JReleaser if you ever need to do this in your own project!

OS: Mac and Linux

$ brew install varabyte/tap/kobweb

OS: Windows

# Note: Adding buckets only has to be done once.

# Feel free to skip java if you already have it
> scoop bucket add java
> scoop install java/openjdk

# Install kobweb
> scoop bucket add varabyte https://github.com/varabyte/scoop-varabyte.git
> scoop install varabyte/kobweb

OS: Windows, Mac, and *nix

$ sdk install kobweb

Thanks a ton to aksh1618 for adding support for this target!

With an AUR helper:

$ trizen -S kobweb

Without an AUR helper:

$ git clone https://aur.archlinux.org/kobweb.git
$ cd kobweb
$ makepkg -si

Please see: https://github.com/varabyte/kobweb/issues/117 and consider leaving a comment!

Our binary artifact is hosted on GitHub. To download latest, you can either grab the zip or tar file from the GitHub or you can fetch it from your terminal:

$ cd /path/to/applications

# You can either pull down the zip file

$ wget https://github.com/varabyte/kobweb-cli/releases/download/v0.9.13/kobweb-0.9.13.zip
$ unzip kobweb-0.9.13.zip

# ... or the tar file

$ wget https://github.com/varabyte/kobweb-cli/releases/download/v0.9.13/kobweb-0.9.13.tar
$ tar -xvf kobweb-0.9.13.tar

and I recommend adding it to your path, either directly:

$ PATH=$PATH:/path/to/applications/kobweb-0.9.13/bin
$ kobweb version # to check it's working

or via symbolic link:

$ cd /path/to/bin # some folder you've created that's in your PATH
$ ln -s /path/to/applications/kobweb-0.9.13/bin/kobweb kobweb

Although we host Kobweb artifacts on GitHub, it's easy enough to build your own.

Building Kobweb requires JDK11 or newer. We'll first discuss how to add it.

If you want full control over your JDK install, manually downloading is a good option.

• Download a JDK for your OS
• Unzip it somewhere
• Update your JAVA_HOME variable to point at it.

JAVA_HOME=/path/to/jdks/corretto-11.0.12
# ... or whatever version or path you chose

For a more automated approach, you can request IntelliJ install a JDK for you.

Follow their instructions here: https://www.jetbrains.com/help/idea/sdk.html#set-up-jdk

The Kobweb CLI is actually maintained in a separate GitHub repo. Once you have the JDK set up, it should be easy to clone and build it:

$ cd /path/to/src/root # some folder you've created for storing src code
$ git clone https://github.com/varabyte/kobweb-cli
$ cd kobweb-cli
$ ./gradlew :kobweb:installDist

Finally, update your PATH:

$ PATH=$PATH:/path/to/src/root/kobweb-cli/kobweb/build/install/kobweb/bin
$ kobweb version # to check it's working

If you previously installed Kobweb and are aware that a new version is available, the way you update it depends on how you installed it.

$ cd /path/to/projects/
$ kobweb create app

You'll be asked a few questions required for setting up your project.

You don't need to create a root folder for your project ahead of time - the setup process will prompt you for one to create.

When finished, you'll have a basic project with three pages - a home page, an about page, and a markdown page - and some components (which are collections of reusable, composable pieces). Your own directory structure should look something like:

my-project
└── site/src/jsMain
    ├── kotlin.org.example.myproject
    │   ├── components
    │   │   ├── layouts
    │   │   │   └── PageLayout.kt
    │   │   ├── sections
    │   │   │   └── NavHeader.kt
    │   │   └── widgets
    │   │       └── GoHomeLink.kt
    │   ├── pages
    │   │   ├── About.kt
    │   │   └── Index.kt
    │   └── MyApp.kt
    └── resources/markdown
        └── Markdown.md

Note that there's no index.html or routing logic anywhere! We generate that for you automatically when you run Kobweb. Which brings us to the next section...

$ cd /path/to/projects/your-project/site
$ kobweb run

This command spins up a webserver at http:https://localhost:8080. If you want to configure the port, you can do so by editing your project's .kobweb/conf.yaml file.

You can open your project in IntelliJ and start editing it. While Kobweb is running, it will detect changes, recompile, and deploy updates to your site automatically.

If you don't want to keep a separate terminal window open beside your IDE window, you may prefer alternate solutions.

You can use the IntelliJ terminal tool window to run kobweb within it. If you run into a compile error, the stack trace lines will get decorated with links, making it easy to navigate to the relevant source.

kobweb itself delegates to Gradle, but nothing is stopping you from calling the commands yourself. You can create Gradle run configurations for each of the Kobweb commands.

• To start a Kobweb server, use the kobwebStart -t command.
  • The -t argument (or, --continuous) tells Gradle to watch for file changes, which gives you live loading behavior.
• To stop a running Kobweb server, use the kobwebStop command.
• To export a site, use
  kobwebExport -PkobwebReuseServer=false -PkobwebEnv=DEV -PkobwebRunLayout=KOBWEB -PkobwebBuildTarget=RELEASE -PkobwebExportLayout=KOBWEB
  • If you want to export a static layout instead, change the last argument to
    -PkobwebExportLayout=STATIC.
• To run an exported site, use
  kobwebStart -PkobwebEnv=PROD -PkobwebRunLayout=KOBWEB
  • If your site was exported using a static layout, change the last argument to
    -PkobwebRunLayout=STATIC.

You can read all about IntelliJ's Gradle integration here. Or to just jump straight into how to create run configurations for any of the commands discussed above, read these instructions.

Kobweb will provide a growing collection of samples for you to learn from. To see what's available, run:

$ kobweb list

You can create the following Kobweb projects by typing `kobweb create ...`

• app: A template for a minimal site that demonstrates the basic features of Kobweb
• examples/jb/counter: A very minimal site with just a counter (based on the Jetbrains tutorial)
• examples/todo: An example TODO app, showcasing client / server interactions

For example, kobweb create examples/todo will instantiate a TODO app locally.

Kobweb, at its core, is a handful of classes responsible for trimming away much of the boilerplate around building a Compose HTML app, such as routing and configuring basic CSS styles.

Kobweb is also a CLI binary of the same name which provides commands to handle the tedious parts of building and / or running a Compose HTML app. We want to get that stuff out of the way, so you can enjoy focusing on the more interesting work!

Creating a page is easy! It's just a normal @Composable method. To upgrade your composable to a page, all you need to do is:

1. Define your composable in a file somewhere under the pages package in your jsMain source directory.
2. Annotate it with @Page

Just from that, Kobweb will create a site entry for you automatically.

For example, if I create the following file:

// jsMain/kotlin/com/mysite/pages/admin/Settings.kt

@Page
@Composable
fun SettingsPage() {
    /* ... */
}

this will create a page that I can then visit by going to mysite.com/admin/settings.

By default, the slug comes from the file name, but this behavior can be overridden (more on that shortly).

The file name Index.kt is special. If a page is defined inside such a file, it will be treated as the default page under that URL. For example, a page defined in .../pages/admin/Index.kt will be visited if the user visits mysite.com/admin/.

If you ever need to change the route generated for a page, you can set the Page annotation's routeOverride field:

// jsMain/kotlin/com/mysite/pages/admin/Settings.kt

@Page(routeOverride = "config")
@Composable
fun SettingsPage() {
    /* ... */
}

The above would create a page you could visit by going to mysite.com/admin/config.

routeOverride can additionally contain slashes, and if the value begins and/or ends with a slash, that has a special meaning.

• Begins with a slash - represent the whole route from the root
• Ends with a slash - a slug will still be generated from the filename and appended to the route.

And if you set the override to "index", that behaves the same as setting the file to Index.kt as described above.

Some examples can clarify these rules (and how they behave when combined). Assuming we're defining a page for our site example.com within the file a/b/c/Slug.kt:

If you don't want to change your slug but you do want to change a part of the route, you don't have to use a Page annotation for this. You can instead register a package mapping with a PackageMapping file annotation. Doing so looks like this:

// site/pages/blog/_2022/PackageMapping.kt
@file:PackageMapping("2022")

package site.pages.blog._2022

import com.varabyte.kobweb.core.PackageMapping

As with the Page route overrides, the main reason you'd want to do this is that Java / Kotlin package naming requirements are much stricter than what you might want to allow in a URL part. site.com/blog/2022/mypost reads way better than site.com/blog/_2022/mypost.

Every page method provides access to its PageContext via the rememberPageContext() method.

A page's context provides it access to a router, allowing you to navigate to other pages, as well as other dynamic information about the current page's URL (discussed in the next section).

@Page
@Composable
fun ExamplePage() {
    val ctx = rememberPageContext()
    Button(onClick = { ctx.router.navigateTo("/other/page") }) {
        Text("Click me")
    }
}

You can use the page context to check the values of any query parameters passed into the current page's URL.

So if you visit site.com/posts?id=12345&mode=edit, you can query those values like so:

enum class Mode {
    EDIT, VIEW;

    companion object {
        fun from(value: String) {
           entries.find { it.name.equals(value, ignoreCase = true) }
               ?: error("Unknown mode: $value")
        }
    }
}

@Page
@Composable
fun Posts() {
    val ctx = rememberPageContext()
    // Here, I'm assuming these params are always present, but you can
    // use `get` instead of `getValue` to handle the nullable case.
    val postId = ctx.route.params.getValue("id").toInt()
    val mode = Mode.from(ctx.route.params.getValue("mode"))
    /* ... */
}

In addition to query parameters, Kobweb supports embedding arguments directly in the URL itself. For example, you might want to register the path users/{user}/posts/{post} which would be visited if the site visitor typed in a URL like users/bitspittle/posts/20211231103156.

How do we set it up? Thankfully, it's fairly easy.

But first, notice that in the example dynamic route users/{user}/posts/{post} there are actually two different dynamic parts, one in the middle and one at the tail end. These can be handled by the PackageMapping and Page annotations, respectively.

Pay attention to the use of the curly braces in the mapping name! That lets Kobweb know that this is a dynamic package.

// pages/users/user/PackageMapping.kt
@file:PackageMapping("{user}") // or @file:PackageMapping("{}")

package site.pages.users.user

import com.varabyte.kobweb.core.PackageMapping

If you pass an empty "{}" into the PackageMapping annotation, it directs Kobweb to use the name of the package itself (i.e. user in this specific case).

Like PackageMapping, the Page annotation can also take curly braces to indicate a dynamic value.

// pages/users/user/posts/Post.kt

@Page("{post}") // Or @Page("{}")
@Composable
fun PostPage() {
   /* ... */
}

An empty "{}" tells Kobweb to use the name of the current file.

Remember that the Page annotation allows you to rewrite the entire route. That value also accepts dynamic parts, so you could even do something like:

// pages/users/user/posts/Post.kt

@Page("/users/{user}/posts/{post}") // Or @Page("/users/{user}/posts/{}")
@Composable
fun PostPage() {
    /* ... */
}

but with great power comes great responsibility. Tricks like this may be hard to find and/or update later, especially as your project gets larger. While it works, you should only use this format in cases where you absolutely need to (perhaps after a code refactor where you have to support legacy URL paths).

You query dynamic route values exactly the same as if you were requesting query parameters. That is, use ctx.params:

@Page("{}")
@Composable
fun PostPage() {
    val ctx = rememberPageContext()
    val postId = ctx.route.params.getValue("post")
    /* ... */
}

Silk is a UI layer included with Kobweb and built upon Compose HTML. (To learn more about Compose HTML, please visit the official tutorials).

While Compose HTML requires you to understand underlying HTML / CSS concepts, Silk attempts to abstract some of that away, providing an API more akin to what you might experience developing a Compose app on Android or Desktop. Less "div, span, flexbox, attrs, styles, classes" and more "Rows, Columns, Boxes, and Modifiers".

We consider Silk a pretty important part of the Kobweb experience, but it's worth pointing out that it's designed as an optional component. You can absolutely use Kobweb without Silk. (You can also use Silk without Kobweb!).

You can also interleave Silk and Compose HTML components easily (as Silk is just composing them itself).

For those new to web dev, it's worth understanding that there are two ways to set styles on your HTML elements: inline and stylesheet.

Inline styles are defined on the element tag itself. In raw HTML, this might look like:

<div style="background-color:black">

Meanwhile, any given HTML page can reference a list of stylesheets which can define a bunch of styles, where each style is tied to a selector (a rule which selects what elements those styles apply to).

A concrete example of a very short stylesheet can help here:

body {
  background-color: black;
  color: magenta
}
#title {
  color: yellow
}

And you could use that stylesheet to style the following document:

<body>
  <!-- Title gets background-color from "body" and foreground color from "#title" -->
  <div id="title">Yellow on black</div>
  Magenta on black
</body>

There's no hard and fast rule, but in general, when writing HTML / CSS by hand, stylesheets are often preferred over inline styles as it better maintains a separation of concerns. That is, the HTML should represent the content of your site, while the CSS controls the look and feel.

However! We're not writing HTML / CSS by hand. We're using Compose HTML! Should we even care about this in Kotlin?

As it turns out, there are times when you have to use stylesheets, because without them, you can't define styles for advanced behaviors (particularly pseudo classes, pseudo elements, and media queries). For example, you can't override the color of visited links without using a stylesheet approach. So it's worth realizing there are fundamental differences.

Finally, it can also be much easier debugging your page with browser tools when you lean on stylesheets over inline styles, as it makes your DOM tree easier to read when your elements are simple (e.g. <div class="title"> vs. <div style="color:yellow; background-color:black; font-size: 24px; ...">).

We'll be introducing and discussing modifiers and component styles in more detail shortly. But in general, when you pass modifiers directly into a composable widget in Silk, those will result in inline styles, whereas if you use a component style to define your styles, those will get embedded into the site's stylesheet:

// Uses inline styles
Box(Modifier.color(Colors.Red)) { /* ... */ }

// Uses a stylesheet
val BoxStyle by ComponentStyle {
    base { Modifier.Color(Colors.Red) }
}
Box(BoxStyle.toModifier()) { /* ... */ }

As a beginner, or even as an advanced user when prototyping, feel free to use inline modifiers as much as you can, pivoting to component styles if you find yourself needing to use pseudo classes, pseudo elements, or media queries. It is fairly easy to migrate inline styles over to stylesheets in Kobweb.

In my own projects, I tend to use inline styles for really simple layout elements (e.g. Row(Modifier.fillMaxWidth())) and component styles for complex and/or re-usable widgets. It actually becomes a nice organizational convention to have all your styles grouped together in one place above the widget itself.

Silk introduces the Modifier class, in order to provide an experience similar to what you find in Jetpack Compose. (You can read more about them here if you're unfamiliar with the concept).

In the world of Compose HTML, you can think of a Modifier as a wrapper on top of CSS styles and attributes.

Please refer to official documentation if you are not familiar with HTML attributes and/or styles.

So this:

Modifier.backgroundColor(Colors.Red).color(Colors.Green).padding(200.px)

when passed into a widget provided by Kobweb, like Box:

Box(Modifier.backgroundColor(Colors.Red).color(Colors.Green).padding(200.px)) {
    /* ... */
}

would generate an HTML tag with a style property like: <div style="background:red;color:green;padding:200px">

There are a bunch of modifier extensions (and they're growing) provided by Kobweb, like background, color, and padding above. But there are also two escape hatches anytime you run into a modifier that's missing: attrsModifier and styleModifier.

At this point, you are interacting with Compose HTML, one layer underneath Kobweb.

Using them looks like this:

// Modify attributes of an element tag
// e.g. the "a", "b", and "c" in <tag a="..." b="..." c="..." />
Modifier.attrsModifier {
    id("example")
}

// Modify styles of an element tag
// e.g. the "x", "y", and "z" in `<tag a="..." b="..." c="..." style="x:...;y:...;z:..." />
Modifier.styleModifier {
    width(100.percent)
    height(50.percent)
}

// Note: Because "style" itself is an attribute, you can define styles in an attrsModifier:
Modifier.attrsModifier {
    id("example")
    style {
        width(100.percent)
        height(50.percent)
    }
}
// ... but in the above case, you should use a styleModifier for simplicity

In the occasional (and hopefully rare!) case where Kobweb doesn't provide a modifier and Compose HTML doesn't provide the attribute or style support you need, you can use attrsModifier plus the attr method or styleModifier plus the property method. This escape hatch within an escape hatch allows you to provide any custom value you need.

The above cases can be rewritten as:

Modifier.attrsModifier {
    attr("id", "example")
}

Modifier.styleModifier {
    property("width", 100.percent)
    // Or even raw CSS:
    // property("width", "100%")
    property("height", 50.percent)
}

If you end up needing to use attr or property in your own codebase, consider filing an issue with us so that we can add the missing modifier to the library.

With Silk, you can define a style like so, using the base block:

val CustomStyle by ComponentStyle {
    base {
        Modifier.background(Colors.Red)
    }
}

and convert it to a modifier by using CustomStyle.toModifier(). At this point, you can pass it into any composable which takes a Modifier parameter:

// Approach #1 (uses inline styles)
Box(Modifier.backgroundColor(Colors.Red)) { /* ... */ }

// Approach #2 (uses stylesheets)
Box(CustomStyle.toModifier()) { /* ... */ }

You can simplify the syntax of basic component styles a bit further with the ComponentStyle.base declaration:

val CustomStyle by ComponentStyle.base {
    Modifier.background(Colors.Red)
}

Just be aware you may have to break this out again if you find yourself needing to support additional states ▼.

Note above we used the by keyword above to create a component style. This automatically generates a name for your style under the hood, derived from the property name itself but using Kebab Case.

For example, if you write val TitleTextStyle by ComponentStyle, its name behind the scenes will be "title-text".

You usually won't need to care about this name, but there are niche cases where it can be useful to understand that is what's going on.

If you need to set a name manually, there's an alternate constructor version (notice the use of assignment instead of the by keyword):

val CustomStyle = ComponentStyle("my-custom-name") {
    base {
        Modifier.background(Colors.Red)
    }
}

So, what's up with the base block?

True, it looks a bit verbose on its own. However, you can define additional styles that take effect conditionally. The base style will always apply first, but then additional styles can be applied based on what state the element is in. (If multiple states are applicable at the same time, they will be applied in the order specified.)

Here, we create a style which is red by default, but green when the mouse hovers over it:

val CustomStyle by ComponentStyle {
    base {
        Modifier.color(Colors.Red)
    }

    hover {
        Modifier.color(Colors.Green)
    }
}

Kobweb provides a bunch of these state blocks for you for convenience, but for those who are CSS-savvy, you can always define the CSS rule directly to enable more complex combinations or reference states that Kobweb hasn't added yet.

For example, this is identical to the above style definition:

val CustomStyle by ComponentStyle {
    base {
        Modifier.color(Colors.Red)
    }

    cssRule(":hover") {
        Modifier.color(Colors.Green)
    }
}

There's a feature in the world of responsive HTML / CSS design called breakpoints, which confusingly have nothing to do with debugging breakpoints. Rather, they specify size boundaries for your site when styles change. This is how sites present content differently on mobile vs. tablet vs. desktop.

Kobweb provides four breakpoint sizes you can use for your project, which, including using no breakpoint size at all, gives you five buckets you can work with when designing your site:

• no breakpoint - mobile (and larger)
• sm - tablets (and larger)
• md - desktops (and larger)
• lg - widescreen (and larger)
• xl - ultra widescreen (and larger)

You can change the default values of breakpoints for your site by adding an "@InitSilk" block to your code:

@InitSilk
fun initializeBreakpoints(ctx: InitSilkContext) {
    ctx.theme.breakpoints = BreakpointSizes(
        sm = 30.cssRem,
        md = 48.cssRem,
        lg = 62.cssRem,
        xl = 80.cssRem,
    )
}

To reference a breakpoint in a ComponentStyle, just invoke it:

val CustomStyle by ComponentStyle {
    base {
        Modifier.fontSize(24.px)
    }

    Breakpoint.MD {
        Modifier.fontSize(32.px)
    }
}

When you define a ComponentStyle, an optional field is available for you to use called colorMode:

val CustomStyle by ComponentStyle {
    base {
        Modifier.color(if (colorMode.isLight) Colors.Red else Colors.Pink)
    }
}

Silk defines a bunch of light and dark colors for all of its widgets, and if you'd like to re-use any of them in your own widget, you can query them using colorMode.toPalette():

val CustomStyle by ComponentStyle {
    base {
        Modifier.color(colorMode.toPalette().link.default)
    }
}

SilkTheme contains very simple (e.g. black and white) defaults, but you can override them in an @InitSilk method, perhaps to something that is more brand aware:

// Assume a bunch of color constants (e.g. BRAND_LIGHT_COLOR) are defined somewhere

@InitSilk
fun overrideSilkTheme(ctx: InitSilkContext) {
    ctx.theme.palettes.light.background = BRAND_LIGHT_BACKGROUND
    ctx.theme.palettes.light.color = BRAND_LIGHT_COLOR
    ctx.theme.palettes.dark.background = BRAND_DARK_BACKGROUND
    ctx.theme.palettes.dark.color = BRAND_DARK_COLOR
}

With a style, you can also create a variant of that style (that is, additional modifications that are always applied on top of the style).

You define one using the ComponentStyle.addVariant method, but otherwise the declaration looks the same as defining a ComponentStyle:

val HighlightedCustomVariant by CustomStyle.addVariant {
    base {
        Modifier.backgroundColor(Colors.Green)
    }
}

Variants can be particularly useful if you're defining a custom widget that has default styles, but you want to give callers an easy way to deviate from it in special cases.

For example, maybe you define a button widget (perhaps you're not happy with the one provided by Silk):

val ButtonStyle by ComponentStyle { /* ... */ }

// Note: Creates a style called "button-outline"
val OutlineButtonVariant by ButtonStyle.addVariant { /* ... */ }

// Note: Creates a style called "button-inverted"
val InvertedButtonVariant by ButtonStyle.addVariant { /* ... */ }

The ComponentStyle.toModifier(...) method, mentioned earlier, optionally takes a variant parameter. When passed in, both styles will be applied -- the base style followed by the variant style.

For example, MyButtonStyle.toModifier(OutlineButtonVariant) applies the main button style first followed by additional outline styling.

Like ComponentStyle.base, variants that don't need to support additional states can use addVariantBase instead to slightly simplify their declaration:

val HighlightedCustomVariant by CustomStyle.addVariantBase {
    Modifier.backgroundColor(Colors.Green)
}

Like component styles created using the by keyword, variants have their name autogenerated for you. If you need to control this name for any reason, you can use assignment instead and pass a name into addVariant, e.g. val InvertedButtonVariant = ButtonStyle.addVariant("custom-name") { /* ... */ }

While Silk methods are all written to support component styles and variants, if you ever want to write your own custom widget that mimics Silk, your code should look something like:

val CustomWidgetStyle by ComponentStyle { /* ... */ }

@Composable
fun CustomWidget(
    modifier: Modifier = Modifier,
    variant: ComponentVariant? = null,
    @Composable content: () -> Unit
) {
    val finalModifier = CustomWidgetStyle.toModifier(variant).then(modifier)
    Box(finalModifier, content)
}

In other words, you should take in an optional ComponentVariant parameter, and then you should apply the modifiers in order of: base style, then variant, then finally user overrides.

A caller might call your widget one of several ways:

// Approach #1: Use default styling
CustomWidget { /* ... */ }

// Approach #2: Tweak default styling with a variant
CustomWidget(variant = TransparentWidgetVariant) { /* ... */ }

// Approach #3: Tweak default styling with user overrides
CustomWidget(Modifier.backgroundColor(Colors.Blue)) { /* ... */ }

// Approach #4: Tweak default styling with a variant and then user overrides
CustomWidget(Modifier.backgroundColor(Colors.Blue), variant = TransparentWidgetVariant) { /* ... */ }

In CSS, animations work by letting you define keyframes in a stylesheet which you then reference, by name, in an animation style. You can read more about them on Mozilla's documentation site.

For example, here's the CSS for an animation of a sliding rectangle (from this tutorial):

div {
  width: 100px;
  height: 100px;
  background: red;
  position: relative;
  animation: mymove 5s infinite;
}

@keyframes mymove {
  from {left: 0px;}
  to {left: 200px;}
}

Kobweb lets you define your keyframes in code by using the by Keyframes pattern:

val ShiftRight by Keyframes {
    from { Modifier.left(0.px) }
    to { Modifier.left(200.px) }
}

// Later
Div(
    Modifier
        .size(100.px).backgroundColor(Colors.Red).position(Position.Relative)
        .animation(ShiftRight.toAnimation(
            duration = 5.s,
            iterationCount = AnimationIterationCount.Infinite
        ))
)

The name of the keyframes block is automatically derived from the property name (here, ShiftRight is converted into "shift-right"). You can then use the toAnimation method to convert your collection of keyframes into an animation that uses them, which you can pass into the Modifier.animation modifier.

Occasionally, you may need access to the raw element backing the Silk widget you've just created. All Silk widgets provide an optional ref parameter which take a listener that provide this information.

Box(
    ref = /* ... */
) {
    /* ... */
}

All ref callbacks (discussed more below) will receive an org.w3c.dom.Element subclass. You can check out the Element class (and its often more relevant HTMLElement inheritor) to see the methods and properties that are available on it.

Raw HTML elements expose a lot of functionality not available through the higher level Compose HTML APIs.

For a trivial but common example, we can use the raw element to capture focus:

Box(
    ref = ref { element ->
        // Triggered when this Box is first added into the DOM
        element.focus()
    }
)

The ref { ... } method can actually take one or more optional keys of any value. If any of these keys change on a subsequent recomposition, the callback will be rerun:

val colorMode by ColorMode.currentState
Box(
    // Callback will get triggered each time the color mode changes
    ref = ref(colorMode) { element -> /* ... */ }
)

If you need to know both when the element enters AND exits the DOM, you can use disposableRef instead. With disposableRef, the very last line in your block must be a call to onDispose:

val activeElements: MutableSet<HTMLElement> = /* ... */

/* ... later ... */

Box(
    ref = disposableRef { element ->
        activeElements.put(element)
        onDispose { activeElements.remove(element) }
    }
)

The disposableRef method can also take keys which rerun the listener if any of them change. The onDispose callback will also be triggered in that case, as the old effect gets discarded.

And, finally, you may want to have multiple listeners that are recreated independently of one another based on different keys. You can use refScope as a way to combine two or more ref and/or disposableRef calls in any combination:

val isFeature1Enabled: Boolean = /* ... */
val isFeature2Enabled: Boolean = /* ... */

Box(
    ref = refScope {
        ref(isFeature1Enabled) { element -> /* ... */ }
        disposableRef(isFeature2Enabled) { element -> /* ... */; onDispose { /* ... */ } }
    }
)

You may occasionally want the backing element of a normal Compose HTML widget, such as a Div or Span. However, these widgets don't have a ref callback, as that's a convenience feature provided by Silk.

You still have a few options in this case.

The official way to retrieve a reference is by using a ref block inside an attrs block. This version of ref is actually more similar to Silk's disposableRef concept than its ref one, as it requires an onDispose block:

Div(attrs = {
    ref { element -> /* ... */; onDispose { /* ... */ } }
})

The above snippet was adapted from the official tutorials.

You could put that exact same logic inside the Modifier.toAttrs block, if you're terminating some modifier chain:

Div(attrs = Modifier.toAttrs {
  ref { element -> /* ... */; onDispose { /* ... */ } }
})

Unlike Silk's version of ref, Compose HTML's version does not accept keys. If you need this behavior and if the Compose HTML widget accepts a content block (many of them do), you can call Silk's registerRefScope method directly within it:

Div {
  registerRefScope(
    disposableRef { element -> /* ... */; onDispose { /* ... */ } }
    // or ref { element -> /* ... */ }
  )
}

Kobweb supports CSS variables (also called CSS custom properties), which is a feature where you can store and retrieve property values from variables declared within your CSS styles. It does this through a class called StyleVariable.

Using variables is fairly simple. You first declare one without a value (but lock it down to a type) and later you can initialize it within a style using Modifier.setVariable(...):

val dialogWidth by StyleVariable<CSSLengthValue>()

// This style will be applied to a div that wraps the whole page
val RootStyle by ComponentStyle {
  base { Modifier.setVariable(dialogWidth, 600.px) }
}

You can later use variables that were previously set, using the value() method to extract their current value:

val DialogStyle by ComponentStyle {
  base { Modifier.width(dialogWidth.value()) }
}

You can also provide a fallback value, which, if present, would be used in the case that a variable hadn't already been set previously:

val DialogStyle by ComponentStyle {
  base { Modifier.width(dialogWidth.value(500.px)) }
}

Additionally, you can also provide a default fallback value when declaring the variable:

val dialogWidth by StyleVariable<CSSLengthValue>(100.px)

// This style will be applied to a div that wraps the whole page
val DialogStyle100 by ComponentStyle {
  // Uses default fallback. width = 100px
  base { Modifier.width(dialogWidth.value()) }
}
val DialogStyle200 by ComponentStyle {
  // Uses specific fallback. width = 200px
  base { Modifier.width(dialogWidth.value(200.px)) } // Uses fallback = 200.px
}
val DialogStyle300 by ComponentStyle {
  // Fallback ignored because variable is set explicitly. width = 300px
  base { Modifier.setVariable(dialogWidth, 300.px).width(dialogWidth.value(400.px)) }
}

To demonstrate these concepts all together, below we declare a background color variable, create a root container scope which sets it, a child style that uses it, and, finally, a child style variant that overrides it:

// Default to a debug color, so if we see it, it indicates we forgot to set it later
val bgColor by StyleVariable<CSSColorValue>(Colors.Magenta)

val ContainerStyle by ComponentStyle {
    base { Modifier.setVariable(bgColor, Colors.Blue) }
}
val SquareStyle by ComponentStyle {
    base { Modifier.size(100.px).backgroundColor(bgColor.value()) }
}
val RedSquareVariant by SquareStyle.addVariant {
    base { Modifier.setVariable(bgColor, Colors.Red) }
}

The following code brings the above styles together (and in some cases uses inline styles to override the background color further):

@Composable
fun ColoredSquares() {
    Box(ContainerStyle.toModifier()) {
        Column {
            Row {
                // 1: Read color from ancestor's component style
                Box(SquareStyle.toModifier())
                // 2: Override color via variant
                Box(SquareStyle.toModifier(RedSquareVariant))
            }
            Row {
                // 3: Override color via inline styles
                Box(SquareStyle.toModifier().setVariable(bgColor, Colors.Green))
                Span(Modifier.setVariable(bgColor, Colors.Yellow).toAttrs()) {
                    // 4: Read color from parent's inline style
                    Box(SquareStyle.toModifier())
                }
            }
        }
    }
}

The above renders the following output:

You can also set CSS variables directly from code, if you have access to the backing HTML element. Below, we use the ref callback to get the backing element for a fullscreen Box and then use a Button to set it to a random color from the colors of the rainbow:

val bgColor by StyleVariable<CSSColorValue>()

val ScreenStyle by ComponentStyle {
    base {
        Modifier.fillMaxSize().backgroundColor(
            // We specify `Red` as a fallback here, since the variable
            // won't otherwise be set when the UI first renders.
            bgColor.value(Colors.Red)
        )
    }
}

@Page
@Composable
fun RainbowBackground() {
    val roygbiv = listOf(Colors.Red, /*...*/ Colors.Violet)

    var screenElement: HTMLElement? by remember { mutableStateOf(null) }
    Box(ScreenStyle.toModifier(), ref = ref { screenElement = it }) {
        Button(onClick = {
            // We have the backing HTML element, so use setProperty to set the variable value directly
            screenElement!!.setVariable(bgColor, roygbiv.random())
        }) {
            Text("Click me")
        }
    }
}

The above results in the following UI:

Most of the time, you can actually get away with not using CSS Variables! Your Kotlin code is often a more natural place to describe dynamic behavior than HTML / CSS is.

Let's revisit the "colored squares" example from above. Note it's much easier to read if we don't try to use variables at all.

val SquareStyle by ComponentStyle {
    base { Modifier.size(100.px) }
}

@Composable
fun ColoredSquares() {
    Column {
        Row {
            Box(SquareStyle.toModifier().backgroundColor(Colors.Blue))
            Box(SquareStyle.toModifier().backgroundColor(Colors.Red))
        }
        Row {
            Box(SquareStyle.toModifier().backgroundColor(Colors.Green))
            Box(SquareStyle.toModifier().backgroundColor(Colors.Yellow))
        }
    }
}

And the "rainbow background" example is similarly easier to read by using Kotlin variables (i.e. var someValue by remember { mutableStateOf(...) }) instead of CSS variables:

val ScreenStyle by ComponentStyle {
    base { Modifier.fillMaxSize() }
}

@Page
@Composable
fun RainbowBackground() {
    val roygbiv = listOf(Colors.Red, /*...*/ Colors.Violet)

    var currColor by remember { mutableStateOf(Colors.Red) }
    Box(ScreenStyle.toModifier().backgroundColor(currColor)) {
        Button(onClick = { currColor = roygbiv.random() }) {
            Text("Click me")
        }
    }
}

Even though you should rarely need CSS variables, there may be occasions where they can be a useful tool in your toolbox. The above examples were artificial scenarios used as a way to show off CSS variables in relatively isolated environments. But here are some situations that might benefit from CSS variables:

• You have a site which allows users to choose from a list of several themes (e.g. primary and secondary colors). It would be trivial enough to add CSS variables for themePrimary and themeSecondary (applied at the site's root) which you can then reference throughout your styles.
• You need more control for colors in your theming than can be provided for by the simple light / dark color mode. For example, Wordle has light / dark + normal / contrast modes.
• You want to create a widget which dynamically changes its behavior based on the context it is added within. For example, maybe your site has a dark area and a light area, and the widget should use white outlines in the dark area and black outlines in the light. This can be accomplished by exposing an outline color variable, which each area of your site is responsible for setting.
• You are using a component style for a pseudo-class selector already (e.g. hover, focus, active) and you want that behavior to be dynamic.
• You have a widget that you ended up creating a bunch of variants for, but instead you realize you could replace them all with one or two CSS variables.

When in doubt, however, lean on Kotlin for handling dynamic behavior. If you want to share common style settings across multiple component styles, just declare a Modifier instance somewhere and share that in each style. If you want behavior to change based on some event, prefer using inline styles instead of hiding the logic inside component styles.

Kobweb provides the silk-icons-fa artifact which you can use in your project if you want access to all the free Font Awesome (v6) icons.

Using it is easy! Search the Font Awesome gallery, choose an icon, and then call it using the associated Font Awesome icon composable.

For example, if I wanted to add the Kobweb-themed spider icon, I could call this in my Kobweb code:

FaSpider()

That's it!

Some icons have a choice between solid and outline versions, such as "Square" (outline and filled). In that case, the default choice will be outline mode, but you can pass in a style enum to control this:

FaSquare(style = IconStyle.FILLED)

All Font Awesome composables accept a modifier parameter, so you can tweak it further:

FaSpider(Modifier.color(Colors.Red))

Kobweb provides the kobweb-silk-icons-mdi artifact which you can use in your project if you want access to all the free Material Design icons.

Using it is easy! Search the Material Icons gallery, choose an icon, and then call it using the associated Material Design Icon composable.

For example, let's say after a search I found and wanted to use their bug report icon, I could call this in my Kobweb code by converting the name to camel case:

MdiBugReport()

That's it!

Most material design icons support multiple styles: outlined, filled, rounded, sharp, and two-tone. Check the gallery search link above to verify what styles are supported by your icon. You can identify the one you want to use by passing it into the method's style parameter:

MdiLightMode(style = IconStyle.TWO_TONE)

All Material Design Icon composables accept a modifier parameter, so you can tweak it further:

MdiError(Modifier.color(Colors.Red))

Outside of pages, it is common to create reusable, composable parts. While Kobweb doesn't enforce any particular rule here, we recommend a convention which, if followed, may make it easier to allow new readers of your codebase to get around.

First, as a sibling to pages, create a folder called components. Within it, add:

• layouts - High-level composables that provide entire page layouts. Most (all?) of your @Page pages will start by calling a page layout function first. It's possible that you will only need a single layout for your entire site.
• sections - Medium-level composables that represent compound areas inside your pages, organizing a collection of many children composables. If you have multiple layouts, it's likely sections would be shared across them. For example, nav headers and footers are great candidates for this subfolder.
• widgets - Low-level composables. Focused UI pieces that you may want to re-use all around your site. For example, a stylized visitor counter would be a good candidate for this subfolder.

By default, Kobweb will automatically root every page to the KobwebApp composable (or, if using Silk, to a SilkApp composable). These perform some minimal common work (e.g. applying CSS styles) that should be present across your whole site.

This means if you register a page:

// jsMain/kotlin/com/mysite/pages/Index.kt

@Page
@Composable
fun HomePage() {
    /* ... */
}

then the final result that actually runs on your site will be:

// In a generated main.kt somewhere...

KobwebApp {
  HomePage()
}

It is likely you'll want to configure this further for your own application. Perhaps you have additional styles you'd like to globally define (e.g. the default font used by your site). Perhaps you have some initialization logic that you'd like to run before any page gets run (like logic for updating saved settings into local storage).

In this case, you can create your own root composable and annotate it with @App. If present, Kobweb will use that instead of its own default. You should, of course, delegate to KobwebApp (or SilkApp if using Silk), as the initialization logic from those methods should still be run.

Here's an example application composable override that I use in one of my own projects:

@App
@Composable
fun MyApp(content: @Composable () -> Unit) {
    SilkApp {
      val colorMode = ColorMode.current
        LaunchedEffect(colorMode) { // Relaunched every time the color mode changes
            localStorage.setItem(COLOR_MODE_KEY, colorMode.name)
        }

        // A full screen Silk surface. Sets the background based on Silk's palette and animates color changes.
        Surface(Modifier.minHeight(100.vh)) {
            content()
        }
    }
}

You can define at most a single @App on your site, or else the Kobweb Application plugin will complain at build time.

There are two flavors of Kobweb sites: static and full stack.

A static site (or, more completely, a static layout site) is one where you export a bunch of frontend files (e.g. html, js, and public resources) into a single, organized folder that gets served in a direct way by a static website hosting provider. In other words, the name static does not refer to the behavior of your site but rather that of your hosting provider solution.

A full stack site is one where you write both the logic that runs on the frontend (i.e. on the user's machine) as well as the logic that runs on the backend (i.e. on a server somewhere). This custom server must serve requested files (much like a static web hosting service does) plus it should also define endpoints providing unique functionality tailored to your site's needs.

When Kobweb was first written, it only provided the full stack solution, as being able to write your own server logic enabled a maximum amount of power and flexibility. The mental model for using Kobweb during this early time was simple and clear.

However, in practice, most projects didn't need this power. A website can give users a very clean, dynamic experience simply by writing responsive frontend logic to make it look good, e.g. with animations and delightful user interactions.

Additionally, many "Feature as a Service" solutions have popped up over the years, which can provide a ton of convenient functionality that used to require a custom server. These days, you can easily integrate auth, database, and analytics solutions all without writing a single line of backend code.

The process for exporting a bunch of files in a way that can be consumed by a static web hosting provider tends to be much faster and cheaper than using a full stack solution. Therefore, you should prefer a static site layout unless you have a specific need for a full stack approach.

Some possible reasons to use a custom server are:

• needing to communicate with other, private backend services in your company.
• intercepting requests as an intermediary for some third-party service where you own a very sensitive API key that you don't want to leak (such as a service that delegates to ChatGPT).
• acting as a hub to connect multiple clients together (such as a chat server).

If you aren't sure which category you fall into, then you should probably be creating a static layout site. It's much easier to migrate from a static layout site to a full stack site later than the other way around.

Both site flavors require an export. To export your site with a static layout, use the kobweb export --layout static command, while for full stack the command is kobweb export --layout kobweb (or just kobweb export since kobweb is the default layout as it originally was the only way).

Once exported, you can test your site by running it locally before uploading. You run a static site with kobweb run --env prod --layout static and a full stack site with kobweb run --env prod --layout kobweb (or just kobweb run --env prod).

A static site gets exported into .kobweb/site by default (you can configure this location in your .kobweb/conf.yaml file if you'd like). You can then upload the contents of that folder to the static web hosting provider of your choice.

Deploying a full stack site is a bit more complex, as different providers have wildly varying setups, and some users may even decide to run their own web server themselves. However, when you export your Kobweb site, scripts are generated for running your server, both for *nix platforms (.kobweb/server/start.sh) and the Windows platform (.kobweb/server/start.bat). If the provider you are using speaks Dockerfile, you can set ENTRYPOINT to either of these scripts (depending on the server's platform).

Going in more detail than this is outside the scope of this README. However, you can read my blog posts for a lot more information and some clear, concrete examples:

• Static site generation and deployment with Kobweb
• Deploying Kobweb into the cloud

Let's say you've decided on creating a full stack website using Kobweb. This section walks you through setting it up as well as introducing the various APIs for communicating to the backend from the frontend.

A Kobweb project will always at least have a JavaScript component, but if you declare a JVM target, that will be used to define custom server logic that can then be used by your Kobweb site.

It's easiest to let Kobweb do it for you. In your site's build script, make sure you've declared configAsKobwebApplication(includeServer = true):

// site/build.gradle.kts
import com.varabyte.kobweb.gradle.application.util.configAsKobwebApplication

plugins {
    alias(libs.plugins.kobweb.multiplatform)
    alias(libs.plugins.jetbrains.compose)
    alias(libs.plugins.kobweb.application)
}

/* ... */

kotlin {
    configAsKobwebApplication(includeServer = true)
    /* ... */
}

The easy way to check if everything is set up correctly is to open your project inside IntelliJ IDEA, wait for it to finish indexing, and check that the jvmMain folder is detected as a module (if so, it will be given a special icon and look the same as the jsMain folder):

You can define and annotate methods which will generate server endpoints you can interact with. To add one:

1. Define your method (optionally suspendable) in a file somewhere under the api package in your jvmMain source directory.
2. The method should take exactly one argument, an ApiContext.
3. Annotate it with @Api

For example, here's a simple method that echoes back an argument passed into it:

// jvmMain/kotlin/com/mysite/api/Echo.kt

@Api
fun echo(ctx: ApiContext) {
    // ctx.req is for the incoming request, ctx.res for responding back to the client

    // Params are parsed from the URL, e.g. here "/api/echo?message=..."
    val msg = ctx.req.params["message"] ?: ""
    ctx.res.setBodyText(msg)
}

After running your project, you can test the endpoint by visiting mysite.com/api/echo?message=hello

You can also trigger the endpoint in your frontend code by using the extension api property added to the kotlinx.browser.window class:

@Page
@Composable
fun ApiDemoPage() {
  val coroutineScope = rememberCoroutineScope()

  Button(onClick = {
    coroutineScope.launch {
      println("Echoed: " + window.api.get("echo?message=hello").decodeToString())
    }
  }) { Text("Click me") }
}

All the HTTP methods are supported (post, put, etc.).

These methods will throw an exception if the request fails for any reason. Note that for every HTTP method, there's a corresponding "try" version that will return null instead (tryPost, tryPut, etc.).

If you know what you're doing, you can of course always use window.fetch(...) directly.

Kobweb servers also support persistent connections via streams. Streams are essentially named channels that maintain continuous contact between the client and the server, allowing either to send messages to the other at any time. This is especially useful if you want your server to be able to communicate updates to your client without needing to poll.

Additionally, multiple clients can connect to the same stream. In this case, the server can choose to not only send a message back to your client, but also to broadcast messages to all users (or a filtered subset of users) on the same stream. You could use this, for example, to implement a chat server with rooms.

Like API routes, API streams must be defined under the api package in your jvmMain source directory. By default, the name of the stream will be derived from the file name and path that it's declared in.

Unlike API routes, API streams are defined as properties, not methods. This is because API streams need to be a bit more flexible than routes, since streams consist of multiple distinct events: client connection, client messages, and client disconnection.

Streams do not have to be annotated. The Kobweb Application plugin can automatically detect them.

For example, here's a simple stream that echoes back any argument passed into it:

// jvmMain/kotlin/com/mysite/api/Echo.kt

val echo = object : ApiStream {
  override suspend fun onClientConnected(ctx: ClientConnectedContext) {
    // Optional: ctx.stream.broadcast a message to all other clients that ctx.clientId connected
    // Optional: Update ctx.data here, initializing data associated with ctx.clientId
  }
  override suspend fun onTextReceived(ctx: TextReceivedContext) {
    ctx.stream.send(ctx.text)
  }
  override suspend fun onClientDisconnected(ctx: ClientDisconnectedContext) {
    // Optional: ctx.stream.broadcast a message to all other clients that ctx.clientId disconnected
    // Optional: Update ctx.data here, removing data associated with ctx.clientId
  }
}

To communicate with an API stream from your site, you need to create a stream connection there:

@Page
@Composable
fun ApiStreamDemoPage() {
  val echoStream = remember { ApiStream("echo") }
  LaunchedEffect(Unit) {
    echoStream.connect(object : ApiStreamListener {
      override fun onConnected(ctx: ConnectedContext) {  }
      override fun onTextReceived(ctx: TextReceivedContext) { console.log("Echoed: ${ctx.text}") }
      override fun onDisconnected(ctx: DisconnectedContext) {  }
    })
  }

  Button(onClick = {
      echoStream.send("hello!")
  }) { Text("Click me") }
}

After running your project, you can click on the button and check the console logs. If everything is working properly, you should see "Echoed: hello!" for each time you pressed the button.

The above example demonstrated API streams in their most verbose form. However, depending on your use-case, you can elide a fair bit of boilerplate.

First of all, the connect and disconnect handlers are optional, so you can omit them if you don't need them. Let's simplify the echo example:

// Backend
val echo = object : ApiStream {
  override suspend fun onTextReceived(ctx: TextReceivedContext) { ctx.stream.send(ctx.text) }
}

// Frontend
val echoStream = remember { ApiStream("echo") }
LaunchedEffect(Unit) {
  echoStream.connect(object : ApiStreamListener {
    override fun onTextReceived(ctx: TextReceivedContext) { console.log("Echoed: ${ctx.text}") }
  })
}

Additionally, if you only care about the text event, there are convenience overrides for that:

// Backend
val echo = ApiStream { ctx -> ctx.stream.send(ctx.text) }

// Frontend
val echoStream = remember { ApiStream("echo") }
LaunchedEffect(Unit) {
  echoStream.connect { ctx -> console.log("Echoed: ${ctx.text}") }
}

And finally, you can simplify the frontend code even further by using the rememberApiStream composable:

// Frontend
val echoStream = rememberApiStream("echo") { text -> console.log("Echoed: $text") }

// ApiStreamListener version also available:
// val echoStream = rememberApiStream("echo", object : ApiStreamListener { ... })

In practice, your API streams will probably be a bit more involved than the echo example, but it's nice to know that you can handle some cases only needing a one-liner on the server and another on the client to create a persistent client-server connection!

When faced with a choice, use API routes as often as you can. They are conceptually simpler, and you can query API endpoints with a CLI program like curl and sometimes even visit the URL directly in your browser. They are great for handling queries of or updates to server resources in response to user-driven actions (like visiting a page or clicking on a button). Every operation you perform returns a clear response code in addition to some payload information.

Meanwhile, API streams are very flexible and can be a natural choice to handle high-frequency communication. But they are also more complex. Unlike a simple request / response pattern, you are instead opting in to manage a potentially long lifetime during which you can receive any number of events. You may have to concern yourself about interactions between all the clients on the stream as well. API streams are fundamentally stateful.

You often need to make a lot of decisions when using API streams. What should you do if a client or server disconnects earlier than expected? How do you want to communicate to the client that their last action succeeded or failed (and you need to be clear about exactly which action because they might have sent another one in the meantime)? What structure do you want to enforce, if any, between a client and server connection where both sides can send messages to each other at any time?

Most importantly, API streams may not horizontally scale as well as API routes. At some point, you may find yourself in a situation where a new web server is spun up to handle some intense load.

If you're using API routes, you're already probably delegating to a database service as your data backend, so this may just work seamlessly.

But for API streams, you many naturally find yourself writing a bunch of broadcasting code. However, this only works to communicate between all clients that are connected to the same server. Two clients connected to the same stream on different servers are effectively in different, disconnected worlds.

The above situation is often handled by using a pubsub service (like Redis). This feels somewhat equivalent to using a database as a service in the API route situation, but this code might not be as straightforward to migrate.

API routes and API streams are not a you-must-use-one-or-the-other situation. Your project can use both! In general, try to imagine the case where a new server might get spun up, and design your code to handle that situation gracefully. API routes are generally safe to use, so use them often. However, if you have a situation where you need to communicate events in real-time, especially situations where you want your client to be continuously directed what to do by the server via events, API streams are a great choice.

If you create a markdown file under the jsMain/resources/markdown folder, a corresponding page will be created for you at build time, using the filename as its path.

For example, if I create the following file:

// jsMain/resources/markdown/docs/tutorial/Kobweb.kt

# Kobweb Tutorial

...

this will create a page that I can then visit by going to mysite.com/docs/tutorial/kobweb

Front Matter is metadata that you can specify at the beginning of your document, like so:

---
title: Tutorial
author: bitspittle
---

...

In a following section, we'll discuss how to embed code in your markdown, but for now, know that these key / value pairs can be queried in code using the page's context:

@Composable
fun AuthorWidget() {
    val ctx = rememberPageContext()
    // Note: You can use `markdown!!` only if you're sure that
    // this composable is called while inside a page generated
    // from Markdown.
    val author = ctx.markdown!!.frontMatter.getValue("author").single()
    Text("Article by $author")
}

Within your front matter, there's a special value which, if set, will be used to render a root @Composable that wraps the code your markdown file would otherwise create. This is useful for specifying a layout for example:

---
root: .components.layout.DocsLayout
---

# Kobweb Tutorial

The above will generate code like the following:

import com.mysite.components.layout.DocsLayout

@Composable
@Page
fun KobwebPage() {
    DocsLayout {
        H1 {
            Text("Kobweb Tutorial")
        }
    }
}

Kobweb Markdown front matter supports a routeOverride key. If present, its value will be passed into the generated @Page annotation (see the Route Override section ▲ for valid values here).

This allows you to give your URL a name that normal Kotlin filename rules don't allow for, such as a hyphen:

# AStarDemo.md

---
routeOverride: a-star-demo
---

The above will generate code like the following:

@Composable
@Page("a-star-demo")
fun AStarDemoPage() { /* ... */ }

You can additionally override the algorithm used for converting ALL markdown files to their final name, by setting the markdown block's routeOverride callback:

kobweb {
  markdown { //
    // Given "Example.md", name will be "Example" and output will be "post_example"
    routeOverride.set { name -> "post_${name.lowercase()}" }
  }
}

This callback will be triggered on all Markdown pages except Index.md files.

Some common algorithms are provided which you can use instead of writing your own:

import com.varabyte.kobwebx.gradle.markdown.MarkdownBlock.RouteOverride

kobweb {
  markdown {
    routeOverride.set(RouteOverride.KebabCase) // e.g. "ExamplePage" to "example-page"
  }
}

If you specify both a global route override and a local route override in the front matter, the front matter setting will take precedence.

The power of Kotlin + Compose HTML is interactive components, not static text! Therefore, Kobweb Markdown support enables special syntax that can be used to insert Kotlin code.

Usually, you will define widgets that belong in their own section. Just use three triple-curly braces to insert a function that lives in its own block:

# Kobweb Tutorial

...

{{{ .components.widgets.VisitorCounter }}}

which will generate code for you like the following:

@Composable
@Page
fun KobwebPage() {
    /* ... */
    com.mysite.components.widgets.VisitorCounter()
}

You may have noticed that the code path in the markdown file is prefixed with a .. When you do that, the final path will automatically be prepended with your site's full package.

Occasionally, you may want to insert a smaller widget into the flow of a single sentence. For this case, use the ${...} inline syntax:

Press ${.components.widgets.ColorButton} to toggle the site's current color.

You may wish to add imports to the code generated from your markdown. Kobweb Markdown supports registering both global imports (imports that will be added to every generated file) and local imports (those that will only apply to a single target file).

To register a global import, you configure the markdown block in your build script:

// site/build.gradle.kts

kobweb {
  markdown {
    imports.add(".components.widgets.*")
  }
}

Notice that you can begin your path with a "." to tell the Kobweb Markdown plugin to prepend your site's package to it. The above would ensure that every markdown file generated would have the following import:

import com.mysite.components.widgets.*

Imports can help you simplify your Kobweb calls. Revisiting an example from just above:

# Without imports

Press ${.components.widgets.ColorButton} to toggle the site's current color.

# With imports

Press ${ColorButton} to toggle the site's current color.

Local imports are specified in your markdown's Front Matter (and can even affect its root declaration!):

---
root: DocsLayout
imports:
  - .components.sections.DocsLayout
  - .components.widgets.VisitorCounter
---

...

{{{ VisitorCounter }}}

The project templates created by Kobweb all embrace Gradle version catalogs, which are (at the time of writing this README) a relatively new feature, so users may not be aware of it.

There is a file called libs.versions.toml that exists inside your project's root gradle folder. If you find yourself wanting to tweak or add new versions to projects you originally created via kobweb create, that's where you'll find them.

For example, here's the libs.versions.toml we use for our own landing site.

To read more about the feature, please check out the official docs.

The latest available version of Kobweb is declared at the top of this README. If a new version has come out, you can update your own project by visiting libs.version.toml and updating the kobweb version there.

You should also double-check COMPATIBILITY.md to see if you also need to update your kotlin and jetbrains-compose versions at the same time.

For simplicity, new projects can choose to put all their pages and widgets inside a single application module.

However, Kobweb is capable of splitting code up across modules. You can define components and/or pages in separate modules and apply the com.varabyte.kobweb.library plugin on them (in contrast to your main module which applies the com.varabyte.kobweb.application plugin.)

In other words, you can lay out your project like this:

my-project
├── sitelib
│   ├── build.gradle.kts # apply "com.varabyte.kobweb.library"
│   └── src/jsMain
│       └── kotlin.org.example.myproject.sitelib
│           ├── components
│           └── pages
└── site
    ├── build.gradle.kts # apply "com.varabyte.kobweb.application"
    └── src/jsMain
        └── kotlin.org.example.myproject.site
            ├── components
            └── pages

If you'd like to explore a multimodule project example, you can do so by running:

$ kobweb create examples/chat

which demonstrates a chat application with its auth and chat functionality each managed in their own separate modules.

Occasionally, you might find yourself wanting code for your site that is better generated programmatically than written by hand.

The recommended best practice is to create a Gradle task that is associated with its own unique output directory, use the task to write some code to disk under that directory, and then add that task as a source directory for your project.

You want to do this even if you only plan to generate a single file. This is because associating your task with an output directory is what enables it to be used in place of a source directory.

The structure for this approach generally looks like this:

// e.g. site/build.gradle.kts

val generateCodeTask = tasks.register("generateCode") {
  group = "myproject"
  // You may not need an input file or dir for your task, and if so, you can exclude the next line. If you do need one,
  // I'm assuming it is a data file or files in your resources somewhere.
  val resInputDir = layout.projectDirectory.dir("src/jsMain/resources")
  val genOutputDir = layout.buildDirectory.dir("generated/$group/src/jsMain/kotlin")

  inputs.dir(resInputDir).withPathSensitivity(PathSensitivity.RELATIVE)
  outputs.dir(genOutputDir)

  doLast {
    genOutputDir.get().file("org/example/pages/SomeCode.kt").asFile.apply {
      parentFile.mkdirs()
      // find and parse file out of resInputDir and write generated code here:
      writeText(/* ... */)

      println("Generated $absolutePath")
    }
  }
}

kotlin {
  configAsKobwebApplication()
  val commonMain by getting { /* ... */ }
  val jsMain by getting {
    kotlin.srcDir(generateCodeTask) // <----- Set your task here
    dependencies { /* ... */ }
  }
}

If you want to see this working in action, you can check out my blog site's build script here, where it is used to generate a listing page for all blog posts.

Currently, Kobweb is still under active development, and due to our limited resources, we are focusing on improving the path to creating a new project from scratch. However, some users have shown interest in Kobweb but already have an existing project and aren't sure how to add Kobweb into it.

As long as you understand that this path isn't officially supported yet, we'll provide steps below to take which may help people accomplish this manually for now. Honestly, the hardest part is creating a correct .kobweb/conf.yaml, which the following steps help you work around:

1. Be sure to check the Kobweb compatibility matrix (see: COMPATIBILITY.md) to make sure you can match the versions it expects.
2. Create a dummy app project somewhere. Pay attention to the questions it asks you, as you may want to choose a package name that matches your project.

   # In some tmp directory somewhere
   kobweb create app
   # or `kobweb create app/empty`, if you are already
   # experienced with Kobweb and know what you're doing

3. When finished, copy the site subfolder out into your own project. (Once done, you can delete the dummy project, as it has served its usefulness.)

   cp -r app/site /path/to/your/project
   # delete app

4. In your own project's root settings.gradle.kts file, include the new module and add our custom artifact repository link so your project can find the Kobweb Gradle plugins.

   // settings.gradle.kts
   pluginManagement {
     repositories {
       // ... other repositories you already declared ...
       maven("https://us-central1-maven.pkg.dev/varabyte-repos/public")
     }
   }
   // ... other includes you already declared
   include(":site")

5. In your project's root build.gradle.kts file, add our custom artifact repository there as well (so your project can find Kobweb libraries)

   // build.gradle.kts
   subprojects {
     repositories {
       // ... other repositories you already declared ...
       maven("https://us-central1-maven.pkg.dev/varabyte-repos/public")
     }
   }
   
   // If you prefer, you can just declare this directly inside the
   // repositories block in site's `build.gradle.kts` file, but I
   // like declaring my maven repositories globally.

6. Kobweb uses version catalogs for its dependencies. Add or update your version catalog under gradle/libs.versions.toml

   [versions]
   jetbrains-compose = "..." # replace with actual version, see COMPATIBILITY.md!
   kobweb = "..." # replace with actual version
   kotlin = "..." # replace with actual version
   
   [libraries]
   kobweb-api = { module = "com.varabyte.kobweb:kobweb-api", version.ref = "kobweb" }
   kobweb-core = { module = "com.varabyte.kobweb:kobweb-core ", version.ref = "kobweb" }
   kobweb-silk = { module = "com.varabyte.kobweb:kobweb-silk", version.ref = "kobweb" }
   kobwebx-markdown = { module = "com.varabyte.kobwebx:kobwebx-markdown", version.ref = "kobweb" }
   silk-icons-fa = { module = "com.varabyte.kobwebx:silk-icons-fa", version.ref = "kobweb" }
   
   [plugins]
   jetbrains-compose = { id = "org.jetbrains.compose", version.ref = "jetbrains-compose" }
   kobweb-application = { id = "com.varabyte.kobweb.application", version.ref = "kobweb" }
   kobwebx-markdown = { id = "com.varabyte.kobwebx.markdown", version.ref = "kobweb" }
   kotlin-multiplatform = { id = "org.jetbrains.kotlin.multiplatform", version.ref = "kotlin" }

If everything is working as expected, you should be able to run Kobweb within your project now:

# In /path/to/your/project
cd site
kobweb run

If you're still having issues, you may want to connect with us ▼ for support (but understand that getting Kobweb added to complex existing projects may not be something we can currently prioritize).

While you can always export your site manually on your machine, you may want to automate this process. A common solution for this is a GitHub workflow.

For your convenience, we include a sample workflow below that exports your site and then uploads the results (which can be downloaded from a link shown in the workflow summary page):

# .github/workflows/export-site.yml

name: Export Kobweb site

on:
  workflow_dispatch:

jobs:
  export_and_upload:
    runs-on: ubuntu-latest
    defaults:
      run:
        shell: bash

    env:
      KOBWEB_CLI_VERSION: 0.9.13

    steps:
      - uses: actions/checkout@v3
      - uses: actions/setup-java@v3
        with:
          distribution: temurin
          java-version: 11

      # When projects are created on Windows, the executable bit is sometimes lost. So set it back just in case.
      - name: Ensure Gradle is executable
        run: chmod +x gradlew

      - name: Setup Gradle
        uses: gradle/gradle-build-action@v2

      - name: Query Browser Cache ID
        id: browser-cache-id
        run: echo "value=$(./gradlew -q :site:kobwebBrowserCacheId)" >> $GITHUB_OUTPUT

      - name: Cache Browser Dependencies
        uses: actions/cache@v3
        id: playwright-cache
        with:
          path: ~/.cache/ms-playwright
          key: ${{ runner.os }}-playwright-${{ steps.browser-cache-id.outputs.value }}

      - name: Fetch kobweb
        uses: robinraju/[email protected]
        with:
          repository: "varabyte/kobweb-cli"
          tag: "v${{ env.KOBWEB_CLI_VERSION }}"
          fileName: "kobweb-${{ env.KOBWEB_CLI_VERSION }}.zip"
          tarBall: false
          zipBall: false

      - name: Unzip kobweb
        run: unzip kobweb-${{ env.KOBWEB_CLI_VERSION }}.zip

      - name: Run export
        run: |
          cd site
          ../kobweb-${{ env.KOBWEB_CLI_VERSION }}/bin/kobweb export --notty --layout static

      - name: Upload site
        uses: actions/upload-artifact@v3
        with:
          name: site
          path: site/.kobweb/site/
          if-no-files-found: error
          retention-days: 1

You can copy this workflow (or parts of it) into your own GitHub project and then modify it to your needs.

Some notes...

• workflow_dispatch: This means that you can manually trigger this workflow from the GitHub UI, which I suggested here to prevent running an expensive export operation more than you need to. Of course, you can also configure your workflow to run on a schedule, or on push to a branch, etc.
• Setup Gradle: This action is optional but I recommend it because it configures a bunch of caching for you.
• Caching the browser: kobweb export needs to download a browser the first time it is run. This workflow sets up a cache that saves it across runs. The cache is tagged with a unique ID so that future Kobweb releases, which may change the version of browser downloaded, will use a new cache bucket (allowing GitHub to eventually clean up the old one).
• Upload site: This action uploads the exported site as an artifact. You can then download the artifact from the workflow summary page. Your own workflow will likely delete this action and do something else here, like upload to a web server (or some location accessible by your webserver) or copy files over into a gh_pages repository. I've included this here (and set the retention days very low) just so you can verify that the workflow is working for your project.

For a simple site, the above workflow should take about 2 minutes to run.

StyleVariables work in a subtle way that is usually fine until it isn't -- which is often when you try to interact with their values instead of just passing them around.

For context, you can use a style variable interchangeably with the value it represents. For example, code like this works because MyOpacityVar.value() returns something with type Number:

val MyOpacityVar by StyleVariable<Number>()

// later...
Modifier.opacity(MyOpacityVar.value())

This generates the following CSS:

opacity: var(--my-opacity);

How does something of type Number generate output like var(--my-opacity)?

This is accomplished through the use of Kotlin/JS's unsafeCast, where you can tell the compiler to treat a value as a different type than it actually is. In this case, MyOpacityVar.value() returns some object which the Kotlin compiler treats like a Number for compilation purposes, but it is really some class instance whose toString() evaluates to var(--my-opacity).

Therefore, Modifier.opacity(MyOpacityVar.value()) works seemingly like magic! However, if you try to do some arithmetic, like MyOpacityVar.value().toDouble() * 0.5, the compiler might be happy, but things will break silently at runtime, when the JS engine is asked to do math with something that's not really a number.

In CSS, doing math with variables is accomplished by using calc blocks , so Kobweb offers its own calc method to mirror this. When dealing with raw numerical values, you must wrap them in num so we can escape the raw type system which was causing runtime confusion above:

calc { num(MyOpacityVar.value()) * num(0.5) }
// Output: "calc(var(--my-opacity, 1) * 0.5)"

At this point, you can write code like this:

Modifier.opacity(calc { num(MyOpacityVar.value()) * num(0.5) })

It's a little hard to remember to wrap raw values in num, but you will get compile errors if you do it wrong.

Working with variables representing size values inside a calc block doesn't require wrapping methods and feels much more natural:

val MyFontSizeVar by StyleVariable<CSSLengthValue>()
calc { MyFontSizeVar.value() + 1.cssRem }
// Output: "calc(var(--my-font-size) + 1rem)"

Many users who create a full stack application generally expect to completely own both the client- and server-side code.

However, being an opinionated framework, Kobweb provides a custom Ktor server in order to deliver some of its features. For example, it implements logic for handling server API routes▲ as well as some live reloading functionality.

It would not be trivial to refactor this behavior into some library that users could import into their own backend server. As a compromise, some server configuration is exposed by the .kobweb/conf.yaml file, and this has been the main way users could affect the server's behavior.

That said, there will always be some use-cases that Kobweb won't anticipate. So as an escape hatch, Kobweb allows users who know what they're doing to write their own plugins to extend the server.

Creating a Kobweb server plugin is relatively straightforward. You'll need to:

• Create a new module in your project that produces a JAR file which bundles an implementation of the KobwebServerPlugin interface.
• Move a copy of that jar under your project's .kobweb/server/plugins directory.

The following instructions are based on a Kobweb multimodule setup, like the one created by kobweb create app.

• Create a new module in your project.
  • For example, name it "demo-server-plugin".
  • Be sure to update your settings.gradle.kts file to include the new project.
• Add a new entry for the kobweb-server-project library in .gradle/libs.versions.toml:

  [libraries]
  kobweb-server-plugin = { module = "com.varabyte.kobweb:kobweb-server-plugin", version.ref = "kobweb" }

• For all remaining steps, create all files / directories under your new module's directory (e.g. demo-server-plugin/).
• Create build.gradle.kts:

    plugins {
      kotlin("jvm")
    }
    group = "org.example.app" // update to your own project's group
    version = "1.0-SNAPSHOT"
  
    tasks.jar {
      // Remove the version number
      archiveFileName.set("${project.name}.jar")
    }
  
    dependencies {
      compileOnly(libs.kobweb.server.plugin)
    }

  • We omit the version number to prevent the accumulation of multiple versioned copies of the same plugin ended up in the Kobweb server. Instead, each new version should replace the previous one.
• Create src/main/kotlin/DemoKobwebServerPlugin.kt:

  import com.varabyte.kobweb.server.plugin.KobwebServerPlugin
  import io.ktor.server.application.Application
  import io.ktor.server.application.log
  
  class DemoKobwebServerPlugin : KobwebServerPlugin {
    override fun configure(application: Application) {
      application.log.info("REPLACE ME WITH REAL CONFIGURATION")
    }
  }

  • As the Kobweb server is written in Ktor, you should familiarize yourself with Ktor's documentation.
• Create src/main/resources/META-INF/services/com.varabyte.kobweb.server.plugin.KobwebServerPlugin:

  org.example.app.DemoKobwebServerPlugin
  

  • This helps the JDK discover service implementations bundled within a JAR. You can read this helpful article to learn more about this useful Java feature.

After building your JAR (./gradlew :demo-server-plugin:jar), manually copy it from build/libs/ to your Kobweb project's .kobweb/server/plugins directory.

Upon the next Kobweb server run (e.g. via kobweb run), if you check the logs, you should see something like this:

[main] INFO  ktor.application - Autoreload is disabled because the development mode is off.
[main] INFO  ktor.application - REPLACE ME WITH REAL CONFIGURATION
[main] INFO  ktor.application - Application started in 0.112 seconds.
[main] INFO  ktor.application - Responding at http:https://0.0.0.0:8080

For convenience, the Kobweb Gradle Application plugin provides a way to notify it about your JAR task, and it will build and copy it over for you automatically.

In your Kobweb project's build script, include the following notify... line:

// site/build.gradle.kts

kobweb { /* ... */ }

notifyKobwebAboutServerPluginTask(project(":demo-server-plugin").tasks.named("jar", Jar::class))

kotlin { /* ... */ }

Once this is set up, you can modify your Kobweb server plugin, quit the server if one is running, and then rerun kobweb run to have it pick up your changes automatically.

You may already have an existing and complex backend, perhaps written with Ktor or Spring Boot, and, if so, are wondering if you can integrate Kobweb with it.

The recommended solution for now is to export your site using a static layout (read more about static layout sites here ▲) and then add code to your backend to serve the files yourself, as it is fairly trivial.

When you export a site statically, it will generate all files into your .kobweb/site folder. Then, if using Ktor, for example, serving these files is a one-liner:

routing {
    staticFiles("/", File(".kobweb/site"))
}

If you need to access HTTP endpoints exposed by your backend, you can use window.fetch(...) directly, or you can use the convenience http property that Kobweb adds to the window object and which exposes all the HTTP methods (get, post, put, etc.):

@Page
@Composable
fun CustomBackendDemoPage() {
  LaunchedEffect(Unit) {
    val endpointResponse = window.http.get("/my/endpoint?id=123").decodeToString()
    /* ... */
  }
}

Unfortunately, using your own backend does mean you're opting out of using Kobweb's full stack solution, which means you won't have access to Kobweb's API routes, API streams, or live reloading support. This is a situation we'd like to improve someday (link to tracking issue), but we don't have enough resources to be able to prioritize resolving this for a 1.0 release.

Jetbrains is working on a project called "Compose Multiplatform", which will allow developers to use the same Compose API across Android, iOS, Desktop, and the Web. And it may seem like the Kobweb + Silk approach will be obsoleted by it.

It's first worth understanding the core difference between the two approaches. With Multiplatform Compose, the framework owns its own rendering pipeline, drawing to a buffer, while Compose HTML modifies an HTML / CSS DOM tree and leaves it up to the browser to do the final rendering.

This has major implications on how similar the two APIs can get. For example, in Desktop / Android, the order you apply modifiers matters, while in HTML, this action simply sets html style properties under the hood, where order does not matter.

Ditching HTML / CSS entirely at first can seem like a total win, but this approach has several limits:

• robots would lose the ability to crawl and index your site, hurting SEO.
• your initial render may take longer.
• your site will need to allocate a large canvas buffer, which could be very expensive on high res, wide-screen desktops.
• your UI will be opaque to the powerful suite of devtools that come bundled with browsers.
• you won't have the ability to style unvisited vs visited links differently (this information is hidden from you by the browser for security reasons and can only be set through HTML / CSS).
• you won't have the ability to turn elements on / off when printing the page.
• accessibility tools for browsers might not work.

It would also prevent a developer from making use of the rich ecosystem of Javascript libraries out there.

For now, I am making a bet that there will always be value in embracing the web, providing a framework that sticks to HTML / CSS but offers a growing suite of UI widgets that hopefully makes it relatively rare for the developer to need to worry about it.

For example, flexbox is a very powerful CSS concept, but you'll find it's much easier to compose Rows and Columns together than trying to remember if you should be justifying your items or aligning your content, even if Rows and Columns are just creating the correct HTML / CSS for you behind the scenes.

Ultimately, I believe there is room for both approaches. If you want to make an app experience that feels the same on Android, iOS, Desktop, and Web, then "Multiplatform Compose" could be great for you. However, if you just want to make a traditional website but want to use Kotlin instead of TypeScript, Kobweb can provide an excellent development experience for that case.

A Kobweb project always has a frontend and, if configured as a full stack site, a backend as well. Both require different steps to debug them.

At the moment, attaching a debugger to Kotlin/JS code requires IntelliJ Ultimate. If you have it, you can follow these steps in the official docs.

If you do not have access to IntelliJ Ultimate, then you'll have to rely on println debugging. While this is far from great, live reloading plus Kotlin's type system generally help you incrementally build your site up without too many issues.

Debugging the backend first requires configuring the Kobweb server to support remote debugging. This is easy to do by modifying the kobweb block in your build script to enable remote debugging:

kobweb {
  app {
    server {
      remoteDebugging {
        enabled.set(true)
        port.set(5005)
      }
    }
  }
}

Once you've enabled remote debugging support, you can then follow the official documentation to add a remote JVM debug configuration to your IDE.

At this point, start up your Kobweb server using kobweb run.

With your Kobweb server running and your "remote debug" run configuration selected, press the debug button. If everything is set up correctly, you should see a message in the IDE debugger console like: Connected to the target VM, address: 'localhost:5005', transport: 'socket'

If instead you see a red popup with a message like Unable to open debugger port (localhost:5005): java.net.ConnectException "Connection refused", please double-check the values in your conf.yaml file, restart the server, and try again.

The easiest way to use a custom font is if it is already hosted for you. For example, Google Fonts provides a CDN that you can use to load fonts from directly.

The font service should give you HTML to add to your site's <head> tag. For example, Google Fonts suggests the following when I select Roboto Regular 400:

<link rel="preconnect" href="https://fonts.googleapis.com">
<link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
<link href="https://fonts.googleapis.com/css2?family=Roboto&display=swap" rel="stylesheet">

This code should be converted into Kotlin and added to the kobweb block of your site's build.gradle.kts script:

kobweb {
  app {
    index {
      head.add {
        link(rel = "preconnect", href = "https://fonts.googleapis.com")
        link(rel = "preconnect", href = "https://fonts.gstatic.com") { attributes["crossorigin"] = "" }
        link(
          href = "https://fonts.googleapis.com/css2?family=Roboto&display=swap",
          rel = "stylesheet"
        )
      }
    }
  }
}

Once done, you can now reference this new font:

Column(Modifier.fontFamily("Roboto")) {
    Text("Hello world!")
}

Users can flexibly declare a custom font by using CSS's @font-face rule.

In Kobweb, you can normally declare CSS properties in Kotlin (within an @InitSilk block), but unfortunately Firefox doesn't allow you to define or modify @font-face entries in code (relevant Bugzilla issue). Therefore, for guaranteed cross-platform compatibility, you should create a CSS file and reference it from your build script.

To keep the example concrete, let's say you've downloaded the open source font Lobster from Google Fonts (and its license as well, of course).

You need to put the font file inside your public resources directory, so it can be found by the user visiting your site. I recommend the following file organization:

jsMain
└── resources
    └── public
        └── fonts
            ├── faces.css
            └── lobster
                ├── OFL.txt
                └── Lobster-Regular.ttf

where faces.css contains all your @font-face rule definitions (we just have a single one for now):

@font-face {
  font-family: 'Lobster';
  src: url('/fonts/lobster/Lobster-Regular.ttf');
}

Now, you need to reference this CSS file from your build.gradle.kts script:

kobweb {
  app {
    index {
      head.add {
        link(rel = "stylesheet", href = "/fonts/faces.css")
      }
    }
  }
}

Finally, you can reference the font in your code:

Column(Modifier.fontFamily("Lobster")) {
    Text("Hello world!")
}

When you run kobweb run, the spun up web server will, by default, log to the .kobweb/server/logs directory.

You can configure logging behavior by editing the .kobweb/conf.yaml file. Below we show setting all parameters to their default values:

server:
  logging:
    level: DEBUG # ALL, TRACE, DEBUG, INFO, WARN, ERROR, OFF
    logRoot: ".kobweb/server/logs"
    clearLogsOnStart: true # Warning - if true, wipes ALL files in logRoot, so don't put other files in there!
    logFileBaseName: "kobweb-server" # e.g. "kobweb-server.log", "kobweb-server.2023-04-13.log"
    maxFileCount: null # null = unbound. One log file is created per day, so 30 = 1 month of logs
    totalSizeCap: 10MiB # null = unbound. Accepted units: B, K, M, G, KB, MB, GB, KiB, MiB, GiB
    compressHistory: true # If true, old log files are compressed with gzip

The above defaults were chosen to be reasonable for most users running their projects on their local machines in developer mode. However, for production servers, you may want to set clearLogsOnStart to false, bump up the totalSizeCap after reviewing disk limitations of your web server host, and maybe set maxFileCount to a reasonable limit.

Note that most config files assume "10MB" is 10 * 1024 * 1024 bytes, but here it will actually result in 10 * 1000 * 1000 bytes. You probably want to use "KiB", "MiB", or "GiB" when you configure this value.

Current state: Foundations are in place! You may encounter API gaps.

You may wish to refer to our Kobweb 1.0 roadmap document.

Kobweb is becoming quite functional. We are already using it to build https://kobweb.varabyte.com and https://bitspittle.dev. Several users have created working portfolio sites already, and I'm aware of at least two cases where Kobweb was used in a project for a client.

At this point:

• It is easy to set up a new project and get things running quickly.
• The live reloading flow is pretty nice, and you'll miss it when you switch to projects that don't have it.
• It supports generating pages from Markdown that can reference your Composable code.
• While it's not quite server-side rendering, you can export static pages which will get hydrated on load.
• A huge range of CSS properties are supported, along with support for style variables and animations.
• • You can use the Modifier builder for a significant number of css properties.
• Silk components are color mode aware and support responsive behavior.
• There are quite a few widgets available, and it's easy to create your own.

However, there's always more to do.

• I'm trying to add support for every stabilized CSS property, but some are still missing, especially less common ones. (You can use a fallback for such cases in the meanwhile).
• There are still a handful of widgets planned to be added.
• A lot of detailed documentation is planned to go into the Kobweb site (linked just above) but it isn't done yet.

I think there's enough there now to let you do almost anything you'd want to do, as either Kobweb supports it or you can escape hatch to underlying Compose HTML / Kotlin/JS approaches, but there might be some areas where it's still a bit DIY. It would be great to get real world experience to hear what issues users are actually running into.

So, should you use Kobweb at this point? If you are...

• playing around with Compose HTML for the first time and want to get up and running quickly on a toy project:
  • YES!!! Please see the connecting with us ▼ section below, we'd definitely love to hear from you. It's still a good time if you'd want to have a voice in the direction of this project.
• a Kotlin developer who wants to write a small web app or create a new blog from scratch:
  • Probably! I hope if you evaluate Kobweb at this point, you'll find a lot to like. You can get in touch with us at our Discord if you try it and have questions or run into missing features.
• someone who already has an existing project in progress and wants to integrate Kobweb into it:
  • Maybe not? Depending on how much work you've done, it may not be a trivial refactor. You can review this earlier section ▲ if you want to try anyway.
• a company:
  • Probably not? I'm assuming most companies are so risk-averse they would not even use Compose HTML, which Kobweb is built on top of. If you were considering Compose HTML, however, Kobweb is worth a look.

On the fence but not sure? Connect with us, and I'd be happy to help you assess your situation.

I'm pleased to mention that Kobweb has received feedback from some satisfied users. Here are a few:

• "This is a pretty bloody amazing technology you've created here. I have been dreading upgrading [my] website for ages because I didn't want to go back to html and css 🫤 now i can stay with Kotlin 😀"
• "Kobweb looks fantastic and I've been [trying] to use kotlin in all parts of [my] hobby stuff and work, so I got real excited when I saw Kobweb, [even though] I hadn't been satisfied with a web framework in a long time. Incredible work."
• "I started using Kobweb from last week and I have to say this [...] reinvented web development for me. [...] I used to hate html css. After getting hands on kobweb I’m in love with it."
• "Finally got paid -- all thanks to kobweb 🎉💥"

• Join my Discord!
• Follow me on Mastodon: @[email protected]
• You can send direct queries to my email

If you're comfortable with it, using Discord is recommended, because there's a growing community of users in there who can offer help even when I'm not around.

It is still early days, and while we believe we've proven the feasibility of this approach at this point, there's still plenty of work to do to get to a 1.0 launch! We are hungry for the community's feedback, so please don't hesitate to:

• Open an issue
• Contact us (using any of the ways mentioned above) telling us what features you want
• Ask us for guidance, especially as there are no tutorials yet (your questions can help us know what to write first!)

Thank you for your support and interest in Kobweb!