Scaloid is a library that simplifies your Android code. It makes your code easy to understand and maintain by leveraging Scala language.

For example, the code block shown below:

val button = new Button(context)
button.setText("Greet")
button.setOnClickListener(new OnClickListener() {
  def onClick(v: View) {
    Toast.makeText(context, "Hello!", Toast.LENGTH_SHORT).show()
  }
})
layout.addView(button)

is reduced to:

SButton("Greet", toast("Hello!"))

• Write elegant Android software
  Simplicity is number one principle, keeps programmability and type-safety.
• Easy to use
  Check the quick start guide
• Compatible with your legacy code
  You can use both Scaloid and plain-old Java Android API. You can gradually improve your legacy code.
• Production quality
  Not a toy project. The creator of Scaloid uses it to build a millionth downloaded app.

Fork one of this to start a new project:

• Hello world of Scaloid for sbt (recommended, it builds faster)
• Hello world of Scaloid for maven
• Hello world of Scaloid for gradle

Learn how Scaloid can be used in action:

• Scaloid port of apidemos app
• List of projects using Scaloid
• Tutorial by Gaston Hillar - part 1 and part 2

• Core design principle
• UI Layout without XML
  • Layout context
  • Styles for programmers
  • Automatic layout converter
• Lifecycle management
• Asynchronous task processing
• Implicit conversions
  • Shorter listeners
  • Database cursor
• Traits
• Smarter logging
• Improved getters/setters
• Classes
  • Concise dialog builder
  • Beauty ArrayAdapter
  • Dynamically Preferences _{Read in blog}
  • Binding services concisely _{Read in blog}

• Quick start guide
• API doc
• Blog
• Twitter
• FAQs
  • FAQs about Scala on Android
• Inside Scaloid
• We are hiring!

"Being practically simple" is number one principle of Scaloid. Most frequently used things should be written shorter, like Huffman coding. To do this, I first observed Android programs I wrote, and thought that which part of the code is more fundamental than others. For example, what is the most essential part of buttons? Buttons should have some visible things on it, such as title or image, so the buttons are created like this: SButton("Hello"). The second essential part is doing something when it is pressed: SImageButton(R.drawable.hello, toast("World!")). What should be the third one? The answer might not the same for every people, but I think that repetition frequency of press-and-hold action is nice: SButton("Add", n += 1, 500) increases n for every 500 milliseconds when the user holds the button.

Android SDK leverages XML to build UI layouts. However, XML is considered still a bit verbose, and lacks programmability. Scaloid composes UI layout in Scala DSL style, therefore achieve both clarity and programmability. For example, suppose a legacy XML layout as shown below:

<LinearLayout xmlns:android="https://schemas.android.com/apk/res/android"
        android:orientation="vertical" android:layout_width="match_parent"
        android:layout_height="wrap_content" android:padding="20dip">
    <TextView android:layout_width="match_parent"
            android:layout_height="wrap_content" android:text="Sign in"
            android:layout_marginBottom="25dip" android:textSize="24.5sp"/>
    <TextView android:layout_width="match_parent"
            android:layout_height="wrap_content" android:text="ID"/>
    <EditText android:layout_width="match_parent"
            android:layout_height="wrap_content" android:id="@+id/userId"/>
    <TextView android:layout_width="match_parent"
            android:layout_height="wrap_content" android:text="Password"/>
    <EditText android:layout_width="match_parent"
            android:layout_height="wrap_content" android:id="@+id/password"
            android:inputType="textPassword"/>
    <Button android:layout_width="match_parent"
            android:layout_height="wrap_content" android:id="@+id/signin"
            android:text="Sign in"/>
    <LinearLayout android:orientation="horizontal"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content">
        <Button android:text="Help" android:id="@+id/help"
                android:layout_width="match_parent" android:layout_height="wrap_content"/>
        <Button android:text="Sign up" android:id="@+id/signup"
                android:layout_width="match_parent" android:layout_height="wrap_content"/>
    </LinearLayout>
</LinearLayout>

is reduced to:

new SVerticalLayout {
  STextView("Sign in").textSize(24.5.sp).<<.marginBottom(25.dip).>>
  STextView("ID")
  SEditText()
  STextView("Password")
  SEditText() inputType TEXT_PASSWORD
  SButton("Sign in")
  new SLinearLayout {
    SButton("Help")
    SButton("Sign up")
  }.wrap.here
}.padding(20.dip)

The layout description shown above is highly programmable. You can easily wire your logic into the layout:

new SVerticalLayout {
  STextView("Sign in").textSize(24.5.sp).<<.marginBottom(25.dip).>>
  STextView("ID")
  val userId = SEditText()
  STextView("Password")
  val pass = SEditText() inputType TEXT_PASSWORD
  SButton("Sign in", signin(userId.text, pass.text))
  new SLinearLayout {
    SButton("Help", openUri("https://help.url"))
    SButton("Sign up", openUri("https://signup.uri"))
  }.wrap.here
}.padding(20.dip)

Because a Scaloid layout description is plain Scala code, it is type-safe.

This converter turns an Android XML layout into a Scaloid layout:

https://layout.scaloid.org

Scaloid is fully compatible with legacy xml layout files. You can access a widget described in xml layout as:

onCreate {
  setContentView(R.layout.main)
  val name = find[EditText](R.id.name)
  // do something with `name`
}

Basically, a layout written in Scaloid is just an ordinary Scala code, so you can just freely composite the layout according to the device configuration:

import org.scaloid.util.Configuration._

if(long) SButton("This button is shown only for a long screen "
  + "dimension ("+ width + ", " + height + ")")
if(landscape) new SLinearLayout {
  SButton("Buttons for")
  SButton("landscape layout")
  if(dpi <= HDPI) SButton("You have a high resolution display!")
}.here

Please refer to this blog post for more detail:

• Syntactic sugar for multiple device configuration

• Accessing widgets in view class
• Layout context
• In-depth tutorial on styles
• Styles for programmers

With Android API, Registering and unregistering BroadcastReceiver can be done as:

var connectivityListener: BroadcastReceiver = null

def onResume() {
  super.onResume()
  // ...
  connectivityListener = new BroadcastReceiver {
    def onReceive(context: Context, intent: Intent) {
     doSomething()
    }
  } 
  registerReceiver(connectivityListener, new IntentFilter(ConnectivityManager.CONNECTIVITY_ACTION))
}

def onPause() {
  unregisterReceiver(connectivityListener)
  // ...
  super.onPause()
}

In Scaloid, the directly equivalent code is:

broadcastReceiver(ConnectivityManager.CONNECTIVITY_ACTION) {
  doSomething()
}

Scaloid has highly flexible resource register/unregister management architecture. If this code is written in services, registering and unregistering is done in onCreate and onDestroy respectively. If the same code is in activities, registering and unregistering is done in onResume and onPause respectively. This is just a default behavior. You can override a preference that determine when the register/unregister preforms. Overriding it is simple as well:

broadcastReceiver(ConnectivityManager.CONNECTIVITY_ACTION) {
  doSomething()
}(this, onStartStop)

Then, the receiver is registered onStart, and unregistered onStop.

You can declare onDestroy behaviors in many places. This simplifies resource management significantly. Suppose you want to open a stream from a file:

def openFileStream(file: File): InputStream = {
  val stream = new FileInputStream(file)
  onDestroy(stream.close()) // automatically closed when the Activity is destroyed!!
  stream
}

onDestroy is a method that adds a function into the job list triggered when the activity is destroyed. So, we just get a stream from openFileStream() and forget about releasing it. Other lifecycle states (onCreate, onResume, onStop and so on) can be treated in the same way.

Further reading: Refer to this blog post for more details.

Android API provides runOnUiThread() only for class Activity. Scaloid provides a Scala version of runOnUiThread() for anywhere other than Activity.

Instead of:

activity.runOnUiThread {
  new Runnable() {
    def run() {
      debug("Running only in Activity class")
    }
  }
}

In Scaloid, use it like this:

runOnUiThread(debug("Running in any context"))

Running a job asynchronously and notifying the UI thread is a very frequently used pattern. Although Android API provides a helper class AsyncTask, implementing such a simple idea is still painful, even when we use Scala:

new AsyncTask[String, Void, String] {
  def doInBackground(params: Array[String]) = {
    doAJobTakeSomeTime(params)
  }

  override def onPostExecute(result: String) {
    alert("Done!", result)
  }
}.execute("param")

Using scala.concurrent.Future, the asynchronous job shown above can be rewritten like this:

Future {
  val result = doAJobTakeSomeTime(params)
  runOnUiThread(alert("Done!", result))
}

When you don't want to build sophisticate UI interactions, but just want to display something by calling a single Scaloid method (e.g. alert, toast, and spinnerDialog), Scaloid handles runOnUiThread for you. Therefore, the code block shown above is reduced to:

Future {
  alert("Done!", doAJobTakeSomeTime(params))
}

It is a great win as it exposes your idea clearly.

Just like we thrown away AsyncTask, we can also eliminate all other Java helpers for asynchronous job, such as AsyncQueryHandler and AsyncTaskLoader. Compare with the original Java code and a Scala port of ApiDemos example app.

Using Future is just an example of asynchronous task processing in Scaloid. You can freely use any modern task management utilities.

Further reading: Refer to this blog post for an important consideration when using Future in Android.

Scaloid employs several implicit conversions. Some of the available implicit conversions are shown below:

String => Uri

The functions such as play ringtones play() or open URIs openUri() takes an instance of Uri as a parameter. However, we frequently have URIs as a String. Scaloid implicitly converts String into Uri. Therefore, you can freely use String when you play a ringtone:

play("content:https://media/internal/audio/media/50")

, open a URI:

openUri("https://scaloid.org")

, or wherever you want.

Alternatively, you can specify the conversion as:

val uri:Uri = "https://scaloid.org".toUri

Units dip and sp can be converted into the pixel unit.

val inPixel:Int = 32.dip
val inPixel2:Int = 22.sp

Reversely, pixel unit can also be converted into dip and sp unit.

val inDip:Double = 35.px2dip
val inSp:Double = 27.px2sp

Scaloid provides several implicit conversions that convert from Int type resource ID to CharSequence, Array[CharSequence], Array[String], Drawable and Movie. For example:

def toast(msg:CharSequence) = ...

toast(R.string.my_message) // implicit conversion works!

Although Scaloid provides these conversions implicitly, explicit conversion may be required in some context. In this case, methods r2... are provided for the Int type:

warn("Will display the content of the resource: " + R.string.my_message.r2String)

Currently, r2Text, r2TextArray, r2String, r2StringArray, r2Drawable and r2Movie is provided.

Further reading:

• Why implicit conversion of Resource ID is cool?

Many methods in the Android API require an instance of a class Context. Providing this for every method call results in clumsy code. We employ an implicit parameter to eliminate this. Just declare an implicit value that represents current context:

implicit val ctx = ...

or just extend trait SContext, which defines it for you. Then the code that required Context becomes much simpler, for example:

new Intent(context, classOf[MyActivity])

is reduced to:

SIntent[MyActivity]

When a method takes an Intent as a first parameter in which we want to pass the newly created intent object, the parameter can be omitted. For example:

startService(new Intent(context, classOf[MyService]))
stopService(new Intent(context, classOf[MyService]))

is reduced to:

startService[MyService]
stopService[MyService]

or

val intent = // initialize the intent and put some attributes on it
intent.start[MyActivity]

An intent that has a long list of extra attributes:

new Intent().putExtra("valueA", valueA).putExtra("valueB", valueB).putExtra("valueC", valueC)

is reduced to:

new Intent().put(valueA, valueB, valueC)

toast("hi, there!")

If you want a longer toast:

longToast("long toast")

ProgressDialog.show(context, "Dialog", "working...", true)

is reduced to:

spinnerDialog("Dialog", "working...")

When you call toast, longToast or spinnerDialog from non-UI thread, you don't have to mind about threading. The toast example shown above is equivalent to the following Java code:

activity.runOnUiThread(new Runnable() {
    public void run() {
        Toast.makeText(activity, "hi, there!", Toast.LENGTH_SHORT).show();
    }
});

PendingIntent.getActivity(context, 0, new Intent(context, classOf[MyActivity]), 0)
PendingIntent.getService(context, 0, new Intent(context, classOf[MyService]), 0)

is reduced to:

pendingActivity[MyActivity]
pendingService[MyService]

This opens a web browser (or another view assigned to the http protocol).

openUri("https://scaloid.org")

Getting system service objects become much simpler. The following legacy code:

val vibrator = context.getSystemService(Context.VIBRATOR_SERVICE).asInstanceOf[Vibrator]
vibrator.vibrate(500)

is reduced to:

vibrator.vibrate(500)

Under the hood, Scaloid defines a function vibrator like this:

def vibrator(implicit ctx: Context) = ctx.getSystemService(Context.VIBRATOR_SERVICE).asInstanceOf[Vibrator]

All the system service accessors available in Android API level 8 are defined (e.g. audioManager, alarmManager, notificationManager, etc.). The name of a system service accessor is the same as its class name, except that the first character is lowercased.

Suppose an Android class Foo, for example, Scaloid defines an implicit conversion Foo => RichFoo. The class RichFoo defines additional methods for more convenient access to Foo. This is a common pattern in Scala to extend existing API (see pimp-my-library pattern). This section describes various features added on existing Android API classes.

Android API defines many listener interfaces for callback notifications. For example, View.OnClickListener is used to be notified when a view is clicked:

find[Button](R.id.search).setOnClickListener(new View.OnClickListener {
  def onClick(v:View) {
    openUri("https://scaloid.org")
  }
})

Scaloid provides a shortcut that dramatically reduces the length of the code:

find[Button](R.id.search).onClick(openUri("https://scaloid.org"))

All other listener-appending methods such as .onKey(), .onLongClick(), and .onTouch() are defined.

Some conventions we employed for method naming are:

• We omit set..., add..., and ...Listener from the method name, which is less significant.
  For example, .setOnKeyListener() becomes .onKey().
• Every method has two versions of parameters overridden. One is a lazy parameter, and another is a function which has full parameters defined in the original Android API. For example, these two usages are valid:

button.onClick(info("touched"))
button.onClick((v:View) => info("touched a button "+v))

• Methods add... is abbreviated with a method += if it is not a listener-appender.
  For example, layout.addView(button) becomes layout += button.

Methods beforeTextChanged(), onTextChanged(), and afterTextChanged() are defined in RichTextView, which can be implicitly converted from TextView. It is more convenient than using TextWatcher directly. For example:

inputField.beforeTextChanged(saveTextStatus())

is equivalent to:

inputField.addTextChangedListener(new TextWatcher {
  def beforeTextChanged(s: CharSequence, start: Int, before: Int, count: Int) {
    saveTextStatus()
  }

  def onTextChanged(p1: CharSequence, p2: Int, p3: Int, p4: Int) {}

  def afterTextChanged(p1: Editable) {}
})

Also, we override beforeTextChanged() with full parameters defined in the original listener:

inputField.beforeTextChanged((s:CharSequence, _:Int, _:Int) => saveText(s))

Other listeners in Android API can also be accessed in this way.

In Android API, layout information is stored into a View object via the method View.setLayoutParams(ViewGroup.LayoutParams). A specific type of parameter passing into that method is determined by a the type of ...Layout object which contains the View object. For example, let us see some Java code shown below:

LinearLayout layout = new LinearLayout(context);
Button button = new Button(context);
button.setText("Click");
LinearLayout.LayoutParams params = new LinearLayout.LayoutParams();
params.weight = 1.0f;  // sets some value
button.setLayoutParams(params);
layout.addView(button);

Because the button is appended into the LinearLayout, the layout parameter must be LinearLayout.LayoutParams, otherwise a runtime error might be occurred. Meanwhile, Scaloid eliminate this burden, while still preserving rigorous typing of LayoutParams. The code shown below is equivalent to the previous Java code:

val layout = new SLinearLayout {
  SButton("Click").<<.Weight(1.0f).>>
}

In the anonymous constructor of 'SLinearLayout', Scaloid provides an implicit function called "layout context". This affects a return type of the method << defined in the class SButton. If we use SFrameLayout as a layout context, the method << returns FrameLayout.LayoutParams, which does not have Weight method. Therefore, the code below results a syntax error.

val layout = new SFrameLayout {
  SButton("Click").<<.Weight(1.0f).>>   // Syntax error on Weight()
}

Compared with XML layout description, Scaloid layout is simple and type-safe.

The method << is overloaded with parameters <<(width:Int, height:Int) which assigns the size of the view component. For example:

SButton("Click").<<(40.dip, WRAP_CONTENT)

Usually, View components are referenced multiple times in an Activity. For example:

lazy val button = new SButton() text "Click"
onCreate {
  contentView = new SLinearLayout {
    button.here
  }
}
// ... uses the button somewhere in other methods (e.g. changing text or adding listeners)

Prefixed classes in Scaloid (e.g. SButton) have a companion object that implements apply methods that create a new component. These methods also append the component to the layout context that enclose the component. Therefore, the code block from the above example:

button = new SButton() text "Click"
button.here

is equivalent to:

button = SButton("Click")

Because the apply methods access to the layout context, it cannot be called outside of the layout context. In this case, use the new operator instead.

As we noted, the method << returns an object which is a type of ViewGroup.LayoutParams:

val params = SButton("Click").<<   // type LayoutParams

This class provides some setters for chaining:

val params = SButton("Click").<<.marginBottom(100).marginLeft(10)   // type LayoutParams

if we want use the SButton object again, Scaloid provides >> method returning back to the object:

val button = SButton("Click").<<.marginBottom(100).marginLeft(10).>>   // type SButton

When the layout context is nested, inner-most layout's context is applied:

val layout = new SFrameLayout {
  new SLinearLayout {
    SButton("Click").<<.Weight(1.0f).>>   // in context of SLinearLayout
  }.here
}

When we get a LayoutParams from <<, the default values of width and height properties are width = FILL_PARENT and height = WRAP_CONTENT. You can override this when you need it:

SButton("Click").<<(FILL_PARENT, FILL_PARENT)

This is a very frequently used idiom. Therefore we provide further shorthand:

SButton("Click").<<.fill

If you want the View element to be wrapped,

SButton("Click").<<(WRAP_CONTENT, WRAP_CONTENT)

This is also shortened as:

SButton("Click").<<.wrap

Similarly, <<(WRAP_CONTENT, FILL_PARENT) and <<(FILL_PARENT, WRAP_CONTENT) can also be shortend as <<.wf and <<.fw respectively.

Because there are so many occurences <<.wrap.>> pattern in actual Android code, it is allowed to remove .<< and .>> in this case:

SButton("Click").wrap    // returns SButton type

This pattern also usable for .fill, .fw and .wf methods.

Scaloid follows the naming conventions of XML attributes in the Android API with some improvements.

For XML attributes, layout related properties are prefixed with layout_ and as you might have guessed, Scaloid does not need it. For boolean attributes, the default is false. However, Scaloid flags it as true when the attribute is declared explicitly without any parameter. For example:

new SRelativeLayout {
  STextView("hello").<<.centerHorizontal.alignParentBottom.>>
}

Scaloid omits unnecessary ="true" for the attribute centerHorizontal. Equivalent XML layout description for TextView is:

<TextView
    android:id="@+id/helloText"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"
    android:layout_alignParentBottom="true"
    android:text="hello"/>

For layout methods named with four directions (e.g. ...Top, ...Right, ...Bottom and ...Left), Scaloid provides additional methods that specifies all properties at once. For example, Because Android XML layout defines margin... properties(marginTop(v:Int), marginRight(v:Int), marginBottom(v:Int) and marginLeft(v:Int)), Scaloid provides additional margin(top:Int, right:Int, bottom:Int, left:Int) and margin(amount:Int) methods that can be used as:

STextView("hello").<<.margin(5.dip, 10.dip, 5.dip, 10.dip)

or

STextView("hello").<<.margin(10.sp)  // assigns the same value for all directions

Android SDK introduced styles to reuse common properties on XML layout. We repeatedly pointed out that XML is verbose. To apply styles in Scaloid, you do not need to learn any syntax or API library, because Scaloid layout is an ordinary Scala code. Just write a code that work as styles.

Suppose the following code that repeats some properties:

SButton("first").textSize(20.dip).<<.margin(5.dip).>>
SButton("prev").textSize(20.dip).<<.margin(5.dip).>>
SButton("next").textSize(20.dip).<<.margin(5.dip).>>
SButton("last").textSize(20.dip).<<.margin(5.dip).>>

Then we can define a function that applies these properties:

def myStyle = (_: SButton).textSize(20.dip).<<.margin(5.dip).>>
myStyle(SButton("first"))
myStyle(SButton("prev"))
myStyle(SButton("next"))
myStyle(SButton("last"))

Still not satisfying? Here we have a shorter one:

def myStyle = (_: SButton).textSize(20.dip).<<.margin(5.dip).>>
List("first", "prev", "next", "last").foreach(title => myStyle(SButton(title)))

Scaloid provides SViewGroup.style(View => View) method to provide more generic component styling. The parameter is a function which receives a view requested for styling, and returns a view which is finished applying the style. Then the example in the previous subsection becomes:

style {
  case b: SButton => b.textSize(20.dip).<<.margin(5.dip).>>
}

SButton("first")
SButton("prev")
SButton("next")
SButton("last")

Note that individually applying myStyle is reduced. Let us see another example:

style {
  case b: SButton => b.textColor(Color.RED).onClick(toast("Bang!"))
  case t: STextView => t.textSize(10.dip)
  case v => v.backgroundColor(Color.YELLOW)
}

STextView("I am 10.dip tall")
STextView("Me too")
STextView("I am taller than you").textSize(15.dip) // overriding
SEditText("Yellow input field")
SButton("Red alert!")

Similar to CSS, you can assign different styles for each classes using Scala pattern matching. Unlike Android XML styles or even CSS, Scaloid can assign some actions to the component (see onclick(toast(...))), or can do anything that you imagine. Also, you can easily override the property individually, as shown in the example above.

Last thing that you may missed: These are type-safe. If you made a mistake, compiler will check it for you.

Further readings:

• Accessing widgets in view class
• In-depth tutorial on styles

When you register BroadcastReceiver with Context.registerReceiver() you have to unregister it to prevent memory leak. Trait UnregisterReceiver handles these chores for you. All you need to do is append the trait to your class.

class MyService extends SService with UnregisterReceiver {
  def func() {
    // ...
    registerReceiver(receiver, intentFilter)
    // Done! automatically unregistered at UnregisterReceiverService.onDestroy()
  }
}

Instead of

findViewById(R.id.login).asInstanceOf[Button]

use a shorthand:

find[Button](R.id.login)

Although we provide this shorthand, Scaloid recommends programmatically laying out UI, not with XML.

Similar to the implicit context, an Activity typed implicit parameter is also required for some methods. Therefore, you have to define an activity as an implicit value:

implicit val ctx: Activity = ...

Because the class Activity is a subclass of Context, it can also be an implicit context. When you extend SActivity, object this is assigned as the implicit activity by default.

Here we show some example cases of using the implicit activity:

Often, Views are required to have an ID value. Although Android API document specifies that the ID need not be unique, allocating unique ID is virtually mandatory in practice. Scaloid provides a package scope function getUniqueId, which returns Int type ID that is not allocated by any existing View components for given implicit activity.

val newUniqueIdForCurrentActivity = getUniqueId

Using this, Scaloid also extended View class to add a method uniqueId, that assigns a new unique ID if it is not already allocated.

val uniqueIdOfMyView = myView.uniqueId

One of the good use case of uniqueId is SRelativeLayout. Some of the methods in this layout context, such as below, above, leftOf and rightOf, takes another View object as an anchor:

new SRelativeLayout {
  val btn = SButton(R.string.hi)
  SButton("There").<<.below(btn)
}

Here we show the implementation of the below function:

def below(anchor: View)(implicit activity: Activity) = {
  addRule(RelativeLayout.BELOW, anchor.uniqueId)
  this
}

A new unique ID is assigned to the anchor if it is not assigned already, and passes it to addRule function.

Unlike other logging frameworks, Android Logging API requires a String tag for every log call. We eliminate this by introducing an implicit parameter. Define an implicit value type of LoggerTag as shown:

implicit val loggerTag = LoggerTag("MyAppTag")

or, extend trait TagUtil or SContext which defines the tag by default. Then you can simply log like this:

warn("Something happened!")

Other functions for every log level (verbose(), debug(), info(), warn(), error() and wtf()) are available.

info("hello " + world)

A String parameter passed with info() is a by-name parameter, so it is evaluated only if the logging is possible. Therefore, the example shown above is equivalent to:

val tag = "MyAppTag"
if(Log.isLoggable(tag, Log.INFO)) Log.i(tag, "hello " + world)

You can use any of the setters listed below:

• obj.setText("Hello") Java bean style
• obj.text = "Hello" Assignment style
• obj text "Hello" DSL style
• obj.text("Hello") Method calling style

Compared to Java style getters and setters, for example:

new TextView(context) {
  setText("Hello")
  setTextSize(15)
}

that of Scala style clearly reveals the nature of the operations as shown below:

new STextView {
  text = "Hello"
  textSize = 15
}

Or, you can also chain the setters:

new STextView text "Hello" textSize 15

which is a syntactic sugar for:

new STextView.text("Hello").textSize(15)

We recommend "assignment style" and "DSL style". Use assignment style when you emphasize that you are assigning something, or use DSL style when the code length of the assignee is short and needs to be chained.

Note: Using .apply(String) method on object STextView, you can further reduce the code above like this:

STextView("Hello") textSize 15

Further readings:

• Return value of setters
• Prefixed classes
• Sweet-little sugar

A Scala-style builder for AlertDialog.

new AlertDialogBuilder(R.string.title, R.string.message) {
  neutralButton()
}.show()

This displays an alert dialog with given string resources. We provide an equivalent shortcut:

alert(R.string.title, R.string.message)

Also you can build a more complex dialog:

new AlertDialogBuilder("Exit the app", "Do you really want to exit?") {
  positiveButton("Exit", finishTheApplication())
  negativeButton(android.R.string.cancel)
}.show()

The code above is equivalent to:

new AlertDialog.Builder(context)
  .setTitle("Exit the app")
  .setMessage("Do you really want to exit?")
  .setPositiveButton("Exit", new DialogInterface.OnClickListener {
    def onClick(dialog: DialogInterface, which: Int) {
      finishTheApplication()
    }
  })
  .setNegativeButton(android.R.string.cancel, new DialogInterface.OnClickListener {
    def onClick(dialog: DialogInterface, which: Int) {
      dialog.cancel()
    }
  }).show()

When you call show() or alert from non-UI thread, you don't have to mind about threading.

Suppose you want to let the user selects a string from spinner, and larger font should be displayed in the dropdown list. Then the plain-old Android code is consisted of a chunk of XML and its wiring:

<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="https://schemas.android.com/apk/res/android"
    style="?android:attr/spinnerDropDownItemStyle"
    android:id="@+id/spinner_textview"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
	android:textSize="25.dip" />

val adapter = new ArrayAdapter(context, android.R.layout.simple_spinner_item, Array("One", "Two", "Three"))
adapter.setDropDownViewResource(R.layout.spinner_dropdown)

In Scaloid, a directly equivalent code is:

SArrayAdapter("One", "Two", "Three").dropDownStyle(_.textSize(25.dip))

If you want to let the text color in the spinner be blue, use the style method:

SArrayAdapter("Quick", "Brown", "Fox").style(_.textColor(Color.BLUE))

Can it be simpler?

Android Developer Guide on service binding says that we have to write more than 60 lines of code to define and bind an in-process service. With Scaloid, you can concisely define and access local service as shown below:

class MyService extends LocalService {
  private val generator = new Random()

  def getRandomNumber() = generator.nextInt(100)
}

class Activity extends SActivity {
  val random = new LocalServiceConnection[MyService]
 
  def onButtonClick(v:View) {
    random( s => toast("number: " + s.getRandomNumber()))
  }
}

Further reading: Refer to this blog post to see why this is awesome in compared with the existing method.

SharedPreference can be accessed in this way:

val executionCount = preferenceVar(0) // default value 0
val ec = executionCount() // read
executionCount() = ec + 1 // write
executionCount.remove() // remove

Further reading:

• Type-safe SharedPreference
• A simple example: Prompt user to rate your app

Often we need to define a custom view widget for a specific requirement. To do this, we define a class that inherits android.widget.View class or its subclass (e.g. TextView and Button). To enable Scaloid extensions for this custom widget, you can define a class as follows:

class MyView(implicit ctx: Context) extends View(ctx) with TraitView[MyView] {
  def basis = this
  
  // custom code for MyView here
}

Scaloid is an Apache licensed project. If you have any idea to improve Scaloid, feel free to open issues or post patches. If you want look into inside of Scaloid, this document would be helpful:

• Inside Scaloid

The company behind Scaloid, onsquare is hiring Scala developers. We are building a music app and other amazing products. We extensively uses Scaloid in our product, and probably it is the best reference of Scaloid application. For more information about our company, please refer to our website https://o-n2.com . Please send us your CV via email if you are interested in working at onsqure. We are located at Incheon, Korea. [email protected]