CSSLayout is a cross-platform implementation of flexbox. The goal of CSSLayout is allow native developers to have the same expressive layout system as developers developing for the modern web are used to. CSSLayout allows developers for web, android, iOS, and windows to use the same layout primitives across platforms. This saves time, increases collaboration between platform teams, and makes it easier for developers to work on multiple platforms.

The goal of CSSLayout is not to re-implement all of css. CSSLayout only targets flexbox, and does not have any plans on implementing support for tables, floats, or any other css concepts. CSSLayout also does not plan on supporting styling properties which do not affect layout such as color or background properties.

CSSLayout tries to stay as close as possible to the web implementation of flexbox. There are however certain cases where CSSLayout differs from the web implementation.

CSSLayout has chosen to make changes to the default values of certain properties. These default values were chosen based on our usage of the library. When testing layout with tools such as JSFiddle you can apply the following css style to ensure the defaults match those of CSSLayout. Or fork the following JSFiddle.

div, span {
  box-sizing: border-box;
  position: relative;

  display: flex;
  flex-direction: column;
  align-items: stretch;
  flex-shrink: 0;
  align-content: flex-start;

  border: 0 solid black;
  margin: 0;
  padding: 0;
  min-width: 0;
}

• box-sizing: border-box is the most convenient way to express the relation between width and borderWidth.
• Everything is display: flex by default. All the behaviors of block and inline-block can be expressed in term of flex but not the opposite.
• All the flex elements are oriented from top to bottom, left to right and do not shrink. This is how things are laid out using the default CSS settings and what you'd expect.
• Everything is position: relative. This makes position: absolute target the direct parent and not some parent which is either relative or absolute. If you want to position an element relative to something else, you should move it in the DOM instead of relying of CSS. It also makes top, left, right, bottom do something when not specifying position: absolute.

CSSLayout currently only supports pixel sizes. The reason being that we have not seen the need for any other units. We would like to support percentage units sometime in the future.

We think supporting RTL locales is very important. Therefor CSSLayout supports non-standards -start and -end suffixed versions of margin, padding, border, and position.

The full API can be found in CSSLayout/CSSLayout.h.

CSSNodeRef root = CSSNodeNew();
CSSNodeStyleSetWidth(root, 100);
CSSNodeStyleSetHeight(root, 100);

for (uint32_t i = 0; i < 10; i++) {
  CSSNodeRef child = CSSNodeNew();
  CSSNodeStyleSetHeight(child, 10);
  CSSNodeInsertChild(root, child, 0);
}

CSSNodeCalculateLayout(root, CSSUndefined, CSSUndefined, CSSDirectionLTR);

// Get for resulting layout
CSSNodeLayoutGetLeft(root);
CSSNodeLayoutGetTop(root);
CSSNodeLayoutGetWidth(root);
CSSNodeLayoutGetHeight(root);

The full API can be found in java/com/facebook/csslayout/CSSNode.java.

CSSNode root = new CSSNode();
root.setStyleWidth(100);
root.setStyleHeight(100);

for (int i = 0; i < 10; i++) {
  CSSNode child = new CSSNode();
  child.setStyleHeight(10);
  root.addChildAt(child, 0);
}

root.calculateLayout(new CSSLayoutContext());

// Get for resulting layout
root.getLayoutX();
root.getLayoutY();
root.getLayoutWidth();
root.getLayoutHeight();

The full API can be found in uikit/CSSLayout/UIView+CSSLayout.h.

UIView *root = [UIView new];
[root css_setUsesFlexbox:YES];
[root css_setWidth:100];
[root css_setHeight:100];

for (NSUInteger i = 0; i < 10; i++) {
  UIView *child = [UIView new];
  [child css_setUsesFlexbox:YES];
  [child css_setHeight:10];
  [root addSubview:child];
}

// Resulting layout will be set on the UIView hierarchy frames.
[root css_applyLayout];

The full API can be found in csharp/Facebook.CSSLayout/CSSNode.cs.

var root = new CSSNode();
root.StyleWidth = 100;
root.StyleHeight = 100;

for (var i = 0; i < 10; i++)
{
  var child = new CSSNode();
  child.StyleHeight = 10;
  root.Insert(0, child);
}

// Get for resulting layout
root.LayoutX;
root.LayoutY;
root.LayoutWidth;
root.LayoutHeight;

To contribute to CSSLayout you need to first install buck which is the build system used by CSSLayout. CSSLayout is implemented in C with language bindings for Java, Objective-C, and .NET. When making changes to CSSLayout/CSSLayout.h please ensure to update java/jni/CSSJNI.h, java/com/facebook/csslayout/CSSNode.java, uikit/CSSLayout/UIView+CSSLayout.m, and csharp/Facebook.CSSLayout/CSSNode.cs to reflect the API change. Before submitting any code please run format.sh to ensure the code matches the project's code style.

Before making any larger changes to CSSLayout please open an issue with a RFC so the changes can be discussed first. Generally we are very open to changes and improvements that will benefit the community.

For any changes you make you should ensure that all the tests are passing. In case you make any fixes or additions to the library please also add at least one test to ensure we don't break anything in the future. Tests are located in tests/CSSLayoutTest.cpp. Run the tests by executing buck test //:CSSLayout.

Instead of manually writing a test which ensures parity with web implementations of flexbox you can run gentest/gentest.rb to generated a test for you. You can write html which you want to verify in CSSLayout, in gentest/fixtures folder, such as the following.

<div style="width: 100px; height: 100px; align-items: center;">
  <div style="width: 50px; height: 50px;"></div>
</div>

Run gentest/gentest.rb to generate test code and re-run buck test //:CSSLayout to validate the behavior. One test case will be generated for every root div in the input html.

You may need to install the latest watir-webdriver gem (gem install watir-webdriver) and ChromeDriver to run gentest/gentest.rb Ruby script.

.NET testing is not integrated in buck yet, you might need to set up .NET testing environment. We have a script which to launch C# test on macOS, csharp/tests/Facebook.CSSLayout/test_macos.sh.

Benchmarks are located in benchmarks/CSSBenchmark.c and can be run with buck run //benchmarks:benchmarks. If you think your change has affected performance please run this before and after your change to validate that nothing has regressed.