Above this layer is the animation library, which provides core animation primitives, and the gestures/ directory, which define a gesture recognition and disambiguation system.

The next layer consists of the painting library, which provides APIs related to drawing graphics. Some of the code here uses the animation library mentioned above.

Layout primitives are provided in the next layer, found in the rendering library. They use dart:sky and the APIs exposed in the painting library to provide a retained-mode layout and rendering model for applications or documents.

Widgets are provided by the files in the widgets library, using a reactive framework. They use data given in the theme/ directory to select styles consistent with Material Design.

Alongside the above is the mojo/ directory, which contains anything that uses the Mojo IPC mechanism, typically as part of wrapping host operating system features. Some of those Host APIs are implemented in the host system's preferred language.

Here is a diagram summarizing all this:

+-----------------------------+ ------
|           YOUR APP          |
|  +----------------------+---+
|  |  widgets   (theme/)  |   |
| ++---------------------++   |
| |      rendering       |    |  Dart
| |---------+------------+    |
| |         | painting   |    |
+-+         +------------+    |
| gestures/ | animation  |    |
+-----------+------------+    |
|             mojo/           |
+------------+--+-+----+------+ -------
|  dart:sky  |    |    | Host |
+--------+---+    |    | APIs |  C++
|  Skia  |  Dart  |    +------+  ObjC
+--------+--------+           |  Java
|            Mojo             |
+-----------------------------+ -------
|    Host Operating System    |  C/C++
+-----------------------------+ -------

TODO(ianh): document dart:sky and the Host APIs somewhere

Sky Engine API

The Sky engine API provides efficient, immutable wrappers for common Skia C++ types, notably Color, Point, and Rect. Because these wrappers are immutable, they are suitable for storage in final member variables of widget classes. More complex Skia wrappers such as Paint and RRect are mutable, to more closely match the Skia APIs. We recommend constructing wrappers for complex Skia classes dynamically during the painting phase based on the underlying state of the widget.