Sky is an experimental, high-performance UI framework for mobile apps. Sky helps you create apps with beautiful user interfaces and high-quality interactive design that run smoothly at 120 Hz.

Sky consists of two components:

1. The Sky engine. The engine is the core of the system. Written in C++, the engine provides the muscle of the Sky system. The engine provides several primitives, including a soft real-time scheduler and a hierarchial, retained-mode graphics system, that let you build high-quality apps.

2. The Sky framework. The framework makes it easy to build apps using Sky by providing familiar user interface widgets, such as buttons, infinite lists, and animations, on top of the engine using Dart. These extensible components follow a functional programming style inspired by React.

We're still iterating on Sky heavily, which means the framework and underlying engine are both likely to change in incompatible ways several times, but if you're interested in trying out the system, this document can help you get started.

Sky uses Dart and Sky applications are Dart Packages. Application creation starts by creating a new directory and adding a pubspec.yaml:

pubspec.yaml for your app:

name: your_app_name
dependencies:
 sky: any

Once the pubspec is in place, create a lib directory (where your dart code will go) and run pub get to download all necessary dependencies and create the symlinks necessary for 'package:your_app_names/main.dart' includes to work.

Currently the Sky Engine assumes the entry point for your application is a main function is located inside a main.sky file at the root of the package. .sky is an html-like format:

<sky>
<script>
import 'package:your_app_name/main.dart'

void main() {
  new HelloWorldApp();
}
</script>
</sky>

The rest of the application then goes inside the lib directory of the package thus lib/main.dart would be:

import 'package:sky/framework/fn.dart';

class HelloWorldApp extends App {
  UINode build() {
    return new Text('Hello, world!');
  }
}

Execution starts in main, which creates the HelloWorldApp. The framework then marks HelloWorldApp as dirty, which schedules it to build during the next animation frame. Each animation frame, the framework calls build on all the dirty components and diffs the virtual UINode hierarchy returned this frame with the hierarchy returned last frame. Any differences are then applied as mutations to the physical hierarchy retained by the engine.

For examples, please see the examples directory.

Sky apps can access services from the host operating system using Mojo IPC. For example, you can access the network using the network_service.mojom interface. Although you can use these low-level interfaces directly, you might prefer to access these services via libraries in the framework. For example, the fetch.dart library wraps the underlying network_service.mojom in an ergonomic interface:

import 'package:sky/framework/net/fetch.dart';

main() async {
  Response response = await fetch('example.txt');
  print(response.bodyAsString());
}

1. Install the Dart SDK:

• https://www.dartlang.org/tools/download.html

2. Install the adb tool from the Android SDK:

• https://developer.android.com/sdk/installing/index.html

3. Install the Sky SDK:

• git clone https://github.com/domokit/sky_sdk.git

4. Ensure that $DART_SDK is set to the path of your Dart SDK and 'adb' (inside 'platform-tools' in the android sdk) is in your $PATH.

Currently Sky requires an Android device running the Lollipop (or newer) version of the Android operating system.

1. Enable developer mode on your device by visiting Settings > About phone and tapping the Build number field five times.

2. Enable USB debugging in Settings > Developer options.

3. Using a USB cable, plug your phone into your computer. If prompted on your device, authorize your computer to access your device.

The sky pub package includes a sky_tool script to assist in running Sky applications inside the SkyDemo.apk harness. The sky_tool script expects to be run from the root directory of your application pub package. To run one of the examples in this SDK, try:

1. cd examples/stocks

2. pub get to set up a copy of the sky package in the app directory.

3. ./packages/sky/sky_tool start --install The --install flag is only necessary to install SkyDemo.apk if not already installed on the device.

4. Use adb logcat to view any errors or Dart print() output from the app. adb logcat -s chromium can be used to filter only adb messages from SkyDemo.apk (which for legacy reasons still uses the android log tag 'chromium').

Sky has support for generating trace files compatible with Chrome's about:tracing.

packages/sky/sky_tool start_tracing and packages/sky/sky_tool stop_tracing are the commands to use.

Due to https://github.com/domokit/mojo/issues/127 tracing currently requires root access on the device.

Dart's Observatory (VM Debugger & Profiler) support in Sky is in progress and should be released shortly after Dart Summit 2015.

Although it is possible to bundle the Sky Engine in your own app (instead of running your code inside SkyDemo.apk), right now doing so is difficult.

There is one example of doing so if you're feeling brave: https://github.com/domokit/mojo/tree/master/sky/apk/stocks

Eventually we plan to make this much easier and support platforms other than Android, but that work is yet in progress.

Mojo IPC is an inter-process-communication system designed to provide cross-thread, cross-process, and language-agnostic communication between applications. Sky uses Mojo IPC to make it possible to write UI code in Dart and yet depend on networking code, etc. written in another language. Services are replacable, meaning that Dart code written to use the network_service remains portable to any platform (iOS, Android, etc.) by simply providing a 'natively' written network_service.

Embedders of the Sky Engine and consumers of the Sky Framework can use this same mechanism to expose not only existing services like the Keyboard service to allow Sky Framework Dart code to interface with the underlying platform's Keyboard, but also to expose any additional non-Dart business logic to Sky/Dart UI code.

As and example, SkyApplication exposes a mojo network_service (required by Sky Engine C++ code) SkyDemoApplication additionally exposes keyboard_service and sensor_service for use by the Sky Framework from Dart.