Sky

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.

Examples

The simplest Sky app is, appropriately, HelloWorldApp:

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

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

void main() {
  new HelloWorldApp();
}

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 Node hierarchy returned this frame with the hierarchy returned last frame. Any differences are then applied as mutations to the physical heiarchy retained by the engine.

For more examples, please see the examples directory.

Services

Sky apps can access services from the host operating system using Mojo. 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';

void main() {
  fetch('example.txt').then((Response response) {
    print(response.bodyAsString());
  });
}

Set up your computer

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

Set up your device

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.

Running a Sky application

1. packages/sky/lib/sky_tool --install examples/stocks/main.sky The --install flag is only necessary the first time to install SkyDemo.apk.

2. Use adb logcat to view any errors or Dart print() output from the app.