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 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.

Sky is still experimental. We're experimenting with different ideas and exploring various approaches, many of which won't work and will need to be discarded, but, if we're lucky, some of which might turn out to be useful.

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 a more featureful example, please see the example stocks app.

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

Supported platforms

Currently, Sky supports the Android and Mojo operating systems.

Specifications

We're documenting Sky with a set of technical specifications that define precisely the behavior of the engine. Currently both the implementation and the specification are in flux, but hopefully they'll converge over time.

Contributing

Instructions for building and testing Sky are contained in HACKING.md.