Description

Package app lets you write portable all-Go apps for Android and iOS.

There are typically two ways to use Go on Android and iOS. The first is to write a Go library and use `gomobile bind` to generate language bindings for Java and Objective-C. Building a library does not require the app package. The `gomobile bind` command produces output that you can include in an Android Studio or Xcode project. For more on language bindings, see https://golang.org/x/mobile/cmd/gobind.

The second way is to write an app entirely in Go. The APIs are limited to those that are portable between both Android and iOS, in particular OpenGL, audio, and other Android NDK-like APIs. An all-Go app should use this app package to initialize the app, manage its lifecycle, and receive events.

Building apps

Apps written entirely in Go have a main function, and can be built with `gomobile build`, which directly produces runnable output for Android and iOS.

The gomobile tool can get installed with go get. For reference, see https://golang.org/x/mobile/cmd/gomobile.

For detailed instructions and documentation, see https://golang.org/wiki/Mobile.

Event processing in Native Apps

The Go runtime is initialized on Android when NativeActivity onCreate is called, and on iOS when the process starts. In both cases, Go init functions run before the app lifecycle has started.

An app is expected to call the Main function in main.main. When the function exits, the app exits. Inside the func passed to Main, call Filter on every event received, and then switch on its type. Registered filters run when the event is received, not when it is sent, so that filters run in the same goroutine as other code that calls OpenGL.

package main

import (
	"log"

	"golang.org/x/mobile/app"
	"golang.org/x/mobile/event/lifecycle"
	"golang.org/x/mobile/event/paint"
)

func main() {
	app.Main(func(a app.App) {
		for e := range a.Events() {
			switch e := a.Filter(e).(type) {
			case lifecycle.Event:
				// ...
			case paint.Event:
				log.Print("Call OpenGL here.")
				a.Publish()
			}
		}
	})
}

An event is represented by the empty interface type interface{}. Any value can be an event. Commonly used types include Event types defined by the following packages:

- golang.org/x/mobile/event/lifecycle
- golang.org/x/mobile/event/mouse
- golang.org/x/mobile/event/paint
- golang.org/x/mobile/event/size
- golang.org/x/mobile/event/touch

For example, touch.Event is the type that represents touch events. Other packages may define their own events, and send them on an app's event channel.

Other packages can also register event filters, e.g. to manage resources in response to lifecycle events. Such packages should call:

app.RegisterFilter(etc)

in an init function inside that package.