app: Start and Stop callbacks

Change-Id: If8ea6aaf2fb2c62eaf4119526a8bb46b8a84b982
Reviewed-on: https://go-review.googlesource.com/1881
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

app/app.go

@@ -20,6 +20,35 @@ func Run(cb Callbacks) {
 
 // Callbacks is the set of functions called by the app.
 type Callbacks struct {
+	// Start is called when the app enters the foreground.
+	// The app will start receiving Draw and Touch calls.
+	//
+	// Window geometry will be configured and an OpenGL context
+	// will be available.
+	//
+	// Start is an equivalent lifecycle state to onStart() on
+	// Android and applicationDidBecomeActive on iOS.
+	Start func()
+
+	// Stop is called shortly before a program is suspended.
+	//
+	// When Stop is received, the app is no longer visible and not is
+	// receiving events. It should:
+	//
+	//	- Save any state the user expects saved (for example text).
+	//	- Release all resources that are not needed.
+	//
+	// Execution time in the stop state is limited, and the limit is
+	// enforced by the operating system. Stop as quickly as you can.
+	//
+	// An app that is stopped may be started again. For example, the user
+	// opens Recent Apps and switches to your app. A stopped app may also
+	// be terminated by the operating system with no further warning.
+	//
+	// Stop is equivalent to onStop() on Android and
+	// applicationDidEnterBackground on iOS.
+	Stop func()
+
 	// Draw is called by the render loop to draw the screen.
 	//
 	// Drawing is done into a framebuffer, which is then swapped onto the

app/loop_android.go

@@ -101,10 +101,19 @@ func windowDrawLoop(cb Callbacks, w *C.ANativeWindow, queue *C.AInputQueue) {
 	geom.Width = geom.Pt(float32(C.windowWidth) / geom.PixelsPerPt)
 	geom.Height = geom.Pt(float32(C.windowHeight) / geom.PixelsPerPt)
 
+	// We start here rather than onStart so the window exists and the Gl
+	// context is configured.
+	if cb.Start != nil {
+		cb.Start()
+	}
+
 	for {
 		processEvents(cb, queue)
 		select {
 		case <-windowDestroyed:
+			if cb.Stop != nil {
+				cb.Stop()
+			}
 			return
 		default:
 			if cb.Draw != nil {