ui/app: implement Command for system events

And add CommandBack for the Android back button.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/app/GioActivity.java

@@ -50,4 +50,9 @@ public class GioActivity extends Activity {
 		super.onLowMemory();
 		view.lowMemory();
 	}
+
+	@Override public void onBackPressed() {
+		if (!view.backPressed())
+			super.onBackPressed();
+	}
 }

ui/app/GioView.java

@@ -163,6 +163,10 @@ public class GioView extends SurfaceView implements Choreographer.FrameCallback
 		onLowMemory();
 	}
 
+	boolean backPressed() {
+		return onBack(nhandle);
+	}
+
 	static private native long onCreateView(GioView view);
 	static private native void onDestroyView(long handle);
 	static private native void onStartView(long handle);
@@ -174,6 +178,7 @@ public class GioView extends SurfaceView implements Choreographer.FrameCallback
 	static private native void onTouchEvent(long handle, int action, int pointerID, int tool, float x, float y, long time);
 	static private native void onKeyEvent(long handle, int code, int character, long time);
 	static private native void onFrameCallback(long handle, long nanos);
+	static private native boolean onBack(long handle);
 
 	private static class InputConnection extends BaseInputConnection {
 		private final Editable editable;

ui/app/app.go

@@ -10,6 +10,10 @@ import (
 	"gioui.org/ui"
 )
 
+type Event interface {
+	ImplementsEvent()
+}
+
 type Draw struct {
 	Config *ui.Config
 	Size   image.Point
@@ -23,12 +27,16 @@ type ChangeStage struct {
 	Stage Stage
 }
 
-type Stage uint8
-
-type Event interface {
-	ImplementsEvent()
+// Command is a system event.
+type Command struct {
+	Type CommandType
+	// Suppress the default action of the command.
+	Cancel bool
 }
 
+type Stage uint8
+type CommandType uint8
+
 type Input interface {
 	ImplementsInput()
 }
@@ -39,6 +47,12 @@ const (
 	StageVisible
 )
 
+const (
+	// CommandBack is the command for a back action
+	// such as the Android back button.
+	CommandBack CommandType = iota
+)
+
 const (
 	inchPrDp = 1.0 / 160
 	mmPrDp   = 25.4 / 160
@@ -98,6 +112,7 @@ func (l Stage) String() string {
 
 func (_ Draw) ImplementsEvent()        {}
 func (_ ChangeStage) ImplementsEvent() {}
+func (_ *Command) ImplementsEvent()    {}
 
 func init() {
 	args := strings.Split(extraArgs, "|")

ui/app/os_android.c

@@ -72,6 +72,11 @@ JNIEXPORT jint JNI_OnLoad(JavaVM *vm, void *reserver) {
 			.name = "onFrameCallback",
 			.signature = "(JJ)V",
 			.fnPtr = onFrameCallback
+		},
+		{
+			.name = "onBack",
+			.signature = "(J)Z",
+			.fnPtr = onBack
 		}
 	};
 	if ((*env)->RegisterNatives(env, viewClass, methods, sizeof(methods)/sizeof(methods[0])) != 0) {

ui/app/os_android.go

@@ -180,6 +180,17 @@ func onFrameCallback(env *C.JNIEnv, class C.jclass, view C.jlong, nanos C.jlong)
 	}
 }
 
+//export onBack
+func onBack(env *C.JNIEnv, class C.jclass, view C.jlong) C.jboolean {
+	w := views[view]
+	ev := &Command{Type: CommandBack}
+	w.event(ev)
+	if ev.Cancel {
+		return C.JNI_TRUE
+	}
+	return C.JNI_FALSE
+}
+
 func (w *window) setVisible() {
 	win := w.aNativeWindow()
 	width, height := C.ANativeWindow_getWidth(win), C.ANativeWindow_getHeight(win)

ui/app/window.go

@@ -206,6 +206,9 @@ func (w *Window) event(e Event) {
 		needRedraw = true
 	case key.Event:
 		needRedraw = true
+	case *Command:
+		needAck = true
+		needRedraw = true
 	case ChangeStage:
 		w.stage = e.Stage
 		if w.stage > StageDead {