app: [Android] detach previous View when another is created

The Android system can in some cases replace the GioView of our Android
Activity before destroying the previous one. This change makes sure the
previous view is ignored, in particular its destroy event.

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

app/GioView.java

@@ -113,7 +113,9 @@ public final class GioView extends SurfaceView {
 	}
 
 	@Override public boolean onKeyDown(int keyCode, KeyEvent event) {
-		onKeyEvent(nhandle, keyCode, event.getUnicodeChar(), event.getEventTime());
+		if (nhandle != 0) {
+			onKeyEvent(nhandle, keyCode, event.getUnicodeChar(), event.getEventTime());
+		}
 		return false;
 	}
 
@@ -229,6 +231,9 @@ public final class GioView extends SurfaceView {
     }
 
 	private void dispatchMotionEvent(MotionEvent event) {
+		if (nhandle == 0) {
+			return;
+		}
 		for (int j = 0; j < event.getHistorySize(); j++) {
 			long time = event.getHistoricalEventTime(j);
 			for (int i = 0; i < event.getPointerCount(); i++) {
@@ -289,12 +294,16 @@ public final class GioView extends SurfaceView {
 	}
 
 	@Override protected boolean fitSystemWindows(Rect insets) {
-		onWindowInsets(nhandle, insets.top, insets.right, insets.bottom, insets.left);
+		if (nhandle != 0) {
+			onWindowInsets(nhandle, insets.top, insets.right, insets.bottom, insets.left);
+		}
 		return true;
 	}
 
 	@Override protected void onDraw(Canvas canvas) {
-		onFrameCallback(nhandle);
+		if (nhandle != 0) {
+			onFrameCallback(nhandle);
+		}
 	}
 
 	int getDensity() {
@@ -306,25 +315,39 @@ public final class GioView extends SurfaceView {
 	}
 
 	public void start() {
-		onStartView(nhandle);
+		if (nhandle != 0) {
+			onStartView(nhandle);
+		}
 	}
 
 	public void stop() {
-		onStopView(nhandle);
+		if (nhandle != 0) {
+			onStopView(nhandle);
+		}
 	}
 
 	public void destroy() {
+		if (nhandle != 0) {
+			onDestroyView(nhandle);
+		}
+	}
+
+	protected void unregister() {
 		setOnFocusChangeListener(null);
 		getHolder().removeCallback(surfCallbacks);
-		onDestroyView(nhandle);
 		nhandle = 0;
 	}
 
 	public void configurationChanged() {
-		onConfigurationChanged(nhandle);
+		if (nhandle != 0) {
+			onConfigurationChanged(nhandle);
+		}
 	}
 
 	public boolean backPressed() {
+		if (nhandle == 0) {
+			return false;
+		}
 		return onBack(nhandle);
 	}
 

app/os_android.go

@@ -161,6 +161,7 @@ var gioView struct {
 	setNavigationColor C.jmethodID
 	setStatusColor     C.jmethodID
 	setFullscreen      C.jmethodID
+	unregister         C.jmethodID
 }
 
 // ViewEvent is sent whenever the Window's underlying Android view
@@ -315,6 +316,7 @@ func Java_org_gioui_GioView_onCreateView(env *C.JNIEnv, class C.jclass, view C.j
 		m.setNavigationColor = getMethodID(env, class, "setNavigationColor", "(II)V")
 		m.setStatusColor = getMethodID(env, class, "setStatusColor", "(II)V")
 		m.setFullscreen = getMethodID(env, class, "setFullscreen", "(Z)V")
+		m.unregister = getMethodID(env, class, "unregister", "()V")
 	})
 	view = C.jni_NewGlobalRef(env, view)
 	wopts := <-mainWindow.out
@@ -325,6 +327,9 @@ func Java_org_gioui_GioView_onCreateView(env *C.JNIEnv, class C.jclass, view C.j
 		}
 		windows[wopts.window] = w
 	}
+	if w.view != 0 {
+		w.detach(env)
+	}
 	w.view = view
 	w.callbacks.SetDriver(w)
 	handle := C.jlong(view)
@@ -339,11 +344,7 @@ func Java_org_gioui_GioView_onCreateView(env *C.JNIEnv, class C.jclass, view C.j
 //export Java_org_gioui_GioView_onDestroyView
 func Java_org_gioui_GioView_onDestroyView(env *C.JNIEnv, class C.jclass, handle C.jlong) {
 	w := views[handle]
-	w.callbacks.Event(ViewEvent{View: 0})
-	w.callbacks.SetDriver(nil)
-	delete(views, handle)
-	C.jni_DeleteGlobalRef(env, w.view)
-	w.view = 0
+	w.detach(env)
 }
 
 //export Java_org_gioui_GioView_onStopView
@@ -442,6 +443,15 @@ func Java_org_gioui_GioView_onWindowInsets(env *C.JNIEnv, class C.jclass, view C
 	}
 }
 
+func (w *window) detach(env *C.JNIEnv) {
+	callVoidMethod(env, w.view, gioView.unregister)
+	w.callbacks.Event(ViewEvent{})
+	w.callbacks.SetDriver(nil)
+	delete(views, C.jlong(w.view))
+	C.jni_DeleteGlobalRef(env, w.view)
+	w.view = 0
+}
+
 func (w *window) setVisible() {
 	width, height := C.ANativeWindow_getWidth(w.win), C.ANativeWindow_getHeight(w.win)
 	if width == 0 || height == 0 {