app/internal/window: [android] re-apply view state lost on rotate etc.

Android can re-create our Activity and GioView at any time, losing view
and activity state such as the current cursor. Further, setting state
while there is no GioView attached is lost.

Track the current and unapplied state, and apply it when a GioView is
available.

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

app/internal/window/os_android.go

@@ -71,11 +71,20 @@ type window struct {
 	stage   system.Stage
 	started bool
 
+	state, newState windowState
+
+	// mu protects the fields following it.
 	mu        sync.Mutex
 	win       *C.ANativeWindow
 	animating bool
 }
 
+// windowState tracks the View or Activity specific state lost when Android
+// re-creates our Activity.
+type windowState struct {
+	cursor *pointer.CursorName
+}
+
 // gioView hold cached JNI methods for GioView.
 var gioView struct {
 	once              sync.Once
@@ -118,8 +127,12 @@ var android struct {
 	mwakeupMainThread C.jmethodID
 }
 
+// view maps from GioView JNI refenreces to windows.
 var views = make(map[C.jlong]*window)
 
+// windows maps from Callbacks to windows
+var windows = make(map[Callbacks]*window)
+
 var mainWindow = newWindowRendezvous()
 
 var mainFuncs = make(chan func(env *C.JNIEnv), 1)
@@ -209,15 +222,20 @@ func Java_org_gioui_GioView_onCreateView(env *C.JNIEnv, class C.jclass, view C.j
 		m.setCursor = getMethodID(env, class, "setCursor", "(I)V")
 	})
 	view = C.gio_jni_NewGlobalRef(env, view)
-	w := &window{
-		view: view,
-	}
 	wopts := <-mainWindow.out
-	w.callbacks = wopts.window
+	w, ok := windows[wopts.window]
+	if !ok {
+		w = &window{
+			callbacks: wopts.window,
+		}
+		windows[wopts.window] = w
+	}
 	w.callbacks.SetDriver(w)
+	w.view = view
 	handle := C.jlong(view)
 	views[handle] = w
 	w.loadConfig(env, class)
+	applyStateDiff(env, view, windowState{}, w.state)
 	w.setStage(system.StagePaused)
 	w.callbacks.Event(ViewEvent{View: uintptr(view)})
 	return handle
@@ -659,6 +677,33 @@ func (w *window) ReadClipboard() {
 }
 
 func (w *window) SetCursor(name pointer.CursorName) {
+	w.setState(func(state *windowState) {
+		state.cursor = &name
+	})
+}
+
+// setState adjust the window state on the main thread.
+func (w *window) setState(f func(state *windowState)) {
+	runOnMain(func(env *C.JNIEnv) {
+		f(&w.newState)
+		if w.view == 0 {
+			// No View attached. The state will be applied at next onCreateView.
+			return
+		}
+		old := w.state
+		state := w.newState
+		applyStateDiff(env, w.view, old, state)
+		w.state = state
+	})
+}
+
+func applyStateDiff(env *C.JNIEnv, view C.jobject, old, state windowState) {
+	if state.cursor != nil && old.cursor != state.cursor {
+		setCursor(env, view, *state.cursor)
+	}
+}
+
+func setCursor(env *C.JNIEnv, view C.jobject, name pointer.CursorName) {
 	var curID int
 	switch name {
 	default:
@@ -678,9 +723,7 @@ func (w *window) SetCursor(name pointer.CursorName) {
 	case pointer.CursorNone:
 		curID = 0 // TYPE_NULL
 	}
-	runOnMain(func(env *C.JNIEnv) {
-		callVoidMethod(env, w.view, gioView.setCursor, jvalue(curID))
-	})
+	callVoidMethod(env, view, gioView.setCursor, jvalue(curID))
 }
 
 // Close the window. Not implemented for Android.