app/internal/window: [Android] replace RegisterFragment with Do

Do is a function for accessing the underlying Android View in a safe
context, the main thread. Do is

- more general than RegisterFragment and may be expanded to other platforms
- simpler to implement (from the Gio side)

and as a bonus, the Do implementation avoids a race condition where
a call to RegisterFragment during an Activity re-create would be ignored.

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

app/app_android.go

@@ -17,18 +17,32 @@ func AppContext() uintptr {
 	return window.AppContext()
 }
 
-// androidDriver is an interface that allows the Window's run method
-// to call the RegisterFragment method of the Android window driver.
-type androidDriver interface {
-	RegisterFragment(string)
-}
-
-// RegisterFragment constructs a Java instance of the specified class
-// and registers it as a Fragment in the Context in which the View was
-// created.
-func (w *Window) RegisterFragment(del string) {
-	w.driverDo(func() {
-		d := w.driver.(androidDriver)
-		d.RegisterFragment(del)
-	})
+// Do invokes the function with a JNI jobject handle to the underlying
+// Android View. The function is invoked on the main thread, and the
+// handle is invalidated after the function returns.
+//
+// Note: Do may deadlock if called from the same goroutine that receives from
+// Events.
+func (w *Window) Do(f func(view uintptr)) {
+	type androidDriver interface {
+		Do(f func(view uintptr)) bool
+	}
+	success := make(chan bool)
+	for {
+		driver := make(chan androidDriver, 1)
+		// two-stage process: first wait for a valid driver...
+		w.driverDo(func() {
+			driver <- w.driver.(androidDriver)
+		})
+		// .. then run the function on the main thread using the
+		// driver. The driver Do method returns false if the
+		// view was invalidated while switching to the main thread.
+		window.RunOnMain(func() {
+			d := <-driver
+			success <- d.Do(f)
+		})
+		if <-success {
+			break
+		}
+	}
 }