app,app/internal: [Android] replace Window.Do with ViewEvent

An event reporting the most recent Android View attached to the Window is both
simpler to implement and automatically tracks the Activity lifecycle.

The downside is that buggy programs may hold on to a stale references.
Fortunately, JNI references are handles not pointers so the always-on Android
JNI checks will very likely catch stale references on use.

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

app/app_android.go

@@ -6,6 +6,8 @@ import (
 	"gioui.org/app/internal/window"
 )
 
+type ViewEvent = window.ViewEvent
+
 // JavaVM returns the global JNI JavaVM.
 func JavaVM() uintptr {
 	return window.JavaVM()
@@ -16,42 +18,3 @@ func JavaVM() uintptr {
 func AppContext() uintptr {
 	return window.AppContext()
 }
-
-// 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)
-		// two-stage process: first wait for a valid driver...
-		go func() {
-			alive := w.driverDo(func() {
-				driver <- w.driver.(androidDriver)
-			})
-			if !alive {
-				driver <- nil
-			}
-		}()
-		d := <-driver
-		if d == nil {
-			// Window is dead.
-			break
-		}
-		// .. 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() {
-			success <- d.Do(f)
-		})
-		if <-success {
-			break
-		}
-	}
-}