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
+		}
+	}
 }

app/internal/window/GioView.java

@@ -191,31 +191,6 @@ public final class GioView extends SurfaceView implements Choreographer.FrameCal
 		return onBack(nhandle);
 	}
 
-	void registerFragment(String del) {
-		final Class cls;
-		try {
-			cls = getContext().getClassLoader().loadClass(del);
-		} catch (ClassNotFoundException e) {
-			throw new RuntimeException("RegisterFragment: fragment class not found: " + e.getMessage());
-		}
-
-		final Fragment frag;
-		try {
-			frag = (Fragment)cls.newInstance();
-		} catch (IllegalAccessException | InstantiationException | ExceptionInInitializerError | SecurityException | ClassCastException e) {
-			throw new RuntimeException("RegisterFragment: error instantiating fragment: " + e.getMessage());
-		}
-		final FragmentManager fm;
-		try {
-			fm = ((Activity)getContext()).getFragmentManager();
-		} catch (ClassCastException e) {
-			throw new RuntimeException("RegisterFragment: cannot get fragment manager from View Context: " + e.getMessage());
-		}
-		FragmentTransaction ft = fm.beginTransaction();
-		ft.add(frag, del);
-		ft.commitNow();
-	}
-
 	static private native long onCreateView(GioView view);
 	static private native void onDestroyView(long handle);
 	static private native void onStartView(long handle);

app/internal/window/os_android.go

@@ -80,7 +80,6 @@ type window struct {
 	mshowTextInput     C.jmethodID
 	mhideTextInput     C.jmethodID
 	mpostFrameCallback C.jmethodID
-	mRegisterFragment  C.jmethodID
 }
 
 type jvalue uint64 // The largest JNI type fits in 64 bits.
@@ -193,7 +192,6 @@ func Java_org_gioui_GioView_onCreateView(env *C.JNIEnv, class C.jclass, view C.j
 		mshowTextInput:     getMethodID(env, class, "showTextInput", "()V"),
 		mhideTextInput:     getMethodID(env, class, "hideTextInput", "()V"),
 		mpostFrameCallback: getMethodID(env, class, "postFrameCallback", "()V"),
-		mRegisterFragment:  getMethodID(env, class, "registerFragment", "(Ljava/lang/String;)V"),
 	}
 	wopts := <-mainWindow.out
 	w.callbacks = wopts.window
@@ -416,7 +414,6 @@ func runInJVM(jvm *C.JavaVM, f func(env *C.JNIEnv)) {
 	runtime.LockOSThread()
 	defer runtime.UnlockOSThread()
 	var env *C.JNIEnv
-	var detach bool
 	if res := C.gio_jni_GetEnv(jvm, &env, C.JNI_VERSION_1_6); res != C.JNI_OK {
 		if res != C.JNI_EDETACHED {
 			panic(fmt.Errorf("JNI GetEnv failed with error %d", res))
@@ -424,14 +421,9 @@ func runInJVM(jvm *C.JavaVM, f func(env *C.JNIEnv)) {
 		if C.gio_jni_AttachCurrentThread(jvm, &env, nil) != C.JNI_OK {
 			panic(errors.New("runInJVM: AttachCurrentThread failed"))
 		}
-		detach = true
+		defer C.gio_jni_DetachCurrentThread(jvm)
 	}
 
-	if detach {
-		defer func() {
-			C.gio_jni_DetachCurrentThread(jvm)
-		}()
-	}
 	f(env)
 }
 
@@ -537,11 +529,21 @@ func javaString(env *C.JNIEnv, str string) C.jstring {
 	return C.gio_jni_NewString(env, (*C.jchar)(unsafe.Pointer(&utf16Chars[0])), C.int(len(utf16Chars)))
 }
 
-func (w *window) RegisterFragment(del string) {
-	w.runOnMain(func(env *C.JNIEnv) {
-		jstr := javaString(env, del)
-		callVoidMethod(env, w.view, w.mRegisterFragment, jvalue(jstr))
+// Do invokes the function with a global JNI handle to the view. If
+// the view is destroyed, Do returns false and does not invoke the
+// function.
+//
+// NOTE: Do must be invoked on the Android main thread.
+func (w *window) Do(f func(view uintptr)) bool {
+	if w.view == 0 {
+		return false
+	}
+	runInJVM(javaVM(), func(env *C.JNIEnv) {
+		view := C.gio_jni_NewGlobalRef(env, w.view)
+		defer C.gio_jni_DeleteGlobalRef(env, view)
+		f(uintptr(view))
 	})
+	return true
 }
 
 func varArgs(args []jvalue) *C.jvalue {
@@ -645,6 +647,14 @@ func (w *window) ReadClipboard() {
 	})
 }
 
+// RunOnMain is the exported version of runOnMain without a JNI
+// environement.
+func RunOnMain(f func()) {
+	runOnMain(func(_ *C.JNIEnv) {
+		f()
+	})
+}
+
 // runOnMain runs a function on the Java main thread.
 func runOnMain(f func(env *C.JNIEnv)) {
 	go func() {

app/permission/doc.go

@@ -37,10 +37,14 @@ program's source code:
 Android -- Dangerous Permissions
 
 Certain permissions on Android are marked with a protection level of
-"dangerous". This means that, in addition to including the relevant Gio
-permission packages, your app will need to prompt the user specifically
-to request access. This can be done with a java Fragment, installed using
-(*app.Window).RegisterFragment(). For more information on dangerous
-permissions, see: https://developer.android.com/guide/topics/permissions/overview#dangerous_permissions
+"dangerous". This means that, in addition to including the relevant
+Gio permission packages, your app will need to prompt the user
+specifically to request access. To access the Android Activity
+required for prompting, use the Do method on Window. Do exposes
+the underlying Android View, on which the getContext method returns
+the Activity.
+
+For more information on dangerous permissions, see:
+https://developer.android.com/guide/topics/permissions/overview#dangerous_permissions
 */
 package permission