[Android] Fix the issue of blank or frozen pages in shared engine scenarios

shell/platform/android/io/flutter/embedding/android/FlutterSurfaceView.java

@@ -12,6 +12,7 @@
 import android.view.SurfaceView;
 import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
+import androidx.annotation.VisibleForTesting;
 import io.flutter.Log;
 import io.flutter.embedding.engine.renderer.FlutterRenderer;
 import io.flutter.embedding.engine.renderer.FlutterUiDisplayListener;
@@ -168,6 +169,10 @@ public FlutterRenderer getAttachedRenderer() {
     return flutterRenderer;
   }
 
+  @VisibleForTesting
+  /* package */ boolean isSurfaceAvailableForRendering() {
+    return isSurfaceAvailableForRendering;
+  }
   /**
    * Invoked by the owner of this {@code FlutterSurfaceView} when it wants to begin rendering a
    * Flutter UI to this {@code FlutterSurfaceView}.
@@ -215,8 +220,6 @@ public void detachFromRenderer() {
         disconnectSurfaceFromRenderer();
       }
 
-      pause();
-
       // Make the SurfaceView invisible to avoid showing a black rectangle.
       setAlpha(0.0f);
       flutterRenderer.removeIsDisplayingFlutterUiListener(flutterUiDisplayListener);
@@ -248,7 +251,7 @@ public void resume() {
 
     // If we're already attached to an Android window then we're now attached to both a renderer
     // and the Android window. We can begin rendering now.
-    if (isSurfaceAvailableForRendering) {
+    if (isSurfaceAvailableForRendering()) {
       Log.v(
           TAG,
           "Surface is available for rendering. Connecting FlutterRenderer to Android surface.");

shell/platform/android/io/flutter/embedding/android/FlutterTextureView.java

@@ -124,6 +124,11 @@ public FlutterRenderer getAttachedRenderer() {
     return flutterRenderer;
   }
 
+  @VisibleForTesting
+  /* package */ boolean isSurfaceAvailableForRendering() {
+    return isSurfaceAvailableForRendering;
+  }
+
   /**
    * Invoked by the owner of this {@code FlutterTextureView} when it wants to begin rendering a
    * Flutter UI to this {@code FlutterTextureView}.
@@ -170,8 +175,6 @@ public void detachFromRenderer() {
         disconnectSurfaceFromRenderer();
       }
 
-      pause();
-
       flutterRenderer = null;
     } else {
       Log.w(TAG, "detachFromRenderer() invoked when no FlutterRenderer was attached.");
@@ -198,7 +201,7 @@ public void resume() {
 
     // If we're already attached to an Android window then we're now attached to both a renderer
     // and the Android window. We can begin rendering now.
-    if (isSurfaceAvailableForRendering) {
+    if (isSurfaceAvailableForRendering()) {
       Log.v(
           TAG,
           "Surface is available for rendering. Connecting FlutterRenderer to Android surface.");

shell/platform/android/test/io/flutter/embedding/android/FlutterSurfaceViewTest.java

@@ -0,0 +1,72 @@
+package io.flutter.embedding.android;
+
+import static io.flutter.Build.API_LEVELS;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.spy;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+import android.annotation.TargetApi;
+import android.view.Surface;
+import android.view.SurfaceHolder;
+import androidx.test.core.app.ApplicationProvider;
+import androidx.test.ext.junit.runners.AndroidJUnit4;
+import io.flutter.embedding.engine.FlutterJNI;
+import io.flutter.embedding.engine.renderer.FlutterRenderer;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.robolectric.annotation.Config;
+
+@Config(manifest = Config.NONE)
+@RunWith(AndroidJUnit4.class)
+@TargetApi(API_LEVELS.API_30)
+public class FlutterSurfaceViewTest {
+  @Test
+  public void itShouldCreateANewSurfaceWhenReattachedAfterDetachingFromRenderer() {
+    // Consider this scenario: In an add-to-app context, where multiple Flutter activities share the
+    // same engine, a situation occurs. When navigating from FlutterActivity1 to FlutterActivity2,
+    // the Flutter view associated with FlutterActivity1 is detached from the engine. Then, the
+    // Flutter view of FlutterActivity2 is attached. Upon navigating back to FlutterActivity1, its
+    // Flutter view is re-attached to the shared engine.
+    //
+    // The expected behavior is: When a Flutter view detaches from the shared engine, the associated
+    // surface should be released. When the Flutter view re-attaches, a new surface should be
+    // created.
+
+    // Setup the test.
+    final FlutterSurfaceView surfaceView =
+        spy(new FlutterSurfaceView(ApplicationProvider.getApplicationContext()));
+
+    FlutterJNI fakeFlutterJNI = mock(FlutterJNI.class);
+    FlutterRenderer flutterRenderer = new FlutterRenderer(fakeFlutterJNI);
+
+    SurfaceHolder fakeSurfaceHolder = mock(SurfaceHolder.class);
+    Surface fakeSurface = mock(Surface.class);
+    when(surfaceView.getHolder()).thenReturn(fakeSurfaceHolder);
+    when(fakeSurfaceHolder.getSurface()).thenReturn(fakeSurface);
+    when(surfaceView.isSurfaceAvailableForRendering()).thenReturn(true);
+    when(surfaceView.getWindowToken()).thenReturn(mock(android.os.IBinder.class));
+
+    // Execute the behavior under test.
+    surfaceView.attachToRenderer(flutterRenderer);
+
+    // Verify the behavior under test.
+    verify(fakeFlutterJNI, times(1)).onSurfaceCreated(any(Surface.class));
+
+    // Execute the behavior under test.
+    surfaceView.detachFromRenderer();
+
+    // Verify the behavior under test.
+    verify(fakeFlutterJNI, times(1)).onSurfaceDestroyed();
+
+    // Execute the behavior under test.
+    surfaceView.attachToRenderer(flutterRenderer);
+
+    // Verify the behavior under test.
+    verify(fakeFlutterJNI, never()).onSurfaceWindowChanged(any(Surface.class));
+    verify(fakeFlutterJNI, times(2)).onSurfaceCreated(any(Surface.class));
+  }
+}

shell/platform/android/test/io/flutter/embedding/android/FlutterTextureViewTest.java

@@ -1,15 +1,22 @@
 package io.flutter.embedding.android;
 
 import static io.flutter.Build.API_LEVELS;
+import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.spy;
+import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
 
 import android.annotation.TargetApi;
 import android.graphics.SurfaceTexture;
 import android.view.Surface;
 import android.view.TextureView;
 import androidx.test.core.app.ApplicationProvider;
 import androidx.test.ext.junit.runners.AndroidJUnit4;
+import io.flutter.embedding.engine.FlutterJNI;
+import io.flutter.embedding.engine.renderer.FlutterRenderer;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.robolectric.annotation.Config;
@@ -31,4 +38,47 @@ public void surfaceTextureListenerReleasesRenderer() {
 
     verify(mockRenderSurface).release();
   }
+
+  @Test
+  public void itShouldCreateANewSurfaceWhenReattachedAfterDetachingFromRenderer() {
+    // Consider this scenario: In an add-to-app context, where multiple Flutter activities share the
+    // same engine, a situation occurs. When navigating from FlutterActivity1 to FlutterActivity2,
+    // the Flutter view associated with FlutterActivity1 is detached from the engine. Then, the
+    // Flutter view of FlutterActivity2 is attached. Upon navigating back to FlutterActivity1, its
+    // Flutter view is re-attached to the shared engine.
+    //
+    // The expected behavior is: When a Flutter view detaches from the shared engine, the associated
+    // surface should be released. When the Flutter view re-attaches, a new surface should be
+    // created.
+
+    // Setup the test.
+    final FlutterTextureView textureView =
+        spy(new FlutterTextureView(ApplicationProvider.getApplicationContext()));
+
+    FlutterJNI fakeFlutterJNI = mock(FlutterJNI.class);
+    FlutterRenderer flutterRenderer = new FlutterRenderer(fakeFlutterJNI);
+
+    when(textureView.isSurfaceAvailableForRendering()).thenReturn(true);
+    when(textureView.getSurfaceTexture()).thenReturn(mock(SurfaceTexture.class));
+    when(textureView.getWindowToken()).thenReturn(mock(android.os.IBinder.class));
+
+    // Execute the behavior under test.
+    textureView.attachToRenderer(flutterRenderer);
+
+    // Verify the behavior under test.
+    verify(fakeFlutterJNI, times(1)).onSurfaceCreated(any(Surface.class));
+
+    // Execute the behavior under test.
+    textureView.detachFromRenderer();
+
+    // Verify the behavior under test.
+    verify(fakeFlutterJNI, times(1)).onSurfaceDestroyed();
+
+    // Execute the behavior under test.
+    textureView.attachToRenderer(flutterRenderer);
+
+    // Verify the behavior under test.
+    verify(fakeFlutterJNI, never()).onSurfaceWindowChanged(any(Surface.class));
+    verify(fakeFlutterJNI, times(2)).onSurfaceCreated(any(Surface.class));
+  }
 }