[Fireperf][AASA] FirstDrawDoneListener (firebase#4041)

* WIP

* handler

* handle bug prior to api 26

* tests

* copyright

* improve tests

* comment fix

* improve tests

* comment

* backport View.isAttachedToWindow()

* improved registerForNextDraw_delaysAddingListenerForAPIsBelow26

* remove LayoutChangeListener

* add check to mmake sure OnAttachStateChangeListener removes itself

* mcreate helper isAliveAndAttached

* improve test with Robolectric ShadowLooper

firebase-perf/src/main/java/com/google/firebase/perf/util/FirstDrawDoneListener.java

@@ -0,0 +1,101 @@
+// Copyright 2022 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+//
+// You may obtain a copy of the License at
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package com.google.firebase.perf.util;
+
+import android.os.Build;
+import android.os.Handler;
+import android.os.Looper;
+import android.view.View;
+import android.view.ViewTreeObserver;
+import androidx.annotation.RequiresApi;
+import java.util.concurrent.atomic.AtomicReference;
+
+/**
+ * OnDrawListener that unregisters itself and invokes callback when the next draw is done. This API
+ * 16+ implementation is an approximation of the initial display time. {@link
+ * android.view.Choreographer#postFrameCallback} is an Android API that provides a simpler and more
+ * accurate initial display time, but it was bugged before API 30, hence we use this backported
+ * implementation.
+ */
+@RequiresApi(Build.VERSION_CODES.JELLY_BEAN)
+public class FirstDrawDoneListener implements ViewTreeObserver.OnDrawListener {
+  private final Handler mainThreadHandler = new Handler(Looper.getMainLooper());
+  private final AtomicReference<View> viewReference;
+  private final Runnable callback;
+
+  /** Registers a post-draw callback for the next draw of a view. */
+  public static void registerForNextDraw(View view, Runnable drawDoneCallback) {
+    final FirstDrawDoneListener listener = new FirstDrawDoneListener(view, drawDoneCallback);
+    // Handle bug prior to API 26 where OnDrawListener from the floating ViewTreeObserver is not
+    // merged into the real ViewTreeObserver.
+    // https://android.googlesource.com/platform/frameworks/base/+/9f8ec54244a5e0343b9748db3329733f259604f3
+    if (Build.VERSION.SDK_INT < 26 && !isAliveAndAttached(view)) {
+      view.addOnAttachStateChangeListener(
+          new View.OnAttachStateChangeListener() {
+            @Override
+            public void onViewAttachedToWindow(View view) {
+              view.getViewTreeObserver().addOnDrawListener(listener);
+              view.removeOnAttachStateChangeListener(this);
+            }
+
+            @Override
+            public void onViewDetachedFromWindow(View view) {
+              view.removeOnAttachStateChangeListener(this);
+            }
+          });
+    } else {
+      view.getViewTreeObserver().addOnDrawListener(listener);
+    }
+  }
+
+  private FirstDrawDoneListener(View view, Runnable callback) {
+    this.viewReference = new AtomicReference<>(view);
+    this.callback = callback;
+  }
+
+  @Override
+  public void onDraw() {
+    // Set viewReference to null so any onDraw past the first is a no-op
+    View view = viewReference.getAndSet(null);
+    if (view == null) {
+      return;
+    }
+    // OnDrawListeners cannot be removed within onDraw, so we remove it with a
+    // GlobalLayoutListener
+    view.getViewTreeObserver()
+        .addOnGlobalLayoutListener(() -> view.getViewTreeObserver().removeOnDrawListener(this));
+    mainThreadHandler.postAtFrontOfQueue(callback);
+  }
+
+  /**
+   * Helper to avoid <a
+   * href="https://android.googlesource.com/platform/frameworks/base/+/9f8ec54244a5e0343b9748db3329733f259604f3">bug
+   * prior to API 26</a>, where the floating ViewTreeObserver's OnDrawListeners are not merged into
+   * the real ViewTreeObserver during attach.
+   *
+   * @return true if the View is already attached and the ViewTreeObserver is not a floating
+   *     placeholder.
+   */
+  private static boolean isAliveAndAttached(View view) {
+    return view.getViewTreeObserver().isAlive() && isAttachedToWindow(view);
+  }
+
+  /** Backport {@link View#isAttachedToWindow()} which is API 19+ only. */
+  private static boolean isAttachedToWindow(View view) {
+    if (Build.VERSION.SDK_INT >= 19) {
+      return view.isAttachedToWindow();
+    }
+    return view.getWindowToken() != null;
+  }
+}

firebase-perf/src/test/java/com/google/firebase/perf/util/FirstDrawDoneListenerTest.java

@@ -0,0 +1,181 @@
+// Copyright 2022 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+//
+// You may obtain a copy of the License at
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package com.google.firebase.perf.util;
+
+import static com.google.common.truth.Truth.assertThat;
+import static org.mockito.Mockito.inOrder;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.robolectric.Shadows.shadowOf;
+
+import android.os.Build;
+import android.os.Handler;
+import android.os.Looper;
+import android.view.View;
+import android.view.ViewTreeObserver;
+import android.view.ViewTreeObserver.OnDrawListener;
+import androidx.test.core.app.ApplicationProvider;
+import java.lang.reflect.Field;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.InOrder;
+import org.robolectric.RobolectricTestRunner;
+import org.robolectric.annotation.Config;
+import org.robolectric.annotation.LooperMode;
+
+/** Unit tests for {@link FirstDrawDoneListener}. */
+@RunWith(RobolectricTestRunner.class)
+@LooperMode(LooperMode.Mode.PAUSED)
+public class FirstDrawDoneListenerTest {
+  private View testView;
+
+  @Before
+  public void setUp() {
+    testView = new View(ApplicationProvider.getApplicationContext());
+  }
+
+  @Test
+  @Config(sdk = 25)
+  public void registerForNextDraw_delaysAddingListenerForAPIsBelow26()
+      throws NoSuchFieldException, IllegalAccessException, ClassNotFoundException {
+    ArrayList<OnDrawListener> mOnDrawListeners =
+        initViewTreeObserverWithListener(testView.getViewTreeObserver());
+    assertThat(mOnDrawListeners.size()).isEqualTo(0);
+
+    // OnDrawListener is not registered, it is delayed for later
+    FirstDrawDoneListener.registerForNextDraw(testView, () -> {});
+    assertThat(mOnDrawListeners.size()).isEqualTo(0);
+
+    // Register listener after the view is attached to a window
+    List<View.OnAttachStateChangeListener> attachListeners = dispatchAttachedToWindow(testView);
+    assertThat(mOnDrawListeners.size()).isEqualTo(1);
+    assertThat(mOnDrawListeners.get(0)).isInstanceOf(FirstDrawDoneListener.class);
+    assertThat(attachListeners).isEmpty();
+  }
+
+  @Test
+  @Config(sdk = 26)
+  public void registerForNextDraw_directlyAddsListenerForApi26AndAbove()
+      throws NoSuchFieldException, IllegalAccessException {
+    ArrayList<OnDrawListener> mOnDrawListeners =
+        initViewTreeObserverWithListener(testView.getViewTreeObserver());
+    assertThat(mOnDrawListeners.size()).isEqualTo(0);
+
+    // Immediately register an OnDrawListener to ViewTreeObserver
+    FirstDrawDoneListener.registerForNextDraw(testView, () -> {});
+    assertThat(mOnDrawListeners.size()).isEqualTo(1);
+    assertThat(mOnDrawListeners.get(0)).isInstanceOf(FirstDrawDoneListener.class);
+  }
+
+  @Test
+  @Config(sdk = 26)
+  public void onDraw_postsCallbackToFrontOfQueue() {
+    Handler handler = new Handler(Looper.getMainLooper());
+    Runnable drawDoneCallback = mock(Runnable.class);
+    Runnable otherCallback = mock(Runnable.class);
+    InOrder inOrder = inOrder(drawDoneCallback, otherCallback);
+
+    FirstDrawDoneListener.registerForNextDraw(testView, drawDoneCallback);
+    handler.post(otherCallback); // 3rd in queue
+    handler.postAtFrontOfQueue(otherCallback); // 2nd in queue
+    testView.getViewTreeObserver().dispatchOnDraw(); // 1st in queue
+    verify(drawDoneCallback, never()).run();
+    verify(otherCallback, never()).run();
+
+    // Execute all posted tasks
+    shadowOf(Looper.getMainLooper()).idle();
+    inOrder.verify(drawDoneCallback, times(1)).run();
+    inOrder.verify(otherCallback, times(2)).run();
+    inOrder.verifyNoMoreInteractions();
+  }
+
+  @Test
+  @Config(sdk = 26)
+  public void onDraw_unregistersItself_inLayoutChangeListener()
+      throws NoSuchFieldException, IllegalAccessException {
+    ArrayList<OnDrawListener> mOnDrawListeners =
+        initViewTreeObserverWithListener(testView.getViewTreeObserver());
+    FirstDrawDoneListener.registerForNextDraw(testView, () -> {});
+    assertThat(mOnDrawListeners.size()).isEqualTo(1);
+
+    // Does not remove OnDrawListener before onDraw, even if OnGlobalLayout is triggered
+    testView.getViewTreeObserver().dispatchOnGlobalLayout();
+    assertThat(mOnDrawListeners.size()).isEqualTo(1);
+
+    // Removes OnDrawListener in the next OnGlobalLayout after onDraw
+    testView.getViewTreeObserver().dispatchOnDraw();
+    testView.getViewTreeObserver().dispatchOnGlobalLayout();
+    assertThat(mOnDrawListeners.size()).isEqualTo(0);
+  }
+
+  /**
+   * Returns ViewTreeObserver.mOnDrawListeners field through reflection. Since reflections are
+   * employed, prefer to be used in tests with fixed API level using @Config(sdk = X).
+   *
+   * @param vto ViewTreeObserver instance to initialize and return the mOnDrawListeners from
+   */
+  private static ArrayList<OnDrawListener> initViewTreeObserverWithListener(ViewTreeObserver vto)
+      throws NoSuchFieldException, IllegalAccessException {
+    // Adding a listener forces ViewTreeObserver.mOnDrawListeners to be initialized and non-null.
+    OnDrawListener placeHolder = () -> {};
+    vto.addOnDrawListener(placeHolder);
+    vto.removeOnDrawListener(placeHolder);
+
+    // Obtain mOnDrawListeners field through reflection
+    Field mOnDrawListeners =
+        android.view.ViewTreeObserver.class.getDeclaredField("mOnDrawListeners");
+    mOnDrawListeners.setAccessible(true);
+    ArrayList<OnDrawListener> listeners = (ArrayList<OnDrawListener>) mOnDrawListeners.get(vto);
+    assertThat(listeners).isNotNull();
+    assertThat(listeners.size()).isEqualTo(0);
+    return listeners;
+  }
+
+  /**
+   * Simulates {@link View}'s dispatchAttachedToWindow() on API 25 using reflection.
+   *
+   * <p>This only simulates the part where dispatchAttachedToWindow() notifies the list of {@link
+   * View.OnAttachStateChangeListener}.
+   *
+   * @param view the view in which we are simulating dispatchAttachedToWindow().
+   * @return list of {@link View.OnAttachStateChangeListener} from the input {@link View}
+   */
+  private static List<View.OnAttachStateChangeListener> dispatchAttachedToWindow(View view)
+      throws ClassNotFoundException, NoSuchFieldException, IllegalAccessException {
+    assert Build.VERSION.SDK_INT == 25;
+    Class<?> listenerInfo = Class.forName("android.view.View$ListenerInfo");
+    Field mListenerInfo = View.class.getDeclaredField("mListenerInfo");
+    mListenerInfo.setAccessible(true);
+    Object li = mListenerInfo.get(view);
+    assertThat(li).isNotNull();
+    Field mOnAttachStateChangeListeners =
+        listenerInfo.getDeclaredField("mOnAttachStateChangeListeners");
+    mOnAttachStateChangeListeners.setAccessible(true);
+    CopyOnWriteArrayList<View.OnAttachStateChangeListener> listeners =
+        (CopyOnWriteArrayList<View.OnAttachStateChangeListener>)
+            mOnAttachStateChangeListeners.get(li);
+    assertThat(listeners).isNotNull();
+    assertThat(listeners).isNotEmpty();
+    for (View.OnAttachStateChangeListener listener : listeners) {
+      listener.onViewAttachedToWindow(view);
+    }
+    return listeners;
+  }
+}