Call Shell::NotifyLowMemory when backgrounded/memory pressure occurs on Android (flutter#19026)

* Reland "Call Shell::NotifyLowMemoryWarning on Android Trim and LowMemory events (flutter#18979)" (flutter#19023)"

This reverts commit 0a852d8.

Author: dnfield

shell/platform/android/android_shell_holder.cc

@@ -181,4 +181,8 @@ fml::WeakPtr<PlatformViewAndroid> AndroidShellHolder::GetPlatformView() {
   return platform_view_;
 }
 
+void AndroidShellHolder::NotifyLowMemoryWarning() {
+  FML_DCHECK(shell_);
+  shell_->NotifyLowMemoryWarning();
+}
 }  // namespace flutter

shell/platform/android/android_shell_holder.h

@@ -40,6 +40,8 @@ class AndroidShellHolder {
 
   void UpdateAssetManager(fml::RefPtr<flutter::AssetManager> asset_manager);
 
+  void NotifyLowMemoryWarning();
+
  private:
   const flutter::Settings settings_;
   const std::shared_ptr<PlatformViewAndroidJNI> jni_facade_;

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

@@ -629,6 +629,9 @@ void onUserLeaveHint() {
   void onTrimMemory(int level) {
     ensureAlive();
     if (flutterEngine != null) {
+      // This is always an indication that the Dart VM should collect memory
+      // and free any unneeded resources.
+      flutterEngine.getDartExecutor().notifyLowMemoryWarning();
       // Use a trim level delivered while the application is running so the
       // framework has a chance to react to the notification.
       if (level == TRIM_MEMORY_RUNNING_LOW) {
@@ -651,6 +654,7 @@ void onTrimMemory(int level) {
   void onLowMemory() {
     Log.v(TAG, "Forwarding onLowMemory() to FlutterEngine.");
     ensureAlive();
+    flutterEngine.getDartExecutor().notifyLowMemoryWarning();
     flutterEngine.getSystemChannel().sendMemoryPressureWarning();
   }
 

shell/platform/android/io/flutter/embedding/engine/FlutterJNI.java

@@ -845,6 +845,22 @@ public Bitmap getBitmap() {
   // TODO(mattcarroll): determine if this is nonull or nullable
   private native Bitmap nativeGetBitmap(long nativePlatformViewId);
 
+  /**
+   * Notifies the Dart VM of a low memory event, or that the application is in a state such that now
+   * is an appropriate time to free resources, such as going to the background.
+   *
+   * <p>This is distinct from sending a SystemChannel message about low memory, which only notifies
+   * the running Flutter application.
+   */
+  @UiThread
+  public void notifyLowMemoryWarning() {
+    ensureRunningOnMainThread();
+    ensureAttachedToNative();
+    nativeNotifyLowMemoryWarning(nativePlatformViewId);
+  }
+
+  private native void nativeNotifyLowMemoryWarning(long nativePlatformViewId);
+
   private void ensureRunningOnMainThread() {
     if (Looper.myLooper() != mainLooper) {
       throw new RuntimeException(

shell/platform/android/io/flutter/embedding/engine/dart/DartExecutor.java

@@ -232,6 +232,19 @@ public void setIsolateServiceIdListener(@Nullable IsolateServiceIdListener liste
     }
   }
 
+  /**
+   * Notify the Dart VM of a low memory event, or that the application is in a state such that now
+   * is an appropriate time to free resources, such as going to the background.
+   *
+   * <p>This does not notify a Flutter application about memory pressure. For that, use the {@link
+   * SystemChannel#sendMemoryPressureWarning}.
+   */
+  public void notifyLowMemoryWarning() {
+    if (flutterJNI.isAttached()) {
+      flutterJNI.notifyLowMemoryWarning();
+    }
+  }
+
   /**
    * Configuration options that specify which Dart entrypoint function is executed and where to find
    * that entrypoint and other assets required for Dart execution.

shell/platform/android/io/flutter/view/FlutterView.java

@@ -320,6 +320,7 @@ public void onStop() {
   }
 
   public void onMemoryPressure() {
+    mNativeView.getFlutterJNI().notifyLowMemoryWarning();
     systemChannel.sendMemoryPressureWarning();
   }
 

shell/platform/android/platform_view_android_jni_impl.cc

@@ -438,6 +438,12 @@ static void InvokePlatformMessageEmptyResponseCallback(JNIEnv* env,
       );
 }
 
+static void NotifyLowMemoryWarning(JNIEnv* env,
+                                   jobject obj,
+                                   jlong shell_holder) {
+  ANDROID_SHELL_HOLDER->NotifyLowMemoryWarning();
+}
+
 static jboolean FlutterTextUtilsIsEmoji(JNIEnv* env,
                                         jobject obj,
                                         jint codePoint) {
@@ -508,6 +514,11 @@ bool RegisterApi(JNIEnv* env) {
           .fnPtr = reinterpret_cast<void*>(
               &InvokePlatformMessageEmptyResponseCallback),
       },
+      {
+          .name = "nativeNotifyLowMemoryWarning",
+          .signature = "(J)V",
+          .fnPtr = reinterpret_cast<void*>(&NotifyLowMemoryWarning),
+      },
 
       // Start of methods from FlutterView
       {

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

@@ -1,6 +1,6 @@
 package io.flutter.embedding.android;
 
-import static android.content.ComponentCallbacks2.TRIM_MEMORY_RUNNING_LOW;
+import static android.content.ComponentCallbacks2.*;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNull;
 import static org.mockito.Matchers.any;
@@ -442,7 +442,7 @@ public void itForwardsOnUserLeaveHintToFlutterEngine() {
   }
 
   @Test
-  public void itSendsMessageOverSystemChannelWhenToldToTrimMemory() {
+  public void itNotifiesDartExecutorAndSendsMessageOverSystemChannelWhenToldToTrimMemory() {
     // Create the real object that we're testing.
     FlutterActivityAndFragmentDelegate delegate = new FlutterActivityAndFragmentDelegate(mockHost);
 
@@ -451,14 +451,21 @@ public void itSendsMessageOverSystemChannelWhenToldToTrimMemory() {
     delegate.onAttach(RuntimeEnvironment.application);
 
     // Emulate the host and call the method that we expect to be forwarded.
+    delegate.onTrimMemory(TRIM_MEMORY_RUNNING_MODERATE);
     delegate.onTrimMemory(TRIM_MEMORY_RUNNING_LOW);
+    delegate.onTrimMemory(TRIM_MEMORY_RUNNING_CRITICAL);
+    delegate.onTrimMemory(TRIM_MEMORY_BACKGROUND);
+    delegate.onTrimMemory(TRIM_MEMORY_COMPLETE);
+    delegate.onTrimMemory(TRIM_MEMORY_MODERATE);
+    delegate.onTrimMemory(TRIM_MEMORY_UI_HIDDEN);
 
     // Verify that the call was forwarded to the engine.
+    verify(mockFlutterEngine.getDartExecutor(), times(7)).notifyLowMemoryWarning();
     verify(mockFlutterEngine.getSystemChannel(), times(1)).sendMemoryPressureWarning();
   }
 
   @Test
-  public void itSendsMessageOverSystemChannelWhenInformedOfLowMemory() {
+  public void itNotifiesDartExecutorAndSendsMessageOverSystemChannelWhenInformedOfLowMemory() {
     // Create the real object that we're testing.
     FlutterActivityAndFragmentDelegate delegate = new FlutterActivityAndFragmentDelegate(mockHost);
 
@@ -470,6 +477,7 @@ public void itSendsMessageOverSystemChannelWhenInformedOfLowMemory() {
     delegate.onLowMemory();
 
     // Verify that the call was forwarded to the engine.
+    verify(mockFlutterEngine.getDartExecutor(), times(1)).notifyLowMemoryWarning();
     verify(mockFlutterEngine.getSystemChannel(), times(1)).sendMemoryPressureWarning();
   }
 

shell/platform/android/test/io/flutter/embedding/engine/dart/DartExecutorTest.java

@@ -6,6 +6,7 @@
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
 
 import android.content.res.AssetManager;
 import io.flutter.embedding.engine.FlutterJNI;
@@ -38,4 +39,14 @@ public void itSendsBinaryMessages() {
     verify(fakeFlutterJni, times(1))
         .dispatchPlatformMessage(eq("fake_channel"), eq(fakeMessage), anyInt(), anyInt());
   }
+
+  @Test
+  public void itNotifiesLowMemoryWarning() {
+    FlutterJNI mockFlutterJNI = mock(FlutterJNI.class);
+    when(mockFlutterJNI.isAttached()).thenReturn(true);
+
+    DartExecutor dartExecutor = new DartExecutor(mockFlutterJNI, mock(AssetManager.class));
+    dartExecutor.notifyLowMemoryWarning();
+    verify(mockFlutterJNI, times(1)).notifyLowMemoryWarning();
+  }
 }