Let FlutterFragment not pop the whole activity by default when more fragments are in the activity (flutter#22692)

Author: xster

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

@@ -902,11 +902,7 @@ protected Bundle getMetaData() throws PackageManager.NameNotFoundException {
   @Override
   public PlatformPlugin providePlatformPlugin(
       @Nullable Activity activity, @NonNull FlutterEngine flutterEngine) {
-    if (activity != null) {
-      return new PlatformPlugin(getActivity(), flutterEngine.getPlatformChannel());
-    } else {
-      return null;
-    }
+    return new PlatformPlugin(getActivity(), flutterEngine.getPlatformChannel(), this);
   }
 
   /**
@@ -1032,6 +1028,12 @@ public boolean shouldRestoreAndSaveState() {
     return true;
   }
 
+  @Override
+  public boolean popSystemNavigator() {
+    // Hook for subclass. No-op if returns false.
+    return false;
+  }
+
   private boolean stillAttachedForEvent(String event) {
     if (delegate == null) {
       Log.v(TAG, "FlutterActivity " + hashCode() + " " + event + " called after release.");

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

@@ -22,7 +22,6 @@
 import androidx.lifecycle.Lifecycle;
 import io.flutter.FlutterInjector;
 import io.flutter.Log;
-import io.flutter.app.FlutterActivity;
 import io.flutter.embedding.engine.FlutterEngine;
 import io.flutter.embedding.engine.FlutterEngineCache;
 import io.flutter.embedding.engine.FlutterShellArgs;
@@ -752,7 +751,10 @@ private void ensureAlive() {
    * FlutterActivityAndFragmentDelegate}.
    */
   /* package */ interface Host
-      extends SplashScreenProvider, FlutterEngineProvider, FlutterEngineConfigurator {
+      extends SplashScreenProvider,
+          FlutterEngineProvider,
+          FlutterEngineConfigurator,
+          PlatformPlugin.PlatformPluginDelegate {
     /** Returns the {@link Context} that backs the host {@link Activity} or {@code Fragment}. */
     @NonNull
     Context getContext();

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

@@ -432,7 +432,7 @@ private CachedEngineFragmentBuilder(@NonNull String engineId) {
       this(FlutterFragment.class, engineId);
     }
 
-    protected CachedEngineFragmentBuilder(
+    public CachedEngineFragmentBuilder(
         @NonNull Class<? extends FlutterFragment> subclass, @NonNull String engineId) {
       this.fragmentClass = subclass;
       this.engineId = engineId;
@@ -984,7 +984,7 @@ public FlutterEngine getFlutterEngine() {
   public PlatformPlugin providePlatformPlugin(
       @Nullable Activity activity, @NonNull FlutterEngine flutterEngine) {
     if (activity != null) {
-      return new PlatformPlugin(getActivity(), flutterEngine.getPlatformChannel());
+      return new PlatformPlugin(getActivity(), flutterEngine.getPlatformChannel(), this);
     } else {
       return null;
     }
@@ -1110,6 +1110,12 @@ public boolean shouldRestoreAndSaveState() {
     return true;
   }
 
+  @Override
+  public boolean popSystemNavigator() {
+    // Hook for subclass. No-op if returns false.
+    return false;
+  }
+
   private boolean stillAttachedForEvent(String event) {
     if (delegate == null) {
       Log.v(TAG, "FlutterFragment " + hashCode() + " " + event + " called after release.");

shell/platform/android/io/flutter/plugin/platform/PlatformPlugin.java

@@ -15,6 +15,7 @@
 import android.view.View;
 import android.view.Window;
 import android.view.WindowManager;
+import androidx.activity.OnBackPressedDispatcherOwner;
 import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
 import androidx.annotation.VisibleForTesting;
@@ -30,10 +31,30 @@ public class PlatformPlugin {
 
   private final Activity activity;
   private final PlatformChannel platformChannel;
+  private final PlatformPluginDelegate platformPluginDelegate;
   private PlatformChannel.SystemChromeStyle currentTheme;
   private int mEnabledOverlays;
   private static final String TAG = "PlatformPlugin";
 
+  /**
+   * The {@link PlatformPlugin} generally has default behaviors implemented for platform
+   * functionalities requested by the Flutter framework. However, functionalities exposed through
+   * this interface could be customized by the more public-facing APIs that implement this interface
+   * such as the {@link io.flutter.embedding.android.FlutterActivity} or the {@link
+   * io.flutter.embedding.android.FlutterFragment}.
+   */
+  public interface PlatformPluginDelegate {
+    /**
+     * Allow implementer to customize the behavior needed when the Flutter framework calls to pop
+     * the Android-side navigation stack.
+     *
+     * @return true if the implementation consumed the pop signal. If false, a default behavior of
+     *     finishing the activity or sending the signal to {@link
+     *     androidx.activity.OnBackPressedDispatcher} will be executed.
+     */
+    boolean popSystemNavigator();
+  }
+
   @VisibleForTesting
   final PlatformChannel.PlatformMessageHandler mPlatformMessageHandler =
       new PlatformChannel.PlatformMessageHandler() {
@@ -101,9 +122,15 @@ public boolean clipboardHasStrings() {
       };
 
   public PlatformPlugin(Activity activity, PlatformChannel platformChannel) {
+    this(activity, platformChannel, null);
+  }
+
+  public PlatformPlugin(
+      Activity activity, PlatformChannel platformChannel, PlatformPluginDelegate delegate) {
     this.activity = activity;
     this.platformChannel = platformChannel;
     this.platformChannel.setPlatformMessageHandler(mPlatformMessageHandler);
+    this.platformPluginDelegate = delegate;
 
     mEnabledOverlays = DEFAULT_SYSTEM_UI;
   }
@@ -161,13 +188,14 @@ private void setSystemChromeApplicationSwitcherDescription(
       return;
     }
 
-    // Linter refuses to believe we're only executing this code in API 28 unless we use distinct if
+    // Linter refuses to believe we're only executing this code in API 28 unless we
+    // use distinct if
     // blocks and
     // hardcode the API 28 constant.
     if (Build.VERSION.SDK_INT < Build.VERSION_CODES.P
         && Build.VERSION.SDK_INT > Build.VERSION_CODES.LOLLIPOP) {
       activity.setTaskDescription(
-          new TaskDescription(description.label, /*icon=*/ null, description.color));
+          new TaskDescription(description.label, /* icon= */ null, description.color));
     }
     if (Build.VERSION.SDK_INT >= 28) {
       TaskDescription taskDescription =
@@ -178,14 +206,16 @@ private void setSystemChromeApplicationSwitcherDescription(
 
   private void setSystemChromeEnabledSystemUIOverlays(
       List<PlatformChannel.SystemUiOverlay> overlaysToShow) {
-    // Start by assuming we want to hide all system overlays (like an immersive game).
+    // Start by assuming we want to hide all system overlays (like an immersive
+    // game).
     int enabledOverlays =
         DEFAULT_SYSTEM_UI
             | View.SYSTEM_UI_FLAG_FULLSCREEN
             | View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
             | View.SYSTEM_UI_FLAG_HIDE_NAVIGATION;
 
-    // The SYSTEM_UI_FLAG_IMMERSIVE_STICKY flag was introduced in API 19, so we apply it
+    // The SYSTEM_UI_FLAG_IMMERSIVE_STICKY flag was introduced in API 19, so we
+    // apply it
     // if desired, and if the current Android version is 19 or greater.
     if (overlaysToShow.size() == 0 && Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
       enabledOverlays |= View.SYSTEM_UI_FLAG_IMMERSIVE_STICKY;
@@ -233,7 +263,8 @@ private void setSystemChromeSystemUIOverlayStyle(
     View view = window.getDecorView();
     int flags = view.getSystemUiVisibility();
     // You can change the navigation bar color (including translucent colors)
-    // in Android, but you can't change the color of the navigation buttons until Android O.
+    // in Android, but you can't change the color of the navigation buttons until
+    // Android O.
     // LIGHT vs DARK effectively isn't supported until then.
     // Build.VERSION_CODES.O
     if (Build.VERSION.SDK_INT >= 26) {
@@ -279,7 +310,16 @@ private void setSystemChromeSystemUIOverlayStyle(
   }
 
   private void popSystemNavigator() {
-    activity.finish();
+    if (platformPluginDelegate.popSystemNavigator()) {
+      // A custom behavior was executed by the delegate. Don't execute default behavior.
+      return;
+    }
+
+    if (activity instanceof OnBackPressedDispatcherOwner) {
+      ((OnBackPressedDispatcherOwner) activity).getOnBackPressedDispatcher().onBackPressed();
+    } else {
+      activity.finish();
+    }
   }
 
   private CharSequence getClipboardData(PlatformChannel.ClipboardContentFormat format) {

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

@@ -382,5 +382,10 @@ public void onFlutterUiNoLongerDisplayed() {}
 
     @Override
     public void detachFromFlutterEngine() {}
+
+    @Override
+    public boolean popSystemNavigator() {
+      return false;
+    }
   }
 }

shell/platform/android/test/io/flutter/plugin/platform/PlatformPluginTest.java

@@ -6,6 +6,9 @@
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
 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.mockito.Mockito.when;
 
 import android.app.Activity;
@@ -18,9 +21,12 @@
 import android.os.Build;
 import android.view.View;
 import android.view.Window;
+import androidx.activity.OnBackPressedDispatcher;
+import androidx.fragment.app.FragmentActivity;
 import io.flutter.embedding.engine.systemchannels.PlatformChannel;
 import io.flutter.embedding.engine.systemchannels.PlatformChannel.ClipboardContentFormat;
 import io.flutter.embedding.engine.systemchannels.PlatformChannel.SystemChromeStyle;
+import io.flutter.plugin.platform.PlatformPlugin.PlatformPluginDelegate;
 import java.io.ByteArrayInputStream;
 import java.io.IOException;
 import java.io.InputStream;
@@ -133,4 +139,87 @@ public void setNavigationBarDividerColor() {
       assertEquals(0XFF000000, fakeActivity.getWindow().getNavigationBarColor());
     }
   }
+
+  @Test
+  public void popSystemNavigatorFlutterActivity() {
+    Activity mockActivity = mock(Activity.class);
+    PlatformChannel mockPlatformChannel = mock(PlatformChannel.class);
+    PlatformPluginDelegate mockPlatformPluginDelegate = mock(PlatformPluginDelegate.class);
+    when(mockPlatformPluginDelegate.popSystemNavigator()).thenReturn(false);
+    PlatformPlugin platformPlugin =
+        new PlatformPlugin(mockActivity, mockPlatformChannel, mockPlatformPluginDelegate);
+
+    platformPlugin.mPlatformMessageHandler.popSystemNavigator();
+
+    verify(mockPlatformPluginDelegate, times(1)).popSystemNavigator();
+    verify(mockActivity, times(1)).finish();
+  }
+
+  @Test
+  public void doesNotDoAnythingByDefaultIfPopSystemNavigatorOverridden() {
+    Activity mockActivity = mock(Activity.class);
+    PlatformChannel mockPlatformChannel = mock(PlatformChannel.class);
+    PlatformPluginDelegate mockPlatformPluginDelegate = mock(PlatformPluginDelegate.class);
+    when(mockPlatformPluginDelegate.popSystemNavigator()).thenReturn(true);
+    PlatformPlugin platformPlugin =
+        new PlatformPlugin(mockActivity, mockPlatformChannel, mockPlatformPluginDelegate);
+
+    platformPlugin.mPlatformMessageHandler.popSystemNavigator();
+
+    verify(mockPlatformPluginDelegate, times(1)).popSystemNavigator();
+    // No longer perform the default action when overridden.
+    verify(mockActivity, never()).finish();
+  }
+
+  @Test
+  public void popSystemNavigatorFlutterFragment() {
+    FragmentActivity mockFragmentActivity = mock(FragmentActivity.class);
+    OnBackPressedDispatcher onBackPressedDispatcher = mock(OnBackPressedDispatcher.class);
+    when(mockFragmentActivity.getOnBackPressedDispatcher()).thenReturn(onBackPressedDispatcher);
+    PlatformChannel mockPlatformChannel = mock(PlatformChannel.class);
+    PlatformPluginDelegate mockPlatformPluginDelegate = mock(PlatformPluginDelegate.class);
+    when(mockPlatformPluginDelegate.popSystemNavigator()).thenReturn(false);
+    PlatformPlugin platformPlugin =
+        new PlatformPlugin(mockFragmentActivity, mockPlatformChannel, mockPlatformPluginDelegate);
+
+    platformPlugin.mPlatformMessageHandler.popSystemNavigator();
+
+    verify(mockFragmentActivity, never()).finish();
+    verify(mockPlatformPluginDelegate, times(1)).popSystemNavigator();
+    verify(mockFragmentActivity, times(1)).getOnBackPressedDispatcher();
+    verify(onBackPressedDispatcher, times(1)).onBackPressed();
+  }
+
+  @Test
+  public void doesNotDoAnythingByDefaultIfFragmentPopSystemNavigatorOverridden() {
+    FragmentActivity mockFragmentActivity = mock(FragmentActivity.class);
+    OnBackPressedDispatcher onBackPressedDispatcher = mock(OnBackPressedDispatcher.class);
+    when(mockFragmentActivity.getOnBackPressedDispatcher()).thenReturn(onBackPressedDispatcher);
+    PlatformChannel mockPlatformChannel = mock(PlatformChannel.class);
+    PlatformPluginDelegate mockPlatformPluginDelegate = mock(PlatformPluginDelegate.class);
+    when(mockPlatformPluginDelegate.popSystemNavigator()).thenReturn(true);
+    PlatformPlugin platformPlugin =
+        new PlatformPlugin(mockFragmentActivity, mockPlatformChannel, mockPlatformPluginDelegate);
+
+    platformPlugin.mPlatformMessageHandler.popSystemNavigator();
+
+    verify(mockPlatformPluginDelegate, times(1)).popSystemNavigator();
+    // No longer perform the default action when overridden.
+    verify(mockFragmentActivity, never()).finish();
+    verify(mockFragmentActivity, never()).getOnBackPressedDispatcher();
+  }
+
+  @Test
+  public void setRequestedOrientationFlutterFragment() {
+    FragmentActivity mockFragmentActivity = mock(FragmentActivity.class);
+    PlatformChannel mockPlatformChannel = mock(PlatformChannel.class);
+    PlatformPluginDelegate mockPlatformPluginDelegate = mock(PlatformPluginDelegate.class);
+    when(mockPlatformPluginDelegate.popSystemNavigator()).thenReturn(false);
+    PlatformPlugin platformPlugin =
+        new PlatformPlugin(mockFragmentActivity, mockPlatformChannel, mockPlatformPluginDelegate);
+
+    platformPlugin.mPlatformMessageHandler.setPreferredOrientations(0);
+
+    verify(mockFragmentActivity, times(1)).setRequestedOrientation(0);
+  }
 }