Wire up channel for restoration data

ci/licenses_golden/licenses_flutter

@@ -736,6 +736,7 @@ FILE: ../../../flutter/shell/platform/android/io/flutter/embedding/engine/system
 FILE: ../../../flutter/shell/platform/android/io/flutter/embedding/engine/systemchannels/NavigationChannel.java
 FILE: ../../../flutter/shell/platform/android/io/flutter/embedding/engine/systemchannels/PlatformChannel.java
 FILE: ../../../flutter/shell/platform/android/io/flutter/embedding/engine/systemchannels/PlatformViewsChannel.java
+FILE: ../../../flutter/shell/platform/android/io/flutter/embedding/engine/systemchannels/RestorationChannel.java
 FILE: ../../../flutter/shell/platform/android/io/flutter/embedding/engine/systemchannels/SettingsChannel.java
 FILE: ../../../flutter/shell/platform/android/io/flutter/embedding/engine/systemchannels/SystemChannel.java
 FILE: ../../../flutter/shell/platform/android/io/flutter/embedding/engine/systemchannels/TextInputChannel.java

shell/platform/android/BUILD.gn

@@ -184,6 +184,7 @@ android_java_sources = [
   "io/flutter/embedding/engine/systemchannels/NavigationChannel.java",
   "io/flutter/embedding/engine/systemchannels/PlatformChannel.java",
   "io/flutter/embedding/engine/systemchannels/PlatformViewsChannel.java",
+  "io/flutter/embedding/engine/systemchannels/RestorationChannel.java",
   "io/flutter/embedding/engine/systemchannels/SettingsChannel.java",
   "io/flutter/embedding/engine/systemchannels/SystemChannel.java",
   "io/flutter/embedding/engine/systemchannels/TextInputChannel.java",
@@ -443,6 +444,7 @@ action("robolectric_tests") {
     "test/io/flutter/embedding/engine/dart/DartExecutorTest.java",
     "test/io/flutter/embedding/engine/plugins/shim/ShimPluginRegistryTest.java",
     "test/io/flutter/embedding/engine/renderer/FlutterRendererTest.java",
+    "test/io/flutter/embedding/engine/systemchannels/RestorationChannelTest.java",
     "test/io/flutter/external/FlutterLaunchTests.java",
     "test/io/flutter/plugin/common/StandardMessageCodecTest.java",
     "test/io/flutter/plugin/common/StandardMethodCodecTest.java",

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

@@ -11,6 +11,7 @@
 import static io.flutter.embedding.android.FlutterActivityLaunchConfigs.EXTRA_BACKGROUND_MODE;
 import static io.flutter.embedding.android.FlutterActivityLaunchConfigs.EXTRA_CACHED_ENGINE_ID;
 import static io.flutter.embedding.android.FlutterActivityLaunchConfigs.EXTRA_DESTROY_ENGINE_WITH_ACTIVITY;
+import static io.flutter.embedding.android.FlutterActivityLaunchConfigs.EXTRA_ENABLE_STATE_RESTORATION;
 import static io.flutter.embedding.android.FlutterActivityLaunchConfigs.EXTRA_INITIAL_ROUTE;
 import static io.flutter.embedding.android.FlutterActivityLaunchConfigs.INITIAL_ROUTE_META_DATA_KEY;
 import static io.flutter.embedding.android.FlutterActivityLaunchConfigs.NORMAL_THEME_META_DATA_KEY;
@@ -63,6 +64,7 @@
  *   <li>Chooses Flutter's initial route.
  *   <li>Renders {@code Activity} transparently, if desired.
  *   <li>Offers hooks for subclasses to provide and configure a {@link FlutterEngine}.
+ *   <li>Save and restore instance state, see {@code #shouldRestoreAndSaveState()};
  * </ul>
  *
  * <p><strong>Dart entrypoint, initial route, and app bundle path</strong>
@@ -949,6 +951,18 @@ public void onFlutterUiNoLongerDisplayed() {
     // no-op
   }
 
+  @Override
+  public boolean shouldRestoreAndSaveState() {
+    if (getIntent().hasExtra(EXTRA_ENABLE_STATE_RESTORATION)) {
+      return getIntent().getBooleanExtra(EXTRA_ENABLE_STATE_RESTORATION, false);
+    }
+    if (getCachedEngineId() != null) {
+      // Prevent overwriting the existing state in a cached engine with restoration state.
+      return false;
+    }
+    return true;
+  }
+
   /**
    * Registers all plugins that an app lists in its pubspec.yaml.
    *

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

@@ -63,6 +63,8 @@
  */
 /* package */ final class FlutterActivityAndFragmentDelegate {
   private static final String TAG = "FlutterActivityAndFragmentDelegate";
+  private static final String FRAMEWORK_RESTORATION_BUNDLE_KEY = "framework";
+  private static final String PLUGINS_RESTORATION_BUNDLE_KEY = "plugins";
 
   // The FlutterActivity or FlutterFragment that is delegating most of its calls
   // to this FlutterActivityAndFragmentDelegate.
@@ -227,7 +229,8 @@ void onAttach(@NonNull Context context) {
         new FlutterEngine(
             host.getContext(),
             host.getFlutterShellArgs().toArray(),
-            /*automaticallyRegisterPlugins=*/ false);
+            /*automaticallyRegisterPlugins=*/ false,
+            /*willProvideRestorationData=*/ host.shouldRestoreAndSaveState());
     isFlutterEngineFromHost = false;
   }
 
@@ -293,11 +296,22 @@ View onCreateView(
   }
 
   void onActivityCreated(@Nullable Bundle bundle) {
-    Log.v(TAG, "onActivityCreated. Giving plugins an opportunity to restore state.");
+    Log.v(TAG, "onActivityCreated. Giving framework and plugins an opportunity to restore state.");
     ensureAlive();
 
+    Bundle pluginState = null;
+    byte[] frameworkState = null;
+    if (bundle != null) {
+      pluginState = bundle.getBundle(PLUGINS_RESTORATION_BUNDLE_KEY);
+      frameworkState = bundle.getByteArray(FRAMEWORK_RESTORATION_BUNDLE_KEY);
+    }
+
+    if (host.shouldRestoreAndSaveState()) {
+      flutterEngine.getRestorationChannel().setRestorationData(frameworkState);
+    }
+
     if (host.shouldAttachEngineToActivity()) {
-      flutterEngine.getActivityControlSurface().onRestoreInstanceState(bundle);
+      flutterEngine.getActivityControlSurface().onRestoreInstanceState(pluginState);
     }
   }
 
@@ -444,11 +458,19 @@ void onDestroyView() {
   }
 
   void onSaveInstanceState(@Nullable Bundle bundle) {
-    Log.v(TAG, "onSaveInstanceState. Giving plugins an opportunity to save state.");
+    Log.v(TAG, "onSaveInstanceState. Giving framework and plugins an opportunity to save state.");
     ensureAlive();
 
+    if (host.shouldRestoreAndSaveState()) {
+      bundle.putByteArray(
+          FRAMEWORK_RESTORATION_BUNDLE_KEY,
+          flutterEngine.getRestorationChannel().getRestorationData());
+    }
+
     if (host.shouldAttachEngineToActivity()) {
-      flutterEngine.getActivityControlSurface().onSaveInstanceState(bundle);
+      final Bundle plugins = new Bundle();
+      flutterEngine.getActivityControlSurface().onSaveInstanceState(plugins);
+      bundle.putBundle(PLUGINS_RESTORATION_BUNDLE_KEY, plugins);
     }
   }
 
@@ -804,5 +826,17 @@ PlatformPlugin providePlatformPlugin(
 
     /** Invoked by this delegate when its {@link FlutterView} stops painting pixels. */
     void onFlutterUiNoLongerDisplayed();
+
+    /**
+     * Whether state restoration is enabled.
+     *
+     * <p>When this returns true, the instance state provided to {@code onActivityCreated(Bundle)}
+     * will be forwarded to the framework via the {@code RestorationChannel} and during {@code
+     * onSaveInstanceState(Bundle)} the current framework instance state obtained from {@code
+     * RestorationChannel} will be stored in the provided bundle.
+     *
+     * <p>This defaults to true, unless a cached engine is used.
+     */
+    boolean shouldRestoreAndSaveState();
   }
 }

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

@@ -23,6 +23,7 @@ public class FlutterActivityLaunchConfigs {
   /* package */ static final String EXTRA_CACHED_ENGINE_ID = "cached_engine_id";
   /* package */ static final String EXTRA_DESTROY_ENGINE_WITH_ACTIVITY =
       "destroy_engine_with_activity";
+  /* package */ static final String EXTRA_ENABLE_STATE_RESTORATION = "enable_state_restoration";
 
   // Default configuration.
   /* package */ static final String DEFAULT_DART_ENTRYPOINT = "main";

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

@@ -113,6 +113,11 @@ public class FlutterFragment extends Fragment implements FlutterActivityAndFragm
    * outlive the {@code FlutterFragment}.
    */
   protected static final String ARG_DESTROY_ENGINE_WITH_FRAGMENT = "destroy_engine_with_fragment";
+  /**
+   * True if the framework state in the engine attached to this engine should be stored and restored
+   * when this fragment is created and destroyed.
+   */
+  protected static final String ARG_ENABLE_STATE_RESTORATION = "enable_state_restoration";
 
   /**
    * Creates a {@code FlutterFragment} with a default configuration.
@@ -1018,6 +1023,17 @@ public void onFlutterUiNoLongerDisplayed() {
     }
   }
 
+  @Override
+  public boolean shouldRestoreAndSaveState() {
+    if (getArguments().containsKey(ARG_ENABLE_STATE_RESTORATION)) {
+      return getArguments().getBoolean(ARG_ENABLE_STATE_RESTORATION);
+    }
+    if (getCachedEngineId() != null) {
+      return false;
+    }
+    return true;
+  }
+
   /**
    * Annotates methods in {@code FlutterFragment} that must be called by the containing {@code
    * Activity}.

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

@@ -24,6 +24,7 @@
 import io.flutter.embedding.engine.systemchannels.MouseCursorChannel;
 import io.flutter.embedding.engine.systemchannels.NavigationChannel;
 import io.flutter.embedding.engine.systemchannels.PlatformChannel;
+import io.flutter.embedding.engine.systemchannels.RestorationChannel;
 import io.flutter.embedding.engine.systemchannels.SettingsChannel;
 import io.flutter.embedding.engine.systemchannels.SystemChannel;
 import io.flutter.embedding.engine.systemchannels.TextInputChannel;
@@ -80,6 +81,7 @@ public class FlutterEngine {
   @NonNull private final LocalizationChannel localizationChannel;
   @NonNull private final MouseCursorChannel mouseCursorChannel;
   @NonNull private final NavigationChannel navigationChannel;
+  @NonNull private final RestorationChannel restorationChannel;
   @NonNull private final PlatformChannel platformChannel;
   @NonNull private final SettingsChannel settingsChannel;
   @NonNull private final SystemChannel systemChannel;
@@ -102,6 +104,7 @@ public void onPreEngineRestart() {
           }
 
           platformViewsController.onPreEngineRestart();
+          restorationChannel.clearData();
         }
       };
 
@@ -159,6 +162,39 @@ public FlutterEngine(
         automaticallyRegisterPlugins);
   }
 
+  /**
+   * Same as {@link #FlutterEngine(Context, String[], boolean)} with added support for configuring
+   * whether the engine will receive restoration data.
+   *
+   * <p>The {@code waitForRestorationData} flag controls whether the engine delays responding to
+   * requests from the framework for restoration data until that data has been provided to the
+   * engine via {@code RestorationChannel.setRestorationData(byte[] data)}. If the flag is false,
+   * the framework may temporarily initialize itself to default values before the restoration data
+   * has been made available to the engine. Setting {@code waitForRestorationData} to true avoids
+   * this extra work by delaying initialization until the data is available.
+   *
+   * <p>When {@code waitForRestorationData} is set, {@code
+   * RestorationChannel.setRestorationData(byte[] data)} must be called at a later point in time. If
+   * it later turns out that no restoration data is available to restore the framework from, that
+   * method must still be called with null as an argument to indicate "no data".
+   *
+   * <p>If the framework never requests the restoration data, this flag has no effect.
+   */
+  public FlutterEngine(
+      @NonNull Context context,
+      @Nullable String[] dartVmArgs,
+      boolean automaticallyRegisterPlugins,
+      boolean waitForRestorationData) {
+    this(
+        context,
+        FlutterLoader.getInstance(),
+        new FlutterJNI(),
+        new PlatformViewsController(),
+        dartVmArgs,
+        automaticallyRegisterPlugins,
+        waitForRestorationData);
+  }
+
   /**
    * Same as {@link #FlutterEngine(Context, FlutterLoader, FlutterJNI, String[])} but with no Dart
    * VM flags.
@@ -194,14 +230,36 @@ public FlutterEngine(
         automaticallyRegisterPlugins);
   }
 
-  /** Fully configurable {@code FlutterEngine} constructor. */
+  /**
+   * Same as {@link #FlutterEngine(Context, FlutterLoader, FlutterJNI, String[], boolean)}, plus the
+   * ability to provide a custom {@code PlatformViewsController}.
+   */
   public FlutterEngine(
       @NonNull Context context,
       @NonNull FlutterLoader flutterLoader,
       @NonNull FlutterJNI flutterJNI,
       @NonNull PlatformViewsController platformViewsController,
       @Nullable String[] dartVmArgs,
       boolean automaticallyRegisterPlugins) {
+    this(
+        context,
+        flutterLoader,
+        flutterJNI,
+        platformViewsController,
+        dartVmArgs,
+        automaticallyRegisterPlugins,
+        false);
+  }
+
+  /** Fully configurable {@code FlutterEngine} constructor. */
+  public FlutterEngine(
+      @NonNull Context context,
+      @NonNull FlutterLoader flutterLoader,
+      @NonNull FlutterJNI flutterJNI,
+      @NonNull PlatformViewsController platformViewsController,
+      @Nullable String[] dartVmArgs,
+      boolean automaticallyRegisterPlugins,
+      boolean waitForRestorationData) {
     this.flutterJNI = flutterJNI;
     flutterLoader.startInitialization(context.getApplicationContext());
     flutterLoader.ensureInitializationComplete(context, dartVmArgs);
@@ -224,6 +282,7 @@ public FlutterEngine(
     mouseCursorChannel = new MouseCursorChannel(dartExecutor);
     navigationChannel = new NavigationChannel(dartExecutor);
     platformChannel = new PlatformChannel(dartExecutor);
+    restorationChannel = new RestorationChannel(dartExecutor, waitForRestorationData);
     settingsChannel = new SettingsChannel(dartExecutor);
     systemChannel = new SystemChannel(dartExecutor);
     textInputChannel = new TextInputChannel(dartExecutor);
@@ -380,6 +439,18 @@ public PlatformChannel getPlatformChannel() {
     return platformChannel;
   }
 
+  /**
+   * System channel to exchange restoration data between framework and engine.
+   *
+   * <p>The engine can obtain the current restoration data from the framework via this channel to
+   * store it on disk and - when the app is relaunched - provide the stored data back to the
+   * framework to recreate the original state of the app.
+   */
+  @NonNull
+  public RestorationChannel getRestorationChannel() {
+    return restorationChannel;
+  }
+
   /**
    * System channel that sends platform/user settings from Android to Flutter, e.g., time format,
    * scale factor, etc.