[firebase_messaging] better handling on background isolate initialization

packages/firebase_messaging/android/src/main/java/io/flutter/plugins/firebasemessaging/FirebaseMessagingPlugin.java

@@ -56,12 +56,7 @@ private void onAttachedToEngine(Context context, BinaryMessenger binaryMessenger
     this.applicationContext = context;
     FirebaseApp.initializeApp(applicationContext);
     channel = new MethodChannel(binaryMessenger, "plugins.flutter.io/firebase_messaging");
-    final MethodChannel backgroundCallbackChannel =
-        new MethodChannel(binaryMessenger, "plugins.flutter.io/firebase_messaging_background");
-
     channel.setMethodCallHandler(this);
-    backgroundCallbackChannel.setMethodCallHandler(this);
-    FlutterFirebaseMessagingService.setBackgroundChannel(backgroundCallbackChannel);
 
     // Register broadcast receiver
     IntentFilter intentFilter = new IntentFilter();
@@ -174,12 +169,10 @@ public void onMethodCall(final MethodCall call, final Result result) {
         e.printStackTrace();
       }
       FlutterFirebaseMessagingService.setBackgroundSetupHandle(mainActivity, setupCallbackHandle);
-      FlutterFirebaseMessagingService.startBackgroundIsolate(mainActivity, setupCallbackHandle);
       FlutterFirebaseMessagingService.setBackgroundMessageHandle(
           mainActivity, backgroundMessageHandle);
-      result.success(true);
-    } else if ("FcmDartService#initialized".equals(call.method)) {
-      FlutterFirebaseMessagingService.onInitialized();
+      FlutterFirebaseMessagingService.startBackgroundIsolate(
+          new BackgroundIsolateMethodCallHandler(), mainActivity, setupCallbackHandle);
       result.success(true);
     } else if ("configure".equals(call.method)) {
       FirebaseInstanceId.getInstance()

packages/firebase_messaging/android/src/main/java/io/flutter/plugins/firebasemessaging/FlutterFirebaseMessagingService.java

@@ -15,6 +15,7 @@
 import androidx.localbroadcastmanager.content.LocalBroadcastManager;
 import com.google.firebase.messaging.FirebaseMessagingService;
 import com.google.firebase.messaging.RemoteMessage;
+import io.flutter.plugin.common.MethodCall;
 import io.flutter.plugin.common.MethodChannel;
 import io.flutter.plugin.common.PluginRegistry;
 import io.flutter.view.FlutterCallbackInformation;
@@ -63,6 +64,13 @@ public class FlutterFirebaseMessagingService extends FirebaseMessagingService {
 
   private static Context backgroundContext;
 
+  /**
+   * flag, so we don't create multiple background views and communication channels as it seems that
+   * multiple {@link MethodChannel}s won't return an answer that is waited by @{link {@link
+   * LatchResult}} to release the thread.
+   */
+  private static AtomicBoolean isBackgroundInitialized = new AtomicBoolean(false);
+
   @Override
   public void onCreate() {
     super.onCreate();
@@ -74,7 +82,8 @@ public void onCreate() {
     if (!isIsolateRunning.get()) {
       SharedPreferences p = backgroundContext.getSharedPreferences(SHARED_PREFERENCES_KEY, 0);
       long callbackHandle = p.getLong(BACKGROUND_SETUP_CALLBACK_HANDLE_KEY, 0);
-      startBackgroundIsolate(backgroundContext, callbackHandle);
+      startBackgroundIsolate(
+          new BackgroundIsolateMethodCallHandler(), backgroundContext, callbackHandle);
     }
   }
 
@@ -134,34 +143,55 @@ public void onNewToken(String token) {
    * side. Called either by the plugin when the app is starting up or when the app receives a
    * message while it is inactive.
    *
+   * @param methodCallHandler a method call handler impl that handles the call from dart side.
+   *     {@link BackgroundIsolateMethodCallHandler}.
    * @param context Registrar or FirebaseMessagingService context.
    * @param callbackHandle Handle used to retrieve the Dart function that sets up background
    *     handling on the dart side.
    */
-  public static void startBackgroundIsolate(Context context, long callbackHandle) {
+  public static void startBackgroundIsolate(
+      final BackgroundIsolateMethodCallHandler methodCallHandler,
+      Context context,
+      long callbackHandle) {
     FlutterMain.ensureInitializationComplete(context, null);
-    String appBundlePath = FlutterMain.findAppBundlePath();
-    FlutterCallbackInformation flutterCallback =
-        FlutterCallbackInformation.lookupCallbackInformation(callbackHandle);
-    if (flutterCallback == null) {
-      Log.e(TAG, "Fatal: failed to find callback");
-      return;
-    }
 
-    // Note that we're passing `true` as the second argument to our
-    // FlutterNativeView constructor. This specifies the FlutterNativeView
-    // as a background view and does not create a drawing surface.
-    backgroundFlutterView = new FlutterNativeView(context, true);
-    if (appBundlePath != null && !isIsolateRunning.get()) {
-      if (pluginRegistrantCallback == null) {
-        throw new RuntimeException("PluginRegistrantCallback is not set.");
+    if (!isBackgroundInitialized.get()) {
+      isBackgroundInitialized.set(true);
+
+      String appBundlePath = FlutterMain.findAppBundlePath();
+      FlutterCallbackInformation flutterCallback =
+          FlutterCallbackInformation.lookupCallbackInformation(callbackHandle);
+      if (flutterCallback == null) {
+        Log.e(TAG, "Fatal: failed to find callback");
+
+        isBackgroundInitialized.set(false);
+        return;
+      }
+
+      // Note that we're passing `true` as the second argument to our
+      // FlutterNativeView constructor. This specifies the FlutterNativeView
+      // as a background view and does not create a drawing surface.
+      backgroundFlutterView = new FlutterNativeView(context, true);
+
+      backgroundChannel =
+          new MethodChannel(
+              backgroundFlutterView.getDartExecutor().getBinaryMessenger(),
+              "plugins.flutter.io/firebase_messaging_background");
+      backgroundChannel.setMethodCallHandler(methodCallHandler);
+
+      if (appBundlePath != null && !isIsolateRunning.get()) {
+        if (pluginRegistrantCallback == null) {
+          isBackgroundInitialized.set(false);
+
+          throw new RuntimeException("PluginRegistrantCallback is not set.");
+        }
+        FlutterRunArguments args = new FlutterRunArguments();
+        args.bundlePath = appBundlePath;
+        args.entrypoint = flutterCallback.callbackName;
+        args.libraryPath = flutterCallback.callbackLibraryPath;
+        backgroundFlutterView.runFromBundle(args);
+        pluginRegistrantCallback.registerWith(backgroundFlutterView.getPluginRegistry());
       }
-      FlutterRunArguments args = new FlutterRunArguments();
-      args.bundlePath = appBundlePath;
-      args.entrypoint = flutterCallback.callbackName;
-      args.libraryPath = flutterCallback.callbackLibraryPath;
-      backgroundFlutterView.runFromBundle(args);
-      pluginRegistrantCallback.registerWith(backgroundFlutterView.getPluginRegistry());
     }
   }
 
@@ -188,9 +218,9 @@ public static void onInitialized() {
    *
    * @param channel Background method channel.
    */
-  public static void setBackgroundChannel(MethodChannel channel) {
-    backgroundChannel = channel;
-  }
+  //  public static void setBackgroundChannel(MethodChannel channel) {
+  //    backgroundChannel = channel;
+  //  }
 
   /**
    * Set the background message handle for future use. When background messages need to be handled
@@ -330,3 +360,14 @@ private static boolean isApplicationForeground(Context context) {
     return false;
   }
 }
+
+class BackgroundIsolateMethodCallHandler implements MethodChannel.MethodCallHandler {
+
+  @Override
+  public void onMethodCall(MethodCall call, MethodChannel.Result result) {
+    if ("FcmDartService#initialized".equals(call.method)) {
+      FlutterFirebaseMessagingService.onInitialized();
+      result.success(true);
+    }
+  }
+}