[firebase_messaging] Add support for handling messages in background

packages/firebase_messaging/README.md

@@ -54,7 +54,79 @@ Note: When you are debugging on Android, use a device or AVD with Google Play se
       <category android:name="android.intent.category.DEFAULT" />
   </intent-filter>
   ```
-
+#### Optionally handle background messages
+
+By default background messaging is not enabled. To handle messages in the background:
+
+1. Add an Application.java class to your app
+
+    ```
+    package io.flutter.plugins.firebasemessagingexample;
+
+    import io.flutter.app.FlutterApplication;
+    import io.flutter.plugin.common.PluginRegistry;
+    import io.flutter.plugin.common.PluginRegistry.PluginRegistrantCallback;
+    import io.flutter.plugins.GeneratedPluginRegistrant;
+    import io.flutter.plugins.firebasemessaging.FlutterFirebaseMessagingService;
+
+    public class Application extends FlutterApplication implements PluginRegistrantCallback {
+      @Override
+      public void onCreate() {
+        super.onCreate();
+        FlutterFirebaseMessagingService.setPluginRegistrant(this);
+      }
+
+      @Override
+      public void registerWith(PluginRegistry registry) {
+        GeneratedPluginRegistrant.registerWith(registry);
+      }
+    }
+    ```
+1. Set name property of application in `AndroidManifest.xml`
+    ```
+    <application android:name=".Application" ...>
+    ```
+1. Define a top level Dart method to handle background messages
+    ```
+    Future<dynamic> myBackgroundMessageHandler(Map<String, dynamic> message) {
+      if (message.containsKey('data')) {
+        // Handle data message
+        dynamic data = message['data'];
+      }
+
+      if (message.containsKey('notification')) {
+        // Handle notification message
+        dynamic notification = message['notification'];
+      }
+
+      // Or do work with other plugins, eg: write to RTDB.
+      FirebaseDatabase.instance.reference().child('foo').set('bar');
+      return Future<void>.value();
+    }
+    ```
+   Note: the protocol of `data` and `notification` are in line with the
+   fields defined by a [RemoteMessage](https://firebase.google.com/docs/reference/android/com/google/firebase/messaging/RemoteMessage). 
+1. Set `onBackgroundMessage` handler when calling `configure`
+    ```
+    _firebaseMessaging.configure(
+          onMessage: (Map<String, dynamic> message) async {
+            print("onMessage: $message");
+            _showItemDialog(message);
+          },
+          onBackgroundMessage: myBackgroundMessageHandler,
+          onLaunch: (Map<String, dynamic> message) async {
+            print("onLaunch: $message");
+            _navigateToItemDetail(message);
+          },
+          onResume: (Map<String, dynamic> message) async {
+            print("onResume: $message");
+            _navigateToItemDetail(message);
+          },
+        );
+    ```
+   Note: `configure` should be called early in the lifecycle of your application
+   so that it can be ready to receive messages as early as possible. See the
+   example app for a demonstration.
 
 ### iOS Integration
 

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

@@ -41,9 +41,14 @@ public class FirebaseMessagingPlugin extends BroadcastReceiver
   public static void registerWith(Registrar registrar) {
     final MethodChannel channel =
         new MethodChannel(registrar.messenger(), "plugins.flutter.io/firebase_messaging");
+    final MethodChannel backgroundCallbackChannel =
+        new MethodChannel(registrar.messenger(), "plugins.flutter.io/android_fcm_background");
     final FirebaseMessagingPlugin plugin = new FirebaseMessagingPlugin(registrar, channel);
     registrar.addNewIntentListener(plugin);
     channel.setMethodCallHandler(plugin);
+    backgroundCallbackChannel.setMethodCallHandler(plugin);
+
+    FlutterFirebaseMessagingService.setBackgroundChannel(backgroundCallbackChannel);
   }
 
   private FirebaseMessagingPlugin(Registrar registrar, MethodChannel channel) {
@@ -99,7 +104,40 @@ private Map<String, Object> parseRemoteMessage(RemoteMessage message) {
 
   @Override
   public void onMethodCall(final MethodCall call, final Result result) {
-    if ("configure".equals(call.method)) {
+    /*  Even when the app is not active the `FirebaseMessagingService` extended by
+     *  `FlutterFirebaseMessagingService` allows incoming FCM messages to be handled.
+     *
+     *  `FcmDartService#start` and `FcmDartService#initialized` are the two methods used
+     *  to optionally setup handling messages received while the app is not active.
+     *
+     *  `FcmDartService#start` sets up the plumbing that allows messages received while
+     *  the app is not active to be handled by a background isolate.
+     *
+     *  `FcmDartService#initialized` is called by the Dart side when the plumbing for
+     *  background message handling is complete.
+     */
+    if ("FcmDartService#start".equals(call.method)) {
+      long setupCallbackHandle = 0;
+      long backgroundMessageHandle = 0;
+      try {
+        Map<String, Long> callbacks = ((Map<String, Long>) call.arguments);
+        setupCallbackHandle = callbacks.get("setupHandle");
+        backgroundMessageHandle = callbacks.get("backgroundHandle");
+      } catch (Exception e) {
+        Log.e(TAG, "There was an exception when getting callback handle from Dart side");
+        e.printStackTrace();
+      }
+      FlutterFirebaseMessagingService.setBackgroundSetupHandle(
+          this.registrar.context(), setupCallbackHandle);
+      FlutterFirebaseMessagingService.startBackgroundIsolate(
+          this.registrar.context(), setupCallbackHandle);
+      FlutterFirebaseMessagingService.setBackgroundMessageHandle(
+          this.registrar.context(), backgroundMessageHandle);
+      result.success(true);
+    } else if ("FcmDartService#initialized".equals(call.method)) {
+      FlutterFirebaseMessagingService.onInitialized();
+      result.success(true);
+    } else if ("configure".equals(call.method)) {
       FirebaseInstanceId.getInstance()
           .getInstanceId()
           .addOnCompleteListener(