[firebase_messaging] Add support for handling messages in background

packages/firebase_messaging/README.md

@@ -54,7 +54,74 @@ 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 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();
+}
+```
+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);
+      },
+    );
+```
 
 ### iOS Integration
 

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

@@ -19,14 +19,17 @@
 import com.google.firebase.iid.InstanceIdResult;
 import com.google.firebase.messaging.FirebaseMessaging;
 import com.google.firebase.messaging.RemoteMessage;
+import io.flutter.plugin.common.JSONMethodCodec;
 import io.flutter.plugin.common.MethodCall;
 import io.flutter.plugin.common.MethodChannel;
 import io.flutter.plugin.common.MethodChannel.MethodCallHandler;
 import io.flutter.plugin.common.MethodChannel.Result;
 import io.flutter.plugin.common.PluginRegistry.NewIntentListener;
 import io.flutter.plugin.common.PluginRegistry.Registrar;
 import java.io.IOException;
+import java.util.ArrayList;
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
 
 /** FirebaseMessagingPlugin */
@@ -41,9 +44,17 @@ 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",
+            JSONMethodCodec.INSTANCE);
     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 +110,28 @@ private Map<String, Object> parseRemoteMessage(RemoteMessage message) {
 
   @Override
   public void onMethodCall(final MethodCall call, final Result result) {
-    if ("configure".equals(call.method)) {
+    if ("FcmDartService.start".equals(call.method)) {
+      long setupCallbackHandle = 0;
+      long backgroundMessageHandle = 0;
+      try {
+        List<Long> callbacks = ((ArrayList<Long>) call.arguments);
+        setupCallbackHandle = callbacks.get(0);
+        backgroundMessageHandle = callbacks.get(1);
+      } 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(