[firebase_messaging] Add support for handling messages in background for iOS

packages/firebase_messaging/README.md

@@ -55,6 +55,90 @@ Note: When you are debugging on Android, use a device or AVD with Google Play se
   </intent-filter>
   ```
 
+#### Optionally handle background messages
+
+ >Background message handling is intended to be performed quickly. Do not perform
+long running tasks as they may not be allowed to finish by the Android system.
+See [Background Execution Limits](https://developer.android.com/about/versions/oreo/background)
+for more.
+ By default background messaging is not enabled. To handle messages in the background:
+
+ 1. For Android 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. For iOS (Swift)
+    ```
+    func callback(registry: FlutterPluginRegistry) {
+        GeneratedPluginRegistrant.register(with: registry)
+    }
+
+    FLTFirebaseMessagingPlugin.setPluginRegistrantCallback(callback)
+    ```
+
+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 other work.
+    }
+    ```
+   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/example/ios/Runner/AppDelegate.m

@@ -4,13 +4,19 @@
 
 #include "AppDelegate.h"
 #include "GeneratedPluginRegistrant.h"
+#import <firebase_messaging/FirebaseMessagingPlugin.h>
 
 @implementation AppDelegate
+
+void callback(NSObject<FlutterPluginRegistry>* registry) {
+    [GeneratedPluginRegistrant registerWithRegistry:registry];
+}
 
 - (BOOL)application:(UIApplication *)application
     didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
-  [GeneratedPluginRegistrant registerWithRegistry:self];
-  return [super application:application didFinishLaunchingWithOptions:launchOptions];
+    [GeneratedPluginRegistrant registerWithRegistry:self];
+    [FLTFirebaseMessagingPlugin setPluginRegistrantCallback:callback];
+    return [super application:application didFinishLaunchingWithOptions:launchOptions];
 }
 
 @end

packages/firebase_messaging/ios/Classes/FirebaseMessagingPlugin.m

@@ -289,7 +289,7 @@ - (void)setupBackgroundHandling:(int64_t)handle {
     NSLog(@"Finished background setup");
 }
 
-- (void) startBackgroundRunner {
+- (void)startBackgroundRunner {
     NSLog(@"Starting background runner");
 
     int64_t handle = [self getCallbackHandle:backgroundMessageCallback];