[firebase_messaging] Add an ArgumentError when passing invalid backgr…

…ound message (firebase#252)

packages/firebase_messaging/CHANGELOG.md

@@ -1,3 +1,8 @@
+## 6.0.1
+
+* `FirebaseMessaging.configure` will throw an `ArgumentError` when `onBackgroundMessage` parameter
+is not a top-level or static function.
+
 ## 6.0.0
 
 * Use `UNUserNotificationCenter` to receive messages on iOS version >= 10.
@@ -11,7 +16,7 @@
 
   and add this line to your iOS project `AppDelegate.m`
 
-  ```objectivec
+  ```swift
   if (@available(iOS 10.0, *)) {
     [UNUserNotificationCenter currentNotificationCenter].delegate = (id<UNUserNotificationCenterDelegate>) self;
   }

packages/firebase_messaging/README.md

@@ -61,7 +61,7 @@ for more.
 
 By default background messaging is not enabled. To handle messages in the background:
 
-1. Add an Application.java class to your app
+1. Add an Application.java class to your app in the same directory as your `MainActivity.java`. This is typically found in `<app-name>/android/app/src/main/java/<app-organization-path>/`.
 
     ```
     package io.flutter.plugins.firebasemessagingexample;
@@ -85,11 +85,11 @@ By default background messaging is not enabled. To handle messages in the backgr
       }
     }
     ```
-1. Set name property of application in `AndroidManifest.xml`
+1. Set name property of application in `AndroidManifest.xml`. This is typically found in `<app-name>/android/app/src/main/`.
     ```
     <application android:name=".Application" ...>
     ```
-1. Define a top level Dart method to handle background messages
+1. Define a **TOP-LEVEL** or **STATIC** function to handle background messages
     ```
     Future<dynamic> myBackgroundMessageHandler(Map<String, dynamic> message) {
       if (message.containsKey('data')) {

packages/firebase_messaging/lib/firebase_messaging.dart

@@ -118,6 +118,15 @@ class FirebaseMessaging {
           PluginUtilities.getCallbackHandle(_fcmSetupBackgroundChannel);
       final CallbackHandle backgroundMessageHandle =
           PluginUtilities.getCallbackHandle(_onBackgroundMessage);
+
+      if (backgroundMessageHandle == null) {
+        throw ArgumentError(
+          '''Failed to setup background message handler! `onBackgroundMessage`
+          should be a TOP-LEVEL OR STATIC FUNCTION and should NOT be tied to a
+          class or an anonymous function.''',
+        );
+      }
+
       _channel.invokeMethod<bool>(
         'FcmDartService#start',
         <String, dynamic>{

packages/firebase_messaging/pubspec.yaml

@@ -3,7 +3,7 @@ description: Flutter plugin for Firebase Cloud Messaging, a cross-platform
   messaging solution that lets you reliably deliver messages on Android and iOS.
 author: Flutter Team <[email protected]>
 homepage: https://github.com/FirebaseExtended/flutterfire/tree/master/packages/firebase_messaging
-version: 6.0.0
+version: 6.0.1
 
 flutter:
   plugin:

packages/firebase_messaging/test/firebase_messaging_test.dart

@@ -97,13 +97,18 @@ void main() {
     final Completer<dynamic> onLaunch = Completer<dynamic>();
     final Completer<dynamic> onResume = Completer<dynamic>();
 
-    firebaseMessaging.configure(onMessage: (dynamic m) async {
-      onMessage.complete(m);
-    }, onLaunch: (dynamic m) async {
-      onLaunch.complete(m);
-    }, onResume: (dynamic m) async {
-      onResume.complete(m);
-    });
+    firebaseMessaging.configure(
+      onMessage: (dynamic m) async {
+        onMessage.complete(m);
+      },
+      onLaunch: (dynamic m) async {
+        onLaunch.complete(m);
+      },
+      onResume: (dynamic m) async {
+        onResume.complete(m);
+      },
+      onBackgroundMessage: validOnBackgroundMessage,
+    );
     final dynamic handler =
         verify(mockChannel.setMethodCallHandler(captureAny)).captured.single;
 
@@ -166,6 +171,17 @@ void main() {
 
     verify(mockChannel.invokeMethod<void>('setAutoInitEnabled', false));
   });
+
+  test('configure bad onBackgroundMessage', () {
+    expect(
+      () => firebaseMessaging.configure(
+        onBackgroundMessage: (dynamic message) => Future<dynamic>.value(),
+      ),
+      throwsArgumentError,
+    );
+  });
 }
 
+Future<dynamic> validOnBackgroundMessage(Map<String, dynamic> message) async {}
+
 class MockMethodChannel extends Mock implements MethodChannel {}