[firebase_messaging]: add additional documentation for the data in notifications

packages/firebase_messaging/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 5.1.2
+
+* Updates to README and example with explanations of differences in data format.
+
 ## 5.1.1
 
 * Update README with more detailed integration instructions.

packages/firebase_messaging/README.md

@@ -93,6 +93,21 @@ Messages are sent to your Flutter app via the `onMessage`, `onLaunch`, and `onRe
 
 Additional reading: Firebase's [About FCM Messages](https://firebase.google.com/docs/cloud-messaging/concept-options).
 
+## Notification messages with additional data
+It is possible to include additional data in notification messages by adding them to the `"data"`-field of the message.
+
+On Android, the message contains an additional field `data` containing the data. On iOS, the data is directly appended to the message and the additional `data`-field is omitted.
+
+To receive the data on both platforms:
+
+````dart
+Future<void> _handleNotification (Map<dynamic, dynamic> message, bool dialog) async {
+    var data = message['data'] ?? message;
+    String expectedAttribute = data['expectedAttribute'];
+    /// [...]
+}
+````
+
 ## Sending Messages
 Refer to the [Firebase documentation](https://firebase.google.com/docs/cloud-messaging/) about FCM for all the details about sending messages to your app. When sending a notification message to an Android device, you need to make sure to set the `click_action` property of the message to `FLUTTER_NOTIFICATION_CLICK`. Otherwise the plugin will be unable to deliver the notification to your app when the users clicks on it in the system tray.
 

packages/firebase_messaging/example/lib/main.dart

@@ -9,9 +9,10 @@ import 'package:flutter/material.dart';
 
 final Map<String, Item> _items = <String, Item>{};
 Item _itemForMessage(Map<String, dynamic> message) {
-  final String itemId = message['data']['id'];
+  final dynamic data = message['data'] ?? message;
+  final String itemId = data['id'];
   final Item item = _items.putIfAbsent(itemId, () => Item(itemId: itemId))
-    ..status = message['data']['status'];
+    ..status = data['status'];
   return item;
 }
 

packages/firebase_messaging/pubspec.yaml

@@ -3,7 +3,8 @@ 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/flutter/plugins/tree/master/packages/firebase_messaging
-version: 5.1.1
+
+version: 5.1.2
 
 flutter:
   plugin: