Document how to extract message body using firebase_messaging

[kbrmimbyl]

There's some confusion w.r.t. whether onLaunch, onResume and onMessage are getting called on iOS or not. Many people are still complaining that one or more of these are not getting called (for e.g. see #18425).

How to reproduce?

Add firebase_messaging to your flutter project, configure for both iOS and Android as explained in the plugin's README. Then send a notification each to your iOS and Android apps using Firebase Console> Cloud Messaging by setting only the "Notification Text" in Step 1 (this is the only mandatory field in this step). Remember to set click_action to FLUTTER_NOTIFICATION_CLICK under Additional Options (optional, Step 5). In your flutter project, try to extract what you put in as "Notification Text" above using the following dart code to configure FirebaseMessaging:

onMessage: (Map<String, dynamic> msg) {
  _message = msg['notification']['body'];
  print(_message);
},
onResume: (Map<String, dynamic> msg) {
  _message = msg['notification']['body'];
  print(_message);
},
onLaunch: (Map<String, dynamic> msg) {
  _message = msg['notification']['body'];
  print(_message);
},

Result

All three messages are printed for Android. To be more clear, onMessage gets executed when the app is in the foreground when it gets a notification, onResume when the app is running in the background and you tap on the notification, onLaunch when the app isn't running and you tap on the notification. But on iOS, none of the print statements are executed. The notifications do arrive when the app is not in the foreground, but nothing happens when you click on them. No print outputs in XCode or VSCode. This leads to the idea that none of the three functions are getting called on iOS.

How I solved it

On looking more closely, I found out that all three functions are actually getting called. Only, execution was apparently getting stopped at _message = msg['notification']['body'];, so the print never got called on iOS. Flutter doesn't log any complaints either.

The issue is, the path to the "Notification Text" is different on iOS and Android, so I ended up using this code:

onMessage: (Map<String, dynamic> msg) {
  if (Theme.of(context).platform == TargetPlatform.iOS)
      _message = msg['aps']['alert'];
  else
      _message = msg['notification']['body'];
  print(_message);
},
onResume: (Map<String, dynamic> msg) {
  if (Theme.of(context).platform == TargetPlatform.iOS)
      _message = msg['aps']['alert'];
  else
      _message = msg['notification']['body'];
  print(_message);
},
onLaunch: (Map<String, dynamic> msg) {
  if (Theme.of(context).platform == TargetPlatform.iOS)
      _message = msg['aps']['alert'];
  else
      _message = msg['notification']['body'];
  print(_message);
},

Now everything works on both iOS and Android.

How this problem can be prevented

Going further, I request the Flutter/Firebase team to document how msg looks on iOS and Android. The firebase_messaging plugin's README is a good place. Also, why isn't Flutter throwing any error when it can't access msg['notification']['body'] on iOS?

Note

#23148 is somewhat different... that one is about keeping the structure of msg the same across onLaunch, onResume and onMessage. I actually find them to be the same! The issue I'm raising here is that it's not clear what the structures on Android and iOS are. It's desirable for the structures to be the same, but it's not as important as knowing what they are

[kbrmimbyl]

Digging further...

When I send a message from the Firebase console, with only the mandatory "Notification text", that text appears as {aps: {alert: <notification text>}} in the notification payload on iOS. This needs to be accessed as msg['aps'][alert'].

If I add the optional "Notification title" in the Firebase console, it becomes {aps: {alert: {title: <notification title>, body: <notification text>}}}. In this case, the notification text needs to be accessed as msg['aps']['alert']['body'] and the notification title as msg['aps']['alert']['title'].

On Android, it's consistently msg['notification']['body'] irrespective of whether I add a title or not. If I do, the title is accessible as msg['notification']['title'].

This means we need different code not only for iOS vs Android, but also title vs no title!

I did fire a cloud function to implement all this. In the cloud function, the above behavior is reproducible by using {notification: {title: <Notification title>, 'body': <Notification text>}} as the payload argument of admin.messaging().sendToDevice().

[nayanbhana]

I have tried all of these steps and still can't get any of the hooks to fire.
ATM I currently will only see the push notification if the app is in the background.
When the app is in the background and a push notification comes through, I can click the notification and it will open the app but I see
2019-02-21 08:13:35.652779+1100 Runner[312:8514] Warning: UNUserNotificationCenter delegate received call to -userNotificationCenter:didReceiveNotificationResponse:withCompletionHandler: but the completion handler was never called.
which leads me to believe my iOS setup is incorrect somewhere.

[nayanbhana]

What version of firebase messaging are you using @kbrmimbyl

[nayanbhana]

I have finally got the hooks to fire. I had to remove flutter_local_notfications from my project completely. I found this information here https://stackoverflow.com/questions/50970343/flutter-fcm-on-ios-not-calling-onmessage-and-onresume/51032413

[kbrmimbyl]

Sorry for the delay, @nayanbhana. Glad you got it working. I had trouble even installing any local notification plugin, so I didn't use any. My firebase_messaging plugin is ^3.0.1

[collinjackson]

Note that #23148 is now fixed, as a breaking change in 4.0.0. It seems like this issue is mostly about documentation of the functionality of the plugin, especially on iOS.

[kbrmimbyl]

@collinjackson "Now fixed"? I see 3.0.1 as the latest version on pub...

[danwulff]

I also have found that the notification model is different on iOS and Android when the optional data attribute is utilized. See #29027

[ghenry]

Anyone done a PR for the FCM plugin docs?