Why doesn't firebase.messaging().onMessage() trigger on iOS when using the REST API for notifications?

When using the REST API your message payload is being interpreted as a notification message rather than a pure data message. On iOS, notification messages (especially when a “notification” key is present) are handled by the system (and potentially by UNUserNotificationCenter delegates) rather than being delivered to your onMessage callback in the foreground.

The Firebase console often sends messages with a different structure (or prioritizes a data‑only message) so that the onMessage callback is triggered when the app is active. In your REST API payload the presence of the "notification" block combined with the APNs payload gives iOS a reason to handle the message as a standard remote notification. This handling means that if the app is in the foreground you might not see the onMessage callback if you haven’t set up your notification presentation properly.

How to Fix

1. Use Data‑Only Messages for onMessage:
   Remove (or avoid including) the "notification" key in your REST API payload so that you send a data message. Data messages are delivered directly to your onMessage handler when the app is in the foreground.

   For example, change your payload to:

   JSONCopied
   {
     "message": {
       "token": "my-token",
       "data": {
         "title": "New Notification",
         "body": "Hello, this is a test notification!",
         "name": "name"
       },
       "apns": {
         "headers": {
           "apns-priority": "10"
         },
         "payload": {
           "aps": {
             "content-available": 1
           }
         }
       }
     }
   }

2. If You Intend to Show a Notification:
   If you want to display a notification (with an alert) and still handle onMessage in the foreground, you’ll need to implement the appropriate methods on the iOS side. This usually means implementing the UNUserNotificationCenter delegate method (for example, userNotificationCenter:willPresentNotification:withCompletionHandler:) so that notifications delivered while the app is active can be handled or presented as needed.

3. Review APNs Headers (if needed):
   iOS now requires that you set the appropriate apns-push-type header in some cases. For alert messages, you might add:

   JSONCopied
   "headers": {
     "apns-priority": "10",
     "apns-push-type": "alert"
   }

   Adjust this only if you intend to show an alert.

Summary

• Data-Only Message: Removing the "notification" field will ensure that your payload is delivered as data-only so that onMessage is triggered in the foreground.
• Foreground Notification Handling: If you want to display notification alerts in the foreground while also handling the message in code, implement the proper delegate methods on iOS.

By adjusting your payload accordingly, you can make sure that your onMessage callback is triggered for iOS when using the REST API.