When you tap a notification from the device's notification centre the operating system will launch the app, provided of course you have set up Firebase messaging correctly as per instructions the onLaunch or onMessage functions will get called.
Now since firebase messaging configured pretty much as early as possible, typically inside the main entry of the flutter app you will need to store the notification somewhere first. This can be a global state object created before anything that uses it.
final notifications = <NotificationData>[]; // globally accessible (i.e. via Provider)
Then in the handler of the messages...
NotificationData onMessage(Map<String, dynamic> data) {
notifications.add(NotificationData.fromJson(data));
}
The flutter app will start and the MaterialApp will get built. At this point you can have a loading screen to check if the user is authenticated or any new messages in the notifications global state. If there are you can route to a page to display the notification, or get the user to sign in then route to the notification page.
Thank you. I've also considered that, but wasn't sure whether the handling speed would be fast enough. Because we cannot exclude that the setup screen is faster than the message handler. Or what's your opinion here?
That is why storing the message data somewhere until you are ready to display it is probably best. if the app is in the background which means that the user will just come back to the page they were on you can listen for when the app resumes then process the notification