How to fix silenced push notifications on Android

The issue

When working on an application that notifies users in the case of an evacuation, my team and I encountered an interesting bug. Our notifications on Android were silent by default, even though they were sent as critical alerts from the backend.

In our code, we hooked into the notification handler and showed the notifications locally using flutter_local_notifications to be able to adjust some of the content.

So I started debugging and found the issue after a while...

Androids importance level

In Android we have to create a notification channel to be able to show local notifications. This notification channel two properties that are of interest: Importance and Priority.

Priority is only used for Android 7.1 and lower according to the documentation, so that was not of interest for us here. The importance setting however, was interesting. But wait - we already set this to max in our code? What could possibly be wrong?

static final _channel = AndroidNotificationChannel(
  'alarm_notification_channel',
  'Alarm Notifications',
  description: 'This channel is used for alarm notifications.',
  importance: Importance.max, // The importance was set to max...
  playSound: true,
  sound: const RawResourceAndroidNotificationSound('alarm'),
  enableVibration: true,
  vibrationPattern: Int64List.fromList([0, 30000]),
);