Notification not shown when setGroup() is called in Android KitKat

Question

I\'m testing the stackable notifications (Stacking Notifications article).

I detected that in some cases the notifications are not shown after the notify()

Answer 1

The short answer to this is that it appears showing a stacked notification on a KitKat device is not automatically supported by the support library.

Since you asked for an enlightenment here are my findings based on testing with two devices running Android 4.4.2. I am also using AppCompat 23.1.1.

When you dig into the source code of the library you will find that when a notification is shown it will either use something called the SideChannel or the NotificationManager directly to show the notification. Below is the NotificationManagerCompat.notify method for reference showing this:

public void notify(String tag, int id, Notification notification) {
    // MY COMMENT: true when the notification has the extra 
    // NotificationCompatJellybean.EXTRA_USE_SIDE_CHANNEL set to true.
    if (useSideChannelForNotification(notification)) {
        pushSideChannelQueue(new NotifyTask(mContext.getPackageName(), id, tag, notification));
        // Cancel this notification in notification manager if it just transitioned to being
        // side channelled.
        IMPL.cancelNotification(mNotificationManager, tag, id);
    } else {
        // MY COMMENT: this calls notificationManager.notify(id, notification); in the base implementation
        IMPL.postNotification(mNotificationManager, tag, id, notification);
    }
}

Now when you show a notification without setting a group the notification is shown using the notification manager which works, but if a group is set it will attempt to use the side channel to send the notification and cancel any notification shown using the notification manager as seen in the above method.

Proof that the side channel is used when a group is set is found in NotificationCompatKitKat.Builder where you will see the following code:

if (groupKey != null) {
    mExtras.putString(NotificationCompatJellybean.EXTRA_GROUP_KEY, groupKey);
    if (groupSummary) {
        mExtras.putBoolean(NotificationCompatJellybean.EXTRA_GROUP_SUMMARY, true);
    } else {
        mExtras.putBoolean(NotificationCompatJellybean.EXTRA_USE_SIDE_CHANNEL, true);
    }
}

This all does not seem like a big deal until you look at what the pushSideChannelQueue(...) method does when showing a notification using the SideChannel.

It ends up looking for a service that can handle the action android.support.BIND_NOTIFICATION_SIDE_CHANNEL of which there is not one by default so the method simply returns. This is what is causing the notification to never be shown.

There is a NotificationCompatSideChannelService abstract class in the compatibility library that you can implement according to the documentation if you want to write your own SideChannelService but it seems like you are better off just not using grouped notifications in KitKat and prior devices.