How to make a heads-up notification when the app is in the background?

Question

Android

1. Instagram manages to send heads-up notifications (a floating card on top of other apps) even when the app is minimized. The same thing, for example, when an SMS arrives or a call is made on WhatsApp. As far as I was able to google, if the application is minimized, then notifications can only come to the tray, but not be shown on top of other applications. I am sending notifications via firebase console. How to make notifications like instagram?
Maybe it's in firebase and instead you need to use another notification provider, for example onesignal?
2. I was able to make heads-up notifications in the emulator when the application is deployed on the screen. But the same conditions on a real device (Huawei 5A) show a notification only in the tray (the android version is the same as in the emulator). What else is the problem here?
3. Heads-up notifications can only be on one line, shortening the text of the notification?

Answer 1

In the onMessageReceived method in your FirebaseMessagingService something like:

if((Build.VERSION.SDK_INT < Build.VERSION_CODES.M  || Settings.canDrawOverlays(this))) {
            new NotificationPopupView(intent, pushNotification.getTitle(), pushNotification.getBody());
}

NotificationPopupView is a normal view. You also need to add permission

<uses-permission android:name="android.permission.SYSTEM_ALERT_WINDOW" />