Countdown Timer - Keep timer running when closing the app - Android Studio Tutorial

Hello guys! In this tutorial, we will learn how to make a countdown timer but when the app was close, the timer keeps running.
Listening is only performed while the activity is running. That's mean you must register to listen before the main activity running and unregister after the main activity close

All Notes:- https://kayraaprogramming.com/

this part source code :-
https://kayraaprogramming.com/countdown-timer-keep-timer-running-when-closing-the-app-android-studio-tutorial/