MainActivity.kt

class MainActivity : AppCompatActivity(), LifecycleObserver {

    private lateinit var binding: ActivityMainBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        ProcessLifecycleOwner.get().lifecycle.addObserver(this)

        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        /* ... */
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
    fun onAppBackgrounded() {
        // Background
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_START)
    fun onAppForegrounded() {
        // Foreground
    }

    /* ... */
}