VisibilityChangeListener

class AppOnVisibilityChangeListener() : LifecycleObserver {

    var isVisible = MutableLiveData<Boolean>()

    init {
        AppLog.d("Initialising")
        ProcessLifecycleOwner.get().lifecycle.addObserver(this)
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
    fun onAppBackgrounded() {
        AppLog.d("App in background")
        onHidden()
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_START)
    fun onAppForegrounded() {
        AppLog.d("App in foreground")
        onVisible()
    }

    private fun onHidden() {
        isVisible.postValue(false)
    }

    private fun onVisible() {
        isVisible.postValue(true)
    }
}

Usage:
In Application On Create (and any other observers):
onVisibilityChangeListener.isVisible.observeForever { // do something on changed }