Detective is a small library that detects Application focus changes

Detective.kt

object Detective : SimpleActivityLifecycleCallbacks() {

    private infix fun <T> List<T>.notContains(element: T): Boolean {
      return !contains(element)
    }

    /** Timeout when app looses focus */
    private val backgroundEnterTimeout: Int = 120
    private val appFocusListeners = ArrayList<ApplicationFocusListener>()
    private var registered = false

    private val appEnteredBackground = AtomicBoolean(false)
    private val appHasFocus = AtomicBoolean(false)

    fun register(context: Context) {
      if (!registered) {
        registered = true
        (context.applicationContext as Application).registerActivityLifecycleCallbacks(this)
      }
    }

    /** Start listening focus changes */
    fun listen(focusListener: ApplicationFocusListener): Boolean {
      if (appFocusListeners notContains focusListener) {
        appFocusListeners += focusListener
        return true
      }

      return false
    }

    /** Stop listening focus changes */
    fun stopListening(focusListener: ApplicationFocusListener): Boolean {
      return appFocusListeners.remove(focusListener)
    }

    override fun onActivityPaused(activity: Activity) {
      appHasFocus.getAndSet(false)

      activity.window.decorView.postDelayed({
        if (!appHasFocus.get()) {
          appEnteredBackground.compareAndSet(false, true)
          appFocusListeners.forEach { it.changed(false) }
        }
      }, backgroundEnterTimeout.toLong())
    }

    override fun onActivityResumed(activity: Activity) {
      appHasFocus.getAndSet(true)
      appEnteredBackground.getAndSet(false)

      appFocusListeners.forEach { it.changed(true) }
    }

    /**
     * Receive application focus changes
     *
     * By application focus means, that at least one window has focus gained,
     * oterwise app entered background state
     * */
    interface ApplicationFocusListener {

      /**
       * Called whenever application loose or gain focus back
       *
       * @param hasFocus True on Application is focused
       */
      fun changed(hasFocus: Boolean)

    }
  }