PushCallbacks.kt

class PushCallbacks(
  val pushStream: PushStream
) : Application.ActivityLifecycleCallbacks, PushStream.OnPush {

  private var foregroundActivity: Activity? = null

  override fun onActivityResumed(activity: Activity) {
    foregroundActivity = activity
    pushStream.register(this)
  }

  override fun onActivityPaused(activity: Activity) {
    pushStream.unregister(this)
    foregroundActivity = null
  }

  override fun onPush(push: PushMessage) {
    foregroundActivity?.let { showMessage(it, push) }
  }

  private fun showMessage(activity: Activity, pushMessage: PushMessage) { ... }

  // Other methods are empty
}