TopActivityProvider.kt

class TopActivityProvider {

  private val pendingActions = mutableListOf<(Activity) -> Unit>()
  var topActivity: Activity? = null
    private set

  @AnyThread
  fun onTopActivity(action: (Activity) -> Unit) {
    val topActivity = this.topActivity

    if (topActivity == null) {
      addPendingAction(action)
    } else {
      if (isMainThread()) {
        action(topActivity)
      } else {
        addPendingAction(action)
        topActivity.runOnUiThread { executePendingActions() }
      }
    }
  }

  @MainThread
  private fun executePendingActions() { ... }

  @AnyThread
  private fun addPendingAction(action: (Activity) -> Unit) { ... }
  
  ...
}