isMovingToParent.swift

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    
    if isMovingToParent {
      // start timer
    }
  }

override func viewWillDisappear(_ animated: Bool) {
    super.viewWillDisappear(animated)
    
    if isMovingFromParent {
      // invalidate timer
    }
  }