UIViewController 第一次 viewDidApear

Perform-In-ViewDidApear-once.md

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        if self.isBeingPresented || self.isMovingToParent {
            //Perform an action that will only be done once
        } 
    }

     if (!self.isBeingPresented && !self.isMovingToParentViewController) {
         //except first time
     }