UIView动画Swift 4 Xcode9的问题

Question

我正在尝试使用弹簧动画块从屏幕外到中心制作UILabel动画。 我下面的代码在viewDidLoad()的第一部分工作正常，但是当我添加动画块时，就像先读取了动画闭包中的代码，并且该动画没有动画，因为标签已经在我想要标签制作动画。 我也尝试将这个确切的代码放入viewDidAppear()但是发生了同样的事情。

@IBOutlet weak var follow: UILabel!
@IBOutlet weak var followX: NSLayoutConstraint!

override func viewDidLoad() {
    super.viewDidLoad()

    self.follow.translatesAutoresizingMaskIntoConstraints = false

    self.followX.constant = self.view.frame.width/2 + follow.frame.width/2
    self.view.layoutIfNeeded()

    UIView.animate(withDuration: 2, delay: 2, usingSpringWithDamping: 5, initialSpringVelocity: 5, options: .curveEaseOut, animations: {
        self.followX.constant = 0
        self.view.layoutIfNeeded()
    })

}

Answer 1

这是正确的方法（尽管我会尝试将动画移动到viewWillAppear或viewDidAppear ）：

@IBOutlet weak var follow: UILabel!
@IBOutlet weak var followX: NSLayoutConstraint!

override func viewDidLoad() {
    super.viewDidLoad()

    self.follow.translatesAutoresizingMaskIntoConstraints = false

    self.followX.constant = self.view.frame.width/2 + follow.frame.width/2
    self.view.layoutIfNeeded()

    self.followX.constant = 0
    self.view.setNeedsLayout()

    UIView.animate(withDuration: 2, delay: 2, usingSpringWithDamping: 5, initialSpringVelocity: 5, options: .curveEaseOut, animations: {
        self.view.layoutIfNeeded()
    })

}

要使用自动版式，请执行以下操作：

1. 首先，请确保准备开始动画：

    self.followX.constant = self.view.frame.width/2 + follow.frame.width/2 self.view.layoutIfNeeded() 

2. 然后将约束更改为动画的最终状态：

    self.followX.constant = 0 

3. 然后，您告诉autolayout约束已更改：

    self.view.setNeedsLayout() 

4. 最后，使用layoutIfNeeded()调用UIView.animate来使更改生效：

    UIView.animate(withDuration: 2, delay: 2, usingSpringWithDamping: 5, initialSpringVelocity: 5, options: .curveEaseOut, animations: { self.view.layoutIfNeeded() }) 

Answer 2

override func viewDidLayoutSubviews() {
   if(once)
   {
      once = false
      self.follow.translatesAutoresizingMaskIntoConstraints = false
      self.followX.constant = self.view.frame.width/2 +   follow.frame.width/2
      self.view.layoutIfNeeded()
   }

}

并在viewDidAppear中

 override func viewDidAppear() {

   self.followX.constant = 0
   UIView.animate(withDuration: 2, delay: 0, usingSpringWithDamping: 5, initialSpringVelocity: 5, options: .curveEaseOut, animations: {
      self.view.layoutIfNeeded()
  })

 }