在循环中添加子视图一次显示所有子视图

Question

我试图在单击按钮时添加 1000 个 UI 标签，但我循环了 1000 次并创建了 1000 个 UI 标签

    @IBAction func display(_ sender: Any) {
      for i in Range(1...1000) {

                      let label =  self.displayLabel(str: "test \(i)",i: i)

                                        )
                        UIView.animate(withDuration: 2.0, delay: 1.0, options: UIView.AnimationOptions.transitionCrossDissolve,animations: {
                            self.view.addSubview(label)
                            label.alpha = 1.0;
                         label.center.y = self.view.center.y/3},
                                       completion: { (value) in label.removeFromSuperview()} )



                      }

}

而displayLabel函数是

    func displayLabel(str:String,i:Int) -> UILabel {

    let label = UILabel.init(frame: CGRect(x: 0, y: 0, width: 200, height: 21))
    label.tag = i

    label.font = UIFont.preferredFont(forTextStyle: .footnote)
    let screenWidth = self.view.frame.size.width
    let screenHeight = self.view.frame.size.height
    label.textColor = .black
    label.center = CGPoint(x: screenWidth * 0.75, y: screenHeight - (screenHeight/3.0))
    label.textAlignment = .center
    label.text = str
    label.backgroundColor = .white
    label.backgroundColor?.withAlphaComponent(0.5)
    return label


}

会发生什么？

屏幕一个接一个地显示标签，然后向上移动并消失。

实际发生的事情

同时添加所有标签，然后它们开始制作动画，使编号为 1000 的标签动画并消失。

你认为为什么会发生这种情况？

这是因为标签的原因还是为什么它等待所有1000个标签并将其添加到子视图中？

我怎样才能达到我想要的？

Answer 1

根据您的代码，这是预期的行为，因为您正在删除 completion 处理程序中的标签对象，它是 @escaping 闭包。

这意味着，执行不会等待removeFromSuperview。它将执行addSubview 操作，无需等待completion 处理程序，它将继续前进。

您可以通过在 completion 处理程序之前和之后添加 print 语句来验证此行为。

现在，关于您的要求，可以通过以下方式满足：

声明一个变量来保存标签标签

var tag = 0

避免for循环，在这里使用递归函数，比如

  //function to add and remove label until the defined count reached
  func showLabel(withTag: Int) {
    if self.tag != 1000 {
      let label =  self.displayLabel(str: "test \(withTag)",i: withTag)
      UIView.animate(withDuration: 2.0,
                     delay: 1.0,
                     options: UIView.AnimationOptions.transitionCrossDissolve,
                     animations: {
                      self.view.addSubview(label)
                      label.alpha = 1.0;
                      label.center.y = self.view.center.y/3
      }) { (value) in
        label.removeFromSuperview()
        self.tag = self.tag + 1
        self.showLabel(withTag: self.tag)
      }
    }
  }

  //initial function to start the process for each label
  func display() {
    showLabel(withTag: tag)
  }

  //function to provide new label object
  //no changes here
  func displayLabel(str:String,i:Int) -> UILabel {
    let label = UILabel.init(frame: CGRect(x: 0, y: 0, width: 200, height: 21))
    label.tag = i
    label.font = UIFont.preferredFont(forTextStyle: .footnote)
    let screenWidth = self.view.frame.size.width
    let screenHeight = self.view.frame.size.height
    label.textColor = .black
    label.center = CGPoint(x: screenWidth * 0.75, y: screenHeight - (screenHeight/3.0))
    label.textAlignment = .center
    label.text = str
    label.backgroundColor = .white
    label.backgroundColor?.withAlphaComponent(0.5)
    return label
  }

Answer 2

您应该在每个 .addSubview 之后布局子视图。所以尝试这样做：

UIView.animate(withDuration: 2.0, delay: 1.0, options: UIView.AnimationOptions.transitionCrossDissolve,animations: {
                        self.view.addSubview(label)
                        label.alpha = 1.0;
                        label.center.y = self.view.center.y/3
                        self.view.layoutSubviews() }