暂停代码，直到获得Swift中UIAlertController的响应

Question

我一直试图从UIAlertController的用户那里得到答案。问题是在显示UIAlertController时代码仍然运行。我想显示警报，然后等到用户给出答案继续代码。

func showPopUp(name:String)->String{
   var genre = ""
   let alert = UIAlertController(title: "What are you "+name+"?", message: nil, preferredStyle: .alert)

   alert.addAction(UIAlertAction(title: "Boy", style: .default, handler: { action in
    genre = "Boy"
}))

   alert.addAction(UIAlertAction(title: "Girl", style: .default, handler: { action in
    genre = "Girl"
}))

   self.present(alert, animated: true)
   return genre
}

override func viewDidLoad() {
        super.viewDidLoad()
        print("This should appear before the alert")
        let a = showPopUp(name: "John)

        print("This should appear after the response")
        print("John is a "+a)
        [...more code using variable 'a' ...]


}

我不能将代码放在警报的操作中，因为它很长，而且它也可以在其他进程中运行。

Answer 1

我无法将代码放在警报的操作中

是的，你可以，这就是你必须做的。该动作是您发出警报已被解除的信号（它还会告诉您在警报中点击了哪个按钮）。您不必将代码本身放在操作中;您只需要调用该代码即可。

例：

alert.addAction(UIAlertAction(title: "Boy", style: .default, handler: { action in
    self.myVeryLongFunction(genre:"Boy")
}))

Answer 2

创建一个单独的函数，并从完成处理程序中为您的操作调用它。

像这样

func showPopUp(name:String)->String{
   var genre = ""
   let alert = UIAlertController(title: "What are you "+name+"?", message: nil, preferredStyle: .alert)

   alert.addAction(UIAlertAction(title: "Boy", style: .default, handler: { action in
    genre = "Boy"
    self.afterPopup()
}))

   alert.addAction(UIAlertAction(title: "Girl", style: .default, handler: { action in
    genre = "Girl"
    self.afterPopup()
}))

   self.present(alert, animated: true)
   return genre
}

override func viewDidLoad() {
        super.viewDidLoad()
        print("This should appear before the alert")
        let a = showPopUp(name: "John)
}

func afterPopup(){
        print("This should appear after the response")
        print("John is a "+a)
        [...more code using variable 'a' ...]
}

另外，请注意在ViewDidLoad中显示弹出窗口。如果正在显示此视图，则会发出警告（并可能导致app的奇怪行为）

Answer 3

警报不会像您期望的那样工作，它假设要求用户并等待他的操作，因此它是异步的

var genre = "Boy" // default

func showPopUp(name:String) {

   let alert = UIAlertController(title: "What are you "+name+"?", message: nil, preferredStyle: .alert)

   alert.addAction(UIAlertAction(title: "Boy", style: .default, handler: { action in
       self.genre = "Boy" 
       self.handleSelection() 
   }))

   alert.addAction(UIAlertAction(title: "Girl", style: .default, handler: { action in
       self.genre = "Girl" 
       self.handleSelection()
   }))

   self.present(alert, animated: true)

}

func handleSelection() {
  print("current selected genre is \(genre)")
}

加上这个

showPopUp(name: "John")

应该在viewDidAppear内部，因为现在在viewDidLoad内展示任何东西还为时尚早，其中vc的观点尚未完全铺设

var once = true
override func viewDidAppear(_ animated:Bool) {
  super.viewDidAppear(animated)
  if once {
     showPopUp(name: "John")
     once = false // to prevent re-showing it again
  }
}