Swift 中的 iOS8 尾随闭包

Question

let callActionHandler = { (action:UIAlertAction!) -> Void) in
        let alertMessage = UIAlertController(title: "Service Unavailable", message: "Sorry, the call feature is not available yet. Please retry later", preferredStyle: UIAlertControllerStyle.Alert)
        alertMessage.addAction(UIAlertAction(title: "OK", style: UIAlertActionStyle.Default, handler: nil))
        self.presentViewController(alertMessage, animated: true, completion: nil)
    };    

// Code Snippet 1
let callAction = UIAlertAction(title: "Call" + "123-000-\(indexPath.row)", style: UIAlertActionStyle.Default ) { (action:UIAlertAction!) -> Void in
        println("check this out")
}

// Code Snippet 2
let callAction = UIAlertAction(title: "Call" + "123-000-\(indexPath.row)", style: UIAlertActionStyle.Default, handler: { (action:UIAlertAction!) -> Void in  
        println("Lets check this out")
})

// Code Snippet 3
let callAction = UIAlertAction(title: "Call" + "123-000-\(indexPath.row)", style: UIAlertActionStyle.Default , handler: callActionHandler)

• 这里有 3 个代码片段，我的疑问是:
  1. 代码片段 1 和代码片段 2 有什么区别？
  2. Snippet 1 或 Snippet 2 中哪个表现更好，应该使用？
  3. 代码片段 1 的确切含义是什么？它是某种属性(完成)在 swift 中观察到的吗？
  4. iOS8 希望我们编写的方式显示在代码片段 1 中，即，当我在 Xcode 自动完成时按回车键时，它会转换为代码片段 1。我们应该使用代码片段 1 还是更喜欢使用代码片段 2/3，因为它们是容易理解吗？

谢谢

Answer 1

你问:

1. What is the difference between Code Snippet 1 and Code Snippet 2 ?

代码片段 1 正在使用片段 2 的“尾随闭包”再现。请参阅 The Swift Programming Language: Closures 中的尾随闭包 讨论。 ，它说:

If you need to pass a closure expression to a function as the function’s final argument and the closure expression is long, it can be useful to write it as a trailing closure instead. A trailing closure is a closure expression that is written outside of (and after) the parentheses of the function call it supports

所以片段 1 和片段 2 做的事情完全一样。

2. Which out of Snippet 1 or Snippet 2 is a better representation and should be used ?

通常人们会更喜欢代码段 1 的尾随闭包语法，因为它更简洁。但是，请使用使您的代码的意图既清晰又简洁的任何内容。

3. What does Code Snippet 1 exactly mean? Is it some sort of property (completion) observing in swift?

不，它与片段 2 完全相同。

4. The way iOS8 wants us to write is shown in Snippet 1, i.e., when I press enter while Xcode autocompletes, it gets converted to Snippet 1. Should we use Code Snippet 1 or still prefer using Snippet 2/3 as they are easily understood?

同样，使用您喜欢的任何内容，并且最简洁明了。通常，人们会尽可能地利用尾随闭包语法。

---

就个人而言，除了尾随闭包语法之外，我可能会利用参数的推断类型和枚举的点语法来进一步简化:

let callAction = UIAlertAction(title: "Call 123-000-\(indexPath.row)", style: .default) { action in
    print("check this out")
}