Swift 3.0 中的 UIView?() 构造函数发生了什么变化? [英] What happened to the UIView?() constructor in Swift 3.0?

问题描述

在 Swift 2.2 中，我经常使用类似于 let x = UIView?() 的简洁语法来声明变量.这给了 x UIView? 类型，并用 nil 初始化它.(当然，在这些示例中，您可以使用任何类型代替 UIView)

In Swift 2.2, I often declared variables using a concise syntax similar to let x = UIView?(). This gave x the UIView? type, and initialised it with nil. (Of course, you could use any type instead of UIView in these examples)

但是，当我在 Swift 3.0 中执行相同操作时，出现错误:无法为UIView"类型调用初始化程序?没有参数.它还说 Overloads for 'UIView?'存在这些部分匹配的参数列表:(Wrapped), (nilLiteral: ()).不知何故，我不认为 UIView?(nilLiteral: ()) 是我所追求的.

However, when I do the same in Swift 3.0, I get an error: Cannot invoke initializer for type 'UIView?' with no arguments. It also says that Overloads for 'UIView?' exist with these partially matching parameter lists: (Wrapped), (nilLiteral: ()). Somehow, I don't think UIView?(nilLiteral: ()) is quite what I'm after.

当然，还有其他替代方法可以做同样的事情，比如 let x: UIView?= nil 和 let x = nil as UIView()，但它们比我之前使用的方法更冗长.UIView?() 构造函数是否在 Swift 3.0 中被删除了，还是被我尚未发现的形式替换了?

Naturally, there are other alternative methods to do the same thing, such as let x: UIView? = nil and let x = nil as UIView(), but they're more verbose than the method I was using previously. Was the UIView?() constructor removed in Swift 3.0, or has it been replaced in a form that I've not yet discovered?

推荐答案

以下确实将 x 初始化为 nil，括号完全是多余的.

The following does initialise x to nil, the brackets are entirely superfluous.

var x: UIView?
return x == nil

会返回真

查看开发者文档了解更多信息.

Check out the developer docs for more information.

https://developer.apple.com/library/content//documentation/Swift/Conceptual/Swift_Programming_Language/TheBasics.html

如果你定义了一个可选变量而不提供默认值，该变量会自动为您设置为零:

If you define an optional variable without providing a default value, the variable is automatically set to nil for you:

var surveyAnswer: String?
// surveyAnswer is automatically set to nil

@Hamish 发表评论澄清 UIView?() 不再有效的原因:

@Hamish posted a comment to clarify the reason why UIView?() no longer works:

UIView 的语法?只是 Optional 的语法糖，因此 UIView?() 只是 Optional.init() 的语法糖.在 Swift 2 中，Optional 的 init() 构造了一个新的可选值，设置为 .None.所以是的，它工作正常.然而，在 Swift 3 中，这个初始化器已被删除，这就是 UIView?() 不再起作用的原因"

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