Swift 3.0中的UIView()构造函数发生了什么? [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?()的简洁语法声明变量.这给x提供了UIView?类型，并用nil对其进行了初始化. (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:
Cannot invoke initializer for type 'UIView?' with no arguments.它还说Overloads for 'UIView?' exist with these partially matching parameter lists: (Wrapped), (nilLiteral: ()).不知何故，我认为UIView?(nilLiteral: ())并不是我想要的.
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.
自然地，还有其他替代方法可以执行相同的操作，例如let x: UIView? = nil和let x = nil as UIView()，但是它们比我以前使用的方法更为冗长.在Swift 3.0中删除了UIView?()构造函数，还是以我尚未发现的形式替换了它?
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
以下将x初始化为nil，括号完全多余.
The following does initialise x to nil, the brackets are entirely superfluous.
var x: UIView?
return x == nil
将返回true
查看开发者文档以获取更多信息.
Check out the developer docs for more information.
如果您在未提供默认值的情况下定义了可选变量， 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发表评论以阐明UIView?()不再起作用的原因:
@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()构造一个新的可选值设置为.None.是的，它工作正常.但是，在Swift 3中，此初始化程序已被删除，这就是UIView?()不再起作用的原因"
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