swift - 枚举数组是 Swift 中的枚举？

Question

我正在关注 Apple's Swift/iOS tutorial它具有以下代码行:

button.setImage( filledStarImage, forState: [UIControlState.Highlighted, UIControlState.Selected] )

但是UIButton.setImage的签名定义是:

public func setImage(image: UIImage?, forState state: UIControlState)

没有接受 [UIControlState]() 数组的 setImage 重载。

显然有些巫术正在发生，谁能解释一下？

Answer 1

在 Swift 中，UIControlState 本身并不是一个枚举，而是一个结构。来自 UIControl 的语言引用:

Control State

The state of a control; a control can have more than one state at a time. States are recognized differently depending on the control. For example, a UIButton instance may be configured (using the setImage:forState: method) to display one image when it is in its normal state and a different image when it is highlighted.

Declaration

struct UIControlState : OptionSetType {
    init(rawValue rawValue: UInt)
    static var Normal: UIControlState { get }
    static var Highlighted: UIControlState { get }
    static var Disabled: UIControlState { get }
    static var Selected: UIControlState { get }
    static var Focused: UIControlState { get }
    static var Application: UIControlState { get }
    static var Reserved: UIControlState { get }
}

这里的关键是 OptionSetType 协议(protocol)(UIControlState 符合)，它允许我们对静态结构属性进行“类似枚举”的访问，前提是它们有一个 RawValue，它是一个 BitwiseOperationsType。例如:

struct MyStruct : OptionSetType {
    private var value: UInt
    var rawValue: UInt { return self.value }
    init(rawValue value: UInt) { self.value = value }

    static var Normal: MyStruct { return self.init(rawValue: 1 << 0) }
    static var Highlighted: MyStruct { return self.init(rawValue: 1 << 1) }
    static var Disable: MyStruct { return self.init(rawValue: 1 << 2) }
        // ...
}

let MyOptionsA : MyStruct = [MyStruct.Normal, MyStruct.Disable]
let MyOptionsB : MyStruct = [.Normal, .Disable]

MyOptionsB.contains(.Highlighted) // false
MyOptionsB.contains(.Normal) // true

查看符合 OptionsSetType 的实例方法(请参阅下面的语言引用链接)。除了成员(member)管理(例如 .contains(...) 如上所述；还有 .remove(...), .insert(..)), union(...) 和 intersect(...) 等整洁集合操作也可用。

总结一下； UIButton.setImage 签名要求 forState state: UIControlState 是对的，但是，由于 OptionSetType 协议(protocol)，数组外观 [UIControlState.Highlighted, UIControlState.Selected] 实际上是调用结构类型参数 UIControlState 的有效参数。

另请注意，根据我上面的示例，结构名称可以省略，因此将 [.Highlighted, .Selected] 作为 forState 参数发送只是一样好。

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引用和好读物:

• OptionSetType Protocol Language Reference
• How do you enumerate OptionSetType in Swift?
• http://www.informit.com/articles/article.aspx?p=2420231