Swift枚举设置为属性时会丢失初始化值?

Swift enum loses initialized values when set as a property?(Swift枚举设置为属性时会丢失初始化值?)

问题描述

我找到了解决方法，但这个问题让我很烦恼，我想我会分享一下，以防其他人遇到同样的问题.很想知道为什么会这样.在下面的代码中，当枚举是局部变量时，我可以在类初始化程序期间很好地打开枚举.我将枚举值存储到一个属性中.但是，当我尝试在下面的示例中以不同的方法(名为 bar())打开存储的属性(名为 foo)时 - 我收到编译器警告和成员无法识别的错误.似乎知道 foo 是 MyEnum 类型，但不知道 .ABC、.DEF 和 .GHI 是成员.

I've found a work-around, but this problem is vexing me and I thought I'd share in case anyone else is having the same problem. Would love to know why this is happening. In the code below, I can switch on the enum just fine during the class initializer when it's a local variable. I store the enum value into a property. But when I try to switch on the stored property (named foo) in a different method (named bar()) in the example below - I get compiler warnings and an error that the member(s) are not recognized. It seems to know that foo is a MyEnum type, but doesn't know that .ABC, .DEF, and .GHI are members.

enum MyEnum {
    case ABC, DEF, GHI
}

class MyClass : NSObject {

    var foo : MyEnum!

    convenience init(foo: MyEnum) {

        self.init()

        self.foo = foo

        switch foo {

        case .ABC: println("ABC foo")
        case .DEF: println("DEF foo")
        case .GHI: println("GHI foo")
        default: println("no foo")

        }
    }

    func bar() {

        switch foo {

        case .ABC: println("ABC foo")
        case .DEF: println("DEF foo")
        case .GHI: println("GHI foo")
        default: println("no foo")

        }
    }
}

解决方法是:

switch foo as MyEnum { }

或者在方法中声明一个局部变量，比如

or declare a local variable in the method like

let x : MyEnum = foo

switch x {  }

再次，很高兴我找到了解决方法，但我很想知道这是否是预期的行为，或者是否需要向 Apple 提交雷达.这是 Xcode 6.2，顺便说一句.

Again, glad I found a workaround, but would sure like to know if this is the expected behavior or if a radar needs to be filed with Apple. This is Xcode 6.2, BTW.

推荐答案

属性 foo 不是 MyEnum，而是 ImplicitlyUnwrappedOptional aka我的枚举！.与许多其他情况不同，switch 不会隐式解包.

Property foo is not MyEnum, but ImplicitlyUnwrappedOptional<MyEnum> aka MyEnum!. Unlike many other cases, switch does not implicitly unwrap it.

你必须手动解包:

    if let foo = foo {
        switch foo {
        case .ABC: println("ABC foo")
        case .DEF: println("DEF foo")
        case .GHI: println("GHI foo")
        default: println("no foo")
        }
    }
    else {
        println("nil foo")
    }

如果您确定 foo 不是 nil，则使用 ! 强制解包，:

OR force unwrap with ! if you are sure foo is not nil, :

    switch foo! {
    case .ABC: println("ABC foo")
    case .DEF: println("DEF foo")
    case .GHI: println("GHI foo")
    default: println("no foo")
    }

按原样与 ImplicitlyUnwrappedOptional 匹配:

    switch foo {
    case .Some(.ABC): println("ABC foo")
    case .Some(.DEF): println("DEF foo")
    case .Some(.GHI): println("GHI foo")
    default: println("no foo")
    }

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