如何在Swift中创建一个指向自身的静态指针变量? [英] How to create a static pointer variable to itself in Swift?

问题描述

在Objective-C中，我经常使用使用static void*作为标识标签的模式.有时，这些标签仅在该函数/方法中使用，因此将变量放置在函数内部很方便.

In Objective-C I often use the pattern of using a static void* as an identification tag. At times these tags are only used within that function/method, hence it's convenient to place the variable inside the function.

例如:

MyObscureObject* GetSomeObscureProperty(id obj) {
    static void* const ObscurePropertyTag = &ObscurePropertyTag;
    MyObscureObject* propValue = objc_getAssociatedObject(id,ObscurePropertyTag);
    if(!propValue) {
        propValue = ... // lazy-instantiate property
        objc_setAssociatedObject(obj,ObscurePropertyTag,propValue, OBJC_ASSOCIATION_RETAIN_NONATOMIC)
    }
    return propValue; 
}

问题是，如何在Swift中自行编写ObscurePropertyTag私有常量指针? (最好是2.1，但将来已经发布的版本应该可以)

The question is, how to write the ObscurePropertyTag private-constant-pointer-to-itself in Swift? (Preferrably 2.1 but future already-announced versions should be okay)

我环顾四周，似乎我必须将此ObscurePropertyTag用作成员变量，并且似乎没有解决方法.

I've looked around and it seems that I have to put this ObscurePropertyTag as a member variable and there doesn't seem to be a way around it.

推荐答案

与(Objective-)C不同，您不能使用 Swift中的未初始化变量.因此，创建一个自引用 指针是一个两步过程:

Unlike (Objective-)C, you cannot take the address of an uninitialized variable in Swift. Therefore creating a self-referencing pointer is a two-step process:

迅速2:

var ptr : UnsafePointer<Void> = nil
withUnsafeMutablePointer(&ptr) { $0.memory = UnsafePointer($0) }

快捷键3:

var ptr = UnsafeRawPointer(bitPattern: 1)!
ptr = withUnsafePointer(to: &ptr) { UnsafeRawPointer($0) }

出于您的目的，将 global 变量的地址与&一起使用是否更容易，请参见 例子

For your purpose, is it easier to use the address of a global variable with &, see for example

• 有没有办法在Swift中设置关联对象?.

如果您想将标签"的范围限制为函数本身 那么您可以在本地struct中使用 static 变量.示例:

If you want to restrict the scope of the "tag" to the function itself then you can use a static variable inside a local struct. Example:

func obscureProperty(obj : AnyObject) -> MyObscureObject {
    struct Tag {
        static var ObscurePropertyTag : Int = 0
    } 
    if let propValue = objc_getAssociatedObject(obj, &Tag.ObscurePropertyTag) as? MyObscureObject {
        return propValue
    }
    let propValue = ... // lazy instantiate property value
    objc_setAssociatedObject(obj, &Tag.ObscurePropertyTag,propValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
    return propValue
}

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