有没有办法在Swift中设置关联对象？

Question

来自目标C的您可以objc_setAssociatedObject在2个对象之间调用函数以使它们维护一个引用，如果在运行时您不希望在删除其引用之前也不要破坏某个对象的话，这会很方便。迅捷有与此类似的东西吗？

Answer 1

它显示了如何向现有类添加新属性。它通过在扩展块中定义计算属性来实现。计算的属性存储为关联的对象：

import ObjectiveC

// Declare a global var to produce a unique address as the assoc object handle

private var AssociatedObjectHandle: UInt8 = 0

extension MyClass {

    var stringProperty:String {

        get {

            return objc_getAssociatedObject(self, &AssociatedObjectHandle) as! String

        }

        set {

            objc_setAssociatedObject(self, &AssociatedObjectHandle, newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)

        }

    }

}

编辑：

如果需要支持获取未初始化属性的值并避免发生错误unexpectedly found nil while unwrapping an Optional value，则可以按以下方式修改getter：

    get {

        return objc_getAssociatedObject(self, &AssociatedObjectHandle) as? String ?? ""

    }

Answer 2

该解决方案还支持所有值类型，不仅支持自动桥接的值类型，例如String，Int，Double等。

包装纸

import ObjectiveC

final class Lifted<T> {

    let value: T

    init(_ x: T) {

        value = x

    }

}

private func lift<T>(x: T) -> Lifted<T>  {

    return Lifted(x)

}

func setAssociatedObject<T>(object: AnyObject, value: T, associativeKey: UnsafePointer<Void>, policy: objc_AssociationPolicy) {

    if let v: AnyObject = value as? AnyObject {

        objc_setAssociatedObject(object, associativeKey, v,  policy)

    }

    else {

        objc_setAssociatedObject(object, associativeKey, lift(value),  policy)

    }

}

func getAssociatedObject<T>(object: AnyObject, associativeKey: UnsafePointer<Void>) -> T? {

    if let v = objc_getAssociatedObject(object, associativeKey) as? T {

        return v

    }

    else if let v = objc_getAssociatedObject(object, associativeKey) as? Lifted<T> {

        return v.value

    }

    else {

        return nil

    }

}

可能的 Class扩展（用法示例）

extension UIView {

    private struct AssociatedKey {

        static var viewExtension = "viewExtension"

    }

    var referenceTransform: CGAffineTransform? {

        get {

            return getAssociatedObject(self, associativeKey: &AssociatedKey.viewExtension)

        }

        set {

            if let value = newValue {

                setAssociatedObject(self, value: value, associativeKey: &AssociatedKey.viewExtension, policy: objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)

            }

        }

    }

}

Answer 3

显然，这仅适用于Objective-C对象。经过一些摆弄之后，以下是在Swift中进行调用的方法：

import ObjectiveC

// Define a variable whose address we'll use as key.

// "let" doesn't work here.

var kSomeKey = "s"

…

func someFunc() {

    objc_setAssociatedObject(target, &kSomeKey, value, UInt(OBJC_ASSOCIATION_RETAIN))

    let value : AnyObject! = objc_getAssociatedObject(target, &kSomeKey)

}