swift 关联属性

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private var key: Void?

var title: String? {
        get {
            return objc_getAssociatedObject(self, &key) as? String
        }
        set {
            objc_setAssociatedObject(self, &key, newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }
    }

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    // 嵌套结构体
       private struct AssociatedKeys {
           static var isShowKey = "isShowKey"
           static var displayNameKey = "displayNameKey"
           static var widthKey = "widthKey"
       }