swift 扩展属性的方法

原理：由于swift扩展不能添加存储属性，所以考虑以计算属性获取另一对象的存储属性，该对象通过动态绑定给原对象

1，创建协议 包含中间对象，两个用于绑定的key

protocol AssociateObject {

    var newInstance: AnyObject {get}

    static var key:UnsafeRawPointer { get }      //key不要用String类型

    static var midKey:UnsafeRawPointer { get }

}

2，创建空类，用于生成中间对象

3，协议扩展，实现中间对象，并绑定

extension AssociateObject {

    var newInstance: AnyObject {

        guard let obj = objc_getAssociatedObject(self, Self.key) else {

            let newInstance = NewClass.init()

            objc_setAssociatedObject(self, Self.key, newInstance, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)

            return newInstance

      }

        return obj as AnyObject

  }

}

4，类的扩展遵守协议，并给出两个key，然后把需要的属性绑定给中间对象

extension UIView:AssociateObject {

    static var key = UnsafeRawPointer("UIViewEx")

    static var midKey: UnsafeRawPointer = UnsafeRawPointer("UIViewMidKey")

    varviewName:String{

        get{

            guardletvalue =objc_getAssociatedObject(newInstance, &UIView.midKey)  else{

                returnString()

            }

            returnvalueas!String

        }

        set{

            objc_setAssociatedObject(newInstance, &UIView.midKey, newValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)

        }

    }

}