swift实现setter和getter的方法

swift_setter_getter.swift

private var key: Void?
extension MAPolyline {
    var isWalk: Bool? {
        get {
            return objc_getAssociatedObject(self, &key) as? Bool
        }

        set {
            objc_setAssociatedObject(self,
                                     &key, newValue,
                                     .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }
    }
}