Swift 中extension 设置属性

extension ADAlertController: ADAlertControllerPriorityQueueProtocol {
   
   private struct AssociatedKeys {
       static var alertPriority: Void?
       static var autoHidenWhenInsertSamePriority: Void?
   }
       
   public var alertPriority: ADAlertPriority? {
       get {
           let alertPriority = objc_getAssociatedObject(self, &AssociatedKeys.alertPriority) as? ADAlertPriority
           return alertPriority ?? ADAlertPriorityDefault
       }
       set {
           objc_setAssociatedObject(self, &AssociatedKeys.alertPriority, newValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
       }
   }
   
   public var autoHidenWhenInsertSamePriority: Bool? {
       get {
           return objc_getAssociatedObject(self, &AssociatedKeys.autoHidenWhenInsertSamePriority) as? Bool ?? false
       }
       set {
           objc_setAssociatedObject(self, &AssociatedKeys.autoHidenWhenInsertSamePriority, newValue, .OBJC_ASSOCIATION_ASSIGN)
       }
   }
}