Bindable Flex中Bindable的原理

 [Bindable]        private var bind_String:String="hi";

    [Bindable(event="propertyChange")]
         private var bind_String:String="hi";
         
         private function onChange():void
         {
              var oldValue:String = bind_String;
              bind_String = "hello";
             if(bind_String!==oldValue) {
                 this.dispatchEvent(PropertyChangeEvent.createUpdateEvent(this,
                 "bind_String", oldValue, bind_String));