清除預設的DragEvent外觀效果
針對Flex 預設的Drag-Drop，List元件會貼心地幫你畫出indicator(預設是一條線)，然而，一旦試著用 DragEvent#preventDefault()時，indicator 會殘留在畫面上，此時，可以利用
ListBase#hideDropFeedback(event:DragEvent)
即可清除掉indicator, highlight 等 DisplayObject.
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To clean up Flex default DragDrop visualization, try ListBase#hideDropFeedback(event), Especially when implementing your own drag-drop event handlers. This method remove all highlight and indicator display objects.
Tags: DragDrop