Flex tree 限制节点同级间拖拽

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<?xml version="1.0" encoding="utf-8"?> <mx:Application xmlns:mx="http://www.adobe.com/2006/mxml">        <mx:Script>    <![CDATA[                      import mx.events.DragEvent;        import mx.managers.DragManager;         import mx.core.DragSource;             import mx.core.UIComponent;             import mx.controls.Tree;                 private var _draggedItem:XML = null;      private function onDragEnter( event:DragEvent ) : void       {                    event.preventDefault();           event.currentTarget.hideDropFeedback(event);     var ds:DragSource = event.dragSource;            var items:Array = ds.dataForFormat("treeItems") as Array;     if (items != null && items.length > 0 && (items[0] is XML))        _draggedItem = items[0];                          DragManager.acceptDragDrop(UIComponent(event.currentTarget));        }                      private function onDragOver( event:DragEvent ) : void       {                    event.preventDefault();                   event.currentTarget.hideDropFeedback(event);           tree.selectedIndex = tree.calculateDropIndex(event);       var node:XML = tree.selectedItem as XML;                    // restrict drag & drop to nodes within same parent        if (_draggedItem.parent() != node.parent())                 {                      DragManager.showFeedback(DragManager.NONE);          return;             }                         DragManager.showFeedback(DragManager.MOVE);   }          private function onDragDrop( event:DragEvent ) : void       {                   event.preventDefault();                event.currentTarget.hideDropFeedback(event);        tree.selectedIndex = tree.calculateDropIndex(event);    var node:XML = tree.selectedItem as XML;                           var addToIndex:int = node.childIndex();            if ((_draggedItem.parent() == node.parent()) && (addToIndex != _draggedItem.childIndex()))             {      tree.dataDescriptor.removeChildAt(node.parent(), _draggedItem, _draggedItem.childIndex());          tree.dataDescriptor.addChildAt(node.parent(), _draggedItem, addToIndex);               }          }                       private function onDragComplete( event:DragEvent ) : void              {                   tree.selectedIndex = -1;         }               ]]>      </mx:Script>       <mx:XML id="treeData" xmlns="">     <root>             <node label="Massachusetts" type="state" data="MA">    <node label="Boston" type="city" >             <node label="Smoke House Grill" type="restaurant" />      <node label="Equator" type="restaurant" />              <node label="Aquataine" type="restaurant" />               <node label="Grill 23" type="restaurant" />              </node>                <node label="Provincetown" type="city" > <node label="Lobster Pot" type="restaurant" />             <node label="The Mews" type="restaurant" />             </node>           </node>      <node label="California" type="state" data="CA">   <node label="San Francisco" type="city" >        <node label="Frog Lane" type="restaurant" />        </node>        </node>      </root>   </mx:XML>   <mx:Tree width="100%" height="100%" id="tree"    labelField="@label"   dataProvider="{treeData.node}"      allowMultipleSelection="false"   dragEnabled="true"      dropEnabled="true"       dragMoveEnabled="false" dragEnter="onDragEnter(event)"      dragOver="onDragOver(event)"      dragDrop="onDragDrop(event)"      dragComplete="onDragComplete(event)"/>  </mx:Application>