datagrid 套combobox

<mx:DataGridColumn headerText="任课教师" dataField="teachername">
		<mx:itemRenderer>
			<mx:Component>
				<mx:HBox> 
        			<mx:Script> 
    	 				 <![CDATA[
						      	  import dto.User; 
						          public function get value():Object 
						          { 
						      		  if(cmb.selectedItem == null)return null; 
						        			var teacherDTO :User = cmb.selectedItem  as User; 
						        			var teachername:String=teacherDTO.username;
						        			data.teachername=teachername;
						       			 return teacherDTO; 
						          } 
						
						          override public function set data(value:Object):void 
						          { 
						      			  //必须要给data复制，否则会导致程序崩溃 
						      			  super.data=value; 
						      				  //给combobox赋初始值 
						       				 for each(var teacherDTO :User in cmb.dataProvider) 
						       				 { 
							         				var teacher: String =teacherDTO.username; 
							           			 if(teacher == value.teachername){  //value DataGrid上面的当前行的数据 
							         					 cmb.selectedItem=teacherDTO; 
							           			} 
						       				 } 
						          } 
    					 ]]> 
        			</mx:Script> 
					        <mx:ComboBox id="cmb" labelField="username" dataProvider="{outerDocument.teacherArray}" close="change()" width="100%">
						        	 <mx:Script>
					        	 	<![CDATA[
					        	 		protected function change():void{
					        	 			data.teachername=cmb.selectedItem.username;
					        	 		}
					        	 	]]>
					        	 </mx:Script> 
				        	</mx:ComboBox>
    					</mx:HBox> 
					</mx:Component>
				</mx:itemRenderer>
		</mx:DataGridColumn>