DataGrid-根据其自身的值对列进行排序，其次基于其他列的值？

我有一个带有两列的AdvancedDatagrid：代码（字符串）和值（数字）。我对每一列使用相同的排序功能。我想做的是根据“值”列（数字数据）对两列进行排序，但是在没有可用数字的地方，我希望对“代码”列按字母顺序进行排序。

我用一个例子来简化我所面临的问题，以代表我想做的事情：

该图显示了两列，并基于“值”列对这两个列进行了排序。在值为NaN的情况下，我希望将“代码”列的值按字母顺序排序。因此，一，二，三，四将保持不变，但BADC将为ABCD。

<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
                layout="absolute" minWidth="955" minHeight="600" initialize="initializeHandler(event)">

    <mx:Script>
        <![CDATA[
            import mx.collections.ArrayCollection;
            import mx.events.FlexEvent;
            import mx.utils.ObjectUtil;

            [Bindable]
            private var ac:ArrayCollection;

            protected function initializeHandler(event:FlexEvent):void
            {
                ac = new ArrayCollection();

                var one:NameValueObject = new NameValueObject("One", 1);
                var two:NameValueObject = new NameValueObject("Two", 2);
                var three:NameValueObject = new NameValueObject("Three", 3);
                var four:NameValueObject = new NameValueObject("Four", 4);
                var a:NameValueObject = new NameValueObject("A", NaN);
                var b:NameValueObject = new NameValueObject("B", NaN);
                var c:NameValueObject = new NameValueObject("C", NaN);
                var d:NameValueObject = new NameValueObject("D", NaN);

                ac.addItem(one);
                ac.addItem(two);
                ac.addItem(three);
                ac.addItem(four);
                ac.addItem(b);
                ac.addItem(a);
                ac.addItem(d);
                ac.addItem(c);              
            }

            private function numericValueSort(obj1:Object, obj2:Object):int
            {
                var value1:Number = (obj1 as NameValueObject).value;
                var value2:Number = (obj2 as NameValueObject).value;

                return ObjectUtil.numericCompare(value1, value2);
            }

            private function codeLabelFunction(item:Object, column:AdvancedDataGridColumn):String
            {
                return (item as NameValueObject).code;
            }

        ]]>
    </mx:Script>

    <mx:AdvancedDataGrid width="500" height="300" dataProvider="{ac}">
        <mx:columns>
            <mx:AdvancedDataGridColumn id="codeColumn" 
                                       headerText="Code"
                                       dataField="value"
                                       labelFunction="codeLabelFunction"
                                       sortCompareFunction="numericValueSort">

            </mx:AdvancedDataGridColumn>
            <mx:AdvancedDataGridColumn id="numericValueColumn" 
                                       headerText="Value"
                                       dataField="value"
                                       sortCompareFunction="numericValueSort">

            </mx:AdvancedDataGridColumn>
        </mx:columns>
    </mx:AdvancedDataGrid>
</mx:Application>

NaveValueObject类

package
{
    public class NameValueObject
    {
        public var code:String;
        public var value:Number;

        public function NameValueObject(aCode:String, aNumber:Number)
        {
            code = aCode;
            value = aNumber;
        }
    }
}