DataGridColumn的属性sortCompareFunction和sortDescending是点击列头后的排序方式，而要确定默认情况下的排序方式是无效的。那么怎么办？
查看datagrid的官方代码就会知道如何给datagrid默认排序，下面是官方代码：
?private function updateSortIndexAndDirection():void
??? {
??????? // Don't show sort indicator if sortableColumns is false or if the
??????? // column sorted on has sortable="false"
??????? if (!sortableColumns)
??????? {
??????????? lastSortIndex = sortIndex;
??????????? sortIndex = -1;
??????????? if (lastSortIndex != sortIndex)
??????????????? invalidateDisplayList();
??????????? return;
??????? }
??????? if (!dataProvider)
??????????? return;
??????? var view:ICollectionView = ICollectionView(dataProvider);
??????? var sort:Sort = view.sort;
??????? if (!sort)
??????? {
??????????? sortIndex = lastSortIndex = -1;
??????????? return;
??????? }
??????? var fields:Array = sort.fields;
??????? if (!fields)
??????????? return;
??????? if (fields.length != 1)
??????? {
??????????? lastSortIndex = sortIndex;
??????????? sortIndex = -1;
??????????? if (lastSortIndex != sortIndex)
??????????????? invalidateDisplayList();
??????????? return;
??????? }
??????? // fields.length == 1,so the collection is sorted on a single field.
??????? var sortField:SortField = fields[0];
??????? var n:int = _columns.length;
??????? sortIndex = -1;
??????? for (var i:int = 0; i < n; i++)
??????? {
??????????? if (_columns[i].dataField == sortField.name)
??????????? {
??????????????? sortIndex = _columns[i].sortable ? i : -1;
??????????????? sortDirection = sortField.descending ? "DESC" : "ASC";
??????????????? return;
??????????? }
??????? }
??? }
?
可以这样实现：
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute" creationComplete="init()">
??? <mx:Script>
??? <![CDATA[
??? import mx.collections.SortField;
??? import mx.collections.Sort;
??? import mx.utils.ObjectUtil;
??? import mx.collections.ArrayCollection;
??? [Bindable]
??? private var adg:ArrayCollection = new ArrayCollection([{name:"张三华",age:15,sex:"男"},
??? ?????????????????????????????????????????????????????? {name:"李四国",age:45,
??? ?????????????????????????????????????????????????????? {name:"王七花",age:24,sex:"女"},
??? ?????????????????????????????????????????????????????? {name:"赵华柳",age:17,
??? ?????????????????????????????????????????????????????? {name:"田三华",age:33,]);
??? ??????????????????????????????????????????????????????
??? ??????????????????????????????????????????????????????
??? private function init():void{
??? adg.sort = new Sort();
??? adg.sort.fields = [new SortField("age",false,true)];
??? adg.refresh();
??? dg.dataProvider = adg;
??? }
??? private function sortRanges(obj1:Object,obj2:Object):int{
??? var value1:Number = obj1.age;
??? var value2:Number = obj2.age;
??? if(value1 > value2 ){
??? return 1;
??? }else if(value1 < value2 ){
??? return -1;
??? }else{
??? return 0;
??? }
??? }????????????
??? private function dataFilter():void{
??? if(adg.filterFunction == null){
??? adg.filterFunction = dataFilter2;
??? }
??? adg.refresh();
??? }??????????
??? private function dataFilter2(item:Object):Boolean{
??? var bool:Boolean = true;
??? var it:String = item["name"];
??? var filter:String = ti.text;
??? if(!it || it.toLowerCase().indexOf(filter.toLowerCase()) < 0){
??? bool = false;
??? }
??? return bool;
??? }??????
??? private function sortCompare(obj1:Object,obj2:Object):int{
??? return ObjectUtil.numericCompare(obj1.age,obj2.age);
??? }????????????????
??? ]]>
??? </mx:Script>
??? <mx:DataGrid id="dg"? x="89" y="46" width="552" height="160">
??? <mx:columns>
??? <mx:DataGridColumn? headerText="name" dataField="name"/>
??? <mx:DataGridColumn headerText="age" dataField="age"/>
??? <mx:DataGridColumn headerText="sex" dataField="sex" />
??? </mx:columns>
??? </mx:DataGrid>
??? <mx:Form>
??? <mx:FormItem>
??? <mx:TextInput id="ti" change="dataFilter()"/>
??? </mx:FormItem>
??? </mx:Form>
</mx:Application>
重点是黑体字部分，SortField第一个参数是排序的字段，第二个参数用来确定是否区分大小写，第三个参数表示是否按降序排列。?
?参考官方SortField构造函数就明白了：
?/**
???? *? Constructor.
???? *
???? *? @param name The name of the property that this field uses for
???? *????????????? comparison.
???? *????????????? If the object is a simple type,pass <code>null</code>.
???? *? @param caseInsensitive When sorting strings,tells the comparitor
???? *????????????? whether to ignore the case of the values.
???? *? @param descending Tells the comparator whether to arrange items in
???? *????????????? descending order.
???? *? @param numeric Tells the comparitor whether to compare sort items as
???? *????????????? numbers,instead of alphabetically.
???? *?
???? *? @langversion 3.0
???? *? @playerversion Flash 9
???? *? @playerversion AIR 1.1
???? *? @productversion Flex 3
???? */
?public function SortField(name:String = null,
????????????????????????????? caseInsensitive:Boolean = false,
????????????????????????????? descending:Boolean = false,
????????????????????????????? numeric:Object = null)
??? {
??????? super();
??????? _name = name;
??????? _caseInsensitive = caseInsensitive;
??????? _descending = descending;
??????? _numeric = numeric;
??????? _compareFunction = stringCompare;
??? }
?
?
?
下面补充： 点击DataGridColumn列头后的排序方式的原因（官方代码），关注属性sortCompareFunction和sortDescending
?? /**
???? *? @private
???? */
??? private function headerReleaseHandler(event:DataGridEvent):void
??? {
??????? if (!event.isDefaultPrevented())
??????? {
??????????? manualSort = true;
??????????? sortByColumn(event.columnIndex);
??????????? manualSort = false;
??????? }
??? }
??? /**
???? *? @private
???? */
??? private function sortByColumn(index:int):void
??? {
??????? var c:DataGridColumn = columns[index];
??????? var desc:Boolean = c.sortDescending;
??????? // do the sort if we're allowed to
??????? if (c.sortable)
??????? {
??????????? var s:Sort = collection.sort;
??????????? var f:SortField;
??????????? if (s)
??????????? {
??????????????? s.compareFunction = null;
??????????????? // analyze the current sort to see what we've been given
??????????????? var sf:Array = s.fields;
??????????????? if (sf)
??????????????? {
??????????????????? for (var i:int = 0; i < sf.length; i++)
??????????????????? {
??????????????????????? if (sf[i].name == c.dataField)
??????????????????????? {
??????????????????????????? // we're part of the current sort
??????????????????????????? f = sf[i]
??????????????????????????? // flip the logic so desc is new desired order
??????????????????????????? desc = !f.descending;
??????????????????????????? break;
??????????????????????? }
??????????????????? }
??????????????? }
??????????? }
??????????? else
??????????????? s = new Sort;
??????????? if (!f)
??????????????? f = new SortField(c.dataField);
??????????? c.sortDescending = desc;
??????????? var dir:String = (desc) ? "DESC" : "ASC";
??????????? sortDirection = dir;
??????????? // set the grid's sortIndex
??????????? lastSortIndex = sortIndex;
??????????? sortIndex = index;
??????????? sortColumn = c;
??????????? // if you have a labelFunction you must supply a sortCompareFunction
??????????? f.name = c.dataField;
??????????? if (c.sortCompareFunction != null)
??????????? {
??????????????? f.compareFunction = c.sortCompareFunction;
??????????? }
??????????? else
??????????? {
??????????????? f.compareFunction = null;
??????????? }
??????????? f.descending = desc;
??????????? s.fields = [f];
??????? }
??????? collection.sort = s;
??????? collection.refresh();
??? }
