How do I perform a multicolumn sort in a Spark DataGrid?

I have used sorting depends on the column click.

On click of location column I have used only that column data sorting.

When clicking on price column it sorts both columns.

I have attached two sample files for sorting the data in the datagrid.

Example : Using SDK 4.5

<?xml version="1.0" encoding="utf-8"?>

<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009" 
  xmlns:s="library://ns.adobe.com/flex/spark" 
  xmlns:mx="library://ns.adobe.com/flex/mx" minWidth="955" minHeight="600"
  creationComplete="application1_creationCompleteHandler(event)">
<fx:Declarations>
<!-- Place non-visual elements (e.g., services, value objects) here -->
</fx:Declarations>
<fx:Script>
<![CDATA[
import mx.collections.ArrayCollection;
import mx.collections.Sort;
import mx.collections.SortField;
import mx.events.FlexEvent;

import spark.events.GridEvent;

private var dataProvider : ArrayCollection = new ArrayCollection(
[{price:"20",location:"india"},
{price:"5",location:"india"},
{price:"40",location:"india"},
{price:"20",location:"germeny"},
{price:"20",location:"spain"},
{price:"10",location:"usa"},
{price:"30",location:"Italy"}]);


private var bool : Boolean = false;
protected function dg_clickHandler(event:GridEvent):void
{
///////////////////////////////////////// use this for individul column click sorting//////////////////////

var sort : Sort = new Sort();
if(event.columnIndex == 1)
{
var locSort : SortField = new SortField("location",true,bool);
sort.fields = [locSort];
}
else
{
var locSort : SortField = new SortField("location",true,bool);
var priceSort : SortField = new SortField("price",false,bool,true);
sort.fields = [locSort,priceSort];
}


dataProvider.sort = sort;
dataProvider.refresh();

bool = !bool;

//////////////////////////////////////////////// combine sorting end////////////////////////////////////////////

/////////////////////////////////////////////// use this for combine sorting////////////////////////////////

var locSort : SortField = new SortField("location",true,bool);
var priceSort : SortField = new SortField("price",false,bool,true);

var sort : Sort = new Sort();
sort.fields = [locSort,priceSort];

dataProvider.sort = sort;
dataProvider.refresh();

bool = !bool;

/////////////////////////////////////////individul sorting end//////////////////////////////////////////////
}


protected function application1_creationCompleteHandler(event:FlexEvent):void
{

dg.columnHeaderGroup.addEventListener(GridEvent.GRID_CLICK, dg_clickHandler);
}

]]>
</fx:Script>

<s:DataGrid id="dg" width="100%"  dataProvider="{dataProvider}">
<s:columns>
<s:ArrayList>
<s:GridColumn headerText="Price"  dataField="price" />
<s:GridColumn headerText="Location"  dataField="location"/>
</s:ArrayList>
</s:columns>
</s:DataGrid>
</s:Application>