[flexcoders] data binding question

[markflex2007]

Hi

I do not know why the data binding doesn't work.Please help me out.

<mx:Script>
        <![CDATA[
        
               private function getTotal():Number{
                
                  var cost1:Number= Number(cost1.text)
                  var cost2:Number= Number(cost2.text)
                  
                  
                  var cost : Number = cost1+cost2;
                  return cost;
                  
                }
                
          ]]>
 </mx:Script> 
                
                
                <mx:TextInput id="cost1"   />
                <mx:TextInput id="cost2"   />
        <mx:TextInput id="course_total"  text="{getTotal()}"/>

Thanks

Mark