Ans:

IResponder interface provides the contract for any service that needs to respond to remote or asynchronous calls.

For example:

<?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" initialize="init();">
<fx:Declarations>
<s:RemoteObject id="contryremote" destination="countryManager"/> 
</fx:Declarations>
<fx:Script> 
<![CDATA[ 
import mx.collections.ArrayCollection;
import mx.rpc.AsyncResponder;
import mx.rpc.AsyncToken;
import mx.controls.Alert;
import mx.rpc.events.ResultEvent;
import mx.rpc.events.FaultEvent;
import mx.rpc.IResponder;
public var token:AsyncToken;
[Bindable]
public var countryList:Object;
public function init():void{ 
var countrylist:ArrayCollection = new ArrayCollection(); 
token=contryremote.getCountryList();
token.addResponder(new AsyncResponder(eventhandler, InflboxFaultEvent, null));
} 
public function eventhandler(event:ResultEvent, token:Object=null):void {
countryList= (ArrayCollection)(event.result);
}
public function InflboxFaultEvent (event:FaultEvent, token:Object=null):void {
Alert.show("Error Data Loding");
}
]]> 
</fx:Script>
<s:Panel title="RemoteObject Example" 
width="356" height="265" 
chromeColor="#555555" 
backgroundColor="#000000" color="#FFFFFF">
<mx:DataGrid dataProvider="{countryList}" width="323" 
height="201" x="15" y="15" color="#000000"/>
</s:Panel>
</s:Application>