We all know the Extenders of the AjaxControlToolkit implement their Client Partial-Refresh action by using their Client behaviors which are generated after they are rendered. In addition, the client classes provide many useful APIs that enable you to bind to events and to provide handlers for those events.

For example, the AutoComplete Extender's client behavior document would like this:

The AutoCompleteExtender

Code
        <AjaxControlToolkit:AutoCompleteExtender ID="AutoCompleteExtender1" runat="server"
            TargetControlID="TextBox2" ServicePath="AutoComplete1.asmx" ServiceMethod="GetCompletionListByRandom"
            MinimumPrefixLength="2" CompletionInterval="10" EnableCaching="true" OnClientPopulated="acePopulated"
            OnClientItemSelected="IAmSelected" />

Its Client Behaviior Document

You may notice that we simply used this method "$find(the Extender's BehaviorID)" to get the Client Behavior Document. If the BehaviorId is not defined, we can use this "$find(<%=the Extender's ClientID%>)".

But sometimes things are not so simple. If we place the Extender inside a DataBind Control's ItemTemplete, such as GridView. The same method would raise error which sames like this:

That's because the BehaviorId shoule be unique.

Then, how can we get the Behaviors in this situation?

Actually, every behavior has been cached into the Sys.Application's Components object after the page client initialize phase in the Life-Cycle. When script in a load event handler runs, all scripts and components have been loaded and are available. That's the key to achieve our goal.

Code
        function pageLoad() {
        //If any Extender is placed in the DataBind Control, it's hard to define the BehaviorId and get the Client behavior. 
        //We can use the method to find all the correct type behaviors.
            var currentBehavior = null;
            var allBehaviors = Sys.Application.getComponents();
            for (var loopIndex = 0; loopIndex < allBehaviors.length; loopIndex++) {
                currentBehavior = allBehaviors[loopIndex];
                if (currentBehavior.get_name() == "AutoCompleteBehavior") {
                    // Now we get the ClientBehavior here: currentBehavior!
                }
            }
        }