ASP.NET UpdatePanel and JavaScript functions

I would like to call JavaScript functions to show/hide a loading animation when a DropDownList is changed in an UpdatePanel.

Here is the DopDownList in an Updat开发者_C百科ePanel

<asp:UpdatePanel ID="UpdatePanel4" runat="server">
  <ContentTemplate>
    <asp:DropDownList ID="ddl_size" runat="server" OnSelectedIndexChanged="ddl_size_SelectedIndexChanged" AutoPostBack="true">/asp:DropDownList>
    ...

Here are the JavaScript functions I want to call

function ShowLoadingAnimation() { ... }
function HideLoadingAnimation() { ... }

Do I use triggers? How do I use it? Thanks in advance.

You can use PageRequestManager's beginRequest and endRequest handlers to show/hide animation. The implementation details are on Sys.WebForms.PageRequestManager beginRequest Event and Walkthrough: Animating ASP.NET UpdatePanel Controls MSDN pages.

Put this code in the JavaScript tag in your aspx page:

Sys.WebForms.PageRequestManager.getInstance().add_endRequest(endRequestHandler); 
function endRequestHandler() { yourJsFunction }