Now that we've seen what the ScriptManager is capable of, let's see how it handles the errors or exceptions that can occur during an asynchronous post back. By default, the ScriptManager traps exceptions and displays them as a message box. This may be not be the best way to convey that information to the user so the developer might want to replace it with one that's more user friendly. ScriptManager provides a way for a developer to do some error handling by defining an OnPageError event handler and assigning a custom error message .The error message can be displayed as an overlay over the page by defining an ErrorTemplate. At a minimum, this error template should contain a span or div element with the pre-set id errorMessageLabel and a button used to dismiss the error message. Before we round up our discussion about the ScriptManager, there's one more control, the ScriptManagerProxy that's a close relative. It's used in case the page already has an instance of the ScriptManager control and one of the controls needs to add a script or a service reference. It can also be used in a scenario where the Master page has a ScriptManager control already defined and the content page needs to add scripts or services.

UpdatePanel
After the script manager, the UpdatePanel is the next most important Atlas server control. It lets the developer divide the page into different sections and lets each section be updated independently without refreshing the entire page. This functionality translates straightaway into better user interactions; partial refreshes are less disruptive than regular full page refreshes. In reality, the UpdatePanel generates a post back similar to the regular post back and in fact most of the page events are triggered.

The Page.IsPostback even returns a true value for an update panel post back. Where the UpdatePanel post back differs from a normal post back in the page lifecycle is in the render event. In the case of an UpdatePanel post back, only those regions defined by the UpdatePanel are rendered and sent back to the client.

On the client side, the child controls of the UpdatePanel are replaced with the new content. This is known as partial rendering. For an UpdatePanel to work in partial rendering mode, it requires a ScriptManager with the EnablePartialRendering attribute set to true.

<atlas:UpdatePanel runat="server" ID="UpdatePanel2" Mode="Always">
   <ContentTemplate>
      <asp:label runat="server" Text="Keep changing me!" ID="Label1" />
   </ContentTemplate>
</atlas:UpdatePanel>

Now that we're familiar with what partial rendering is, let's look at the UpdatePanel control in detail .The code snippet above shows the markup for an UpdatePanel. The UpdatePanel exposes a ContentTemplate. Any content that needs to be replaced by partial rendering needs to be inside the ContentTemplate. The UpdatePanel also has an attribute Mode that defines when the UpdatePanel content would be refreshed. Setting it to Always refreshes the content during each and every post back. If this isn't the case and the UpdatePanel is to be refreshed in response to certain events, it's possible by setting the value of the Mode attribute to conditional. After that, set the trigger that would cause the update. The UpdatePanel can accept a collection of triggers that would cause an update. These triggers are classified into two types: