Develop Asynchronous Web Parts

By default, custom web part uses synchronous mechanism. If in a web part page, there are many web parts, each web part takes some time to retrieve data from either web service or back end database. The overall performance of the web part page will likely be bad. All the web parts rendered in sequence. Second web part must wait until first web part finishes its rendering, third web part must wait until second web part finished its work.

The solution is to use asynchronous mechanism. Asynchronous support is built into the SharePoint web part framework and is very easy to use. You need to follow these steps to use asynchronous call.

1. Derive a custom Web Part class from base WebPart class in Microsoft.SharePoint.dll assembly.
2. Override method named GetRequiresData.
3. Override method named GetData
4. In GetRequiresData method return a value of true. It tells the Web Part framework to call your overridden version of GetData method.
5. In GetData method, register a custom callback method by calling a protected method of the WebPart class named RegisterWorkItemCallback. When calling RegisterWorkItemCallback, you must pass in two parameters—a System.Threading.WaitCallback delegate, which refers to a procedure that gets called when the work item is executed, and a parameter of type System.Object that you can use for providing any data that you want passed to the callback procedure when invoking the delegate. The System.Threading.WaitCallback delegate accept a parameter of type System.Object and no return (void) value.
   

Once you have registered a callback method, the Web Part framework will execute your callback method using a secondary thread.

Handling work item timeouts
By default, work items time out after 5000 milliseconds (5 seconds). Administrators can adjust this timeout period by editing the Timeout attribute of the WebPartWorkItem element in the Web.config file. The following entry creates a timeout period of 15 seconds:

<SharePoint>
<WebPartWorkItem Timeout="15000" />
</SharePoint>

The WebPart class has a RenderWorkItemTimeout method, which you can override to render HTML when the timeout period expires. If you do not override this method, then a default system error is rendered. Also, you can check the value of the Boolean WorkItemTimeout property of the WebPart class—if it is true then at least one pending work item has timed out.