ASP.Net Output Once Control

This is an article about outputting data to the browser only once, specifying where it should be outputted. The result is a control that can write out a single script to the header of the .aspx file (as an example). This control can be downloaded at outputonce.zip.

While programming for the web I often find the need to have some output sent to the client only once. A simple example is to include a javascript file – or function – when it is required. This output need usually arises while developing (dynamic) usercontrols.

Problem setting
1. Imagine a couple of usercontrols using the prototype.js file. The usercontrols are not always loaded and there is thus no need to include this file always and everywhere (don’t include it in the masterpage).

2. Imagine a repeater with formfields and some dynamic calculations using a javascripted function. When the repeater does not contain any data – there is no need to include the javascript function.

Solution
As an attempt to fix these issues I created a small control named OutputOnce. This control makes use of a property called OutputKey and the Page.ClientScript.IsClientScriptBlockRegistered and Page.ClientScript.RegisterClientScriptBlock functions to ensure that the script will only be rendered once. Additionally the control contains an OutputLocation property to specify if the data should be rendered INLINE or in the HEAD.

The ASP.Net code will look like

   1:  <uc:OutputOnce 

   2:      ID=”outputonce” 

   3:      OutputKey=”prototype” 

   4:      OutputLocation=”Head” 

   5:      runat=”server”>

   6:          <script type=”text/javascript” src=”prototype.js”></script>

   7:  </uc:OutputOnce>