Please Wait - Building a WaitScreen control for ASP.NET

Sometimes it happens that a form is processing and you need to make sure that the users don't panic and run away before it finishes.

A splash screen with a "Loading..." indicator can help to calm down frightened users and make life easier for technical support staff.

Back in the days of classic ASP (VBScript) which used a linear programming approach we had to start by setting the response buffer to true:

This line does nothing but instructing the server NOT to send anything back to the client until the page has been finished processing.

An exception can be forced by calling the flush command:

Calling flush lets the server send everything to the client that is processed so far.

This will also speed up your page since the server doesn't have to switch back and forth between executing the page and sending bits to the browser.

Using this we were able to accomplish the following steps:

Send e.g. a div-layer containing a "Loading..." graphic to the client
Process the long running task.
Send the results of the long running task to the client
Send a client side script to the client, that hides the div-layer containing the "Loading..." graphic

   1: <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "">
   2: <html xmlns="">
   3: ...
   4: <body>
   5: <div id="splashScreen">
   6: <img src="wait.gif" width="75" height="15" />
   7: </div>
   8: <script type="text/javascript"></script>
   9: </body>
  10: </html>

I haven’t mentioned yet that client side Javascript initially checks what kind of browser we have to deal with (either up level like Internet Explorer 7.0, other Internet Explorer’s or Netscape 4.x).

But with ASP.NET we have got a complete different way of how the page is executed. Today we have a Page class, which has an event based execution model and controls – so how can this mechanisms be used in ASP.NET?

The difference between classic ASP and ASP.NET, that initially seems to be a problem, is really an advantage. It gives us the ability to write and store our code in a more structured manor.

This way we can separate infrastructure code form application logic code. It lets us focus on the things we, as application developers, really need to do – To get things done.

First of all we need to do a few initial steps:

· Start Visual Studio (or use notepad.exe)

· Creating a new Project of the type “Class Library” (on skip this if you are using notepad)

· Rename Class1.cs to WaitScreen.cs (or just save your file as WaitScreen.cs using notepad)

· Add a reference to the namespace “System.Web” (or remember to add the compiler switch /r pointing to the assembly System.Web.dll)

After our environment has been set up we can start writing the code. First we outline the class to use it as a WebControl by deriving it from the WebControl class.

   1: using System;
   2: using System.ComponentModel;
   3: using System.Web.UI;
   4: using System.Web.UI.WebControls;
   6: namespace devcoach.Web.UI.Controls
   7: {
   8:     /// <summary>
   9:     /// The WaitScreen control displays a grafic while a long running operation 
  10:     /// is running.
  11:     /// </summary>
  12:     public class WaitScreen : WebControl
  13:     {
  14:     }
  15: } 

Controls are reusable components so it is important to let the client side developer (the guy who uses the control in Visual Web Developer – this might also be you…) the opportunities to customize the layout of the control. In our case the only customization that can be done would be a different graphic that is displayed during the long running operation is executed on the server. Here is the code that allows the client side developer to easily set a URL, pointing to an graphics file, form an attribute on the ServerContol or by directly setting the value of the property from the code behind.

   1: // This field holds the URL pointing to an image
   2: private string m_ImageUrl = "~/images/busy.gif";

The private field m_ImageUrl is initialized with a default image on the instantiation of the class so – if the image exists – the client side developer mustn’t explicitly set anything.

   1: /// <summary>
   2: /// Gets/sets the URL pointing to an image.
   3: /// A string containing the URL pointing 
   4: /// to an image..
   5: /// This property gets/sets the URL pointing to an image.
   6: /// </summary>
   7: [Description("Gets/sets the URL pointing to an image.")]
   8: public string ImageUrl
   9: {
  10:     get { return m_ImageUrl; }
  11:     set { m_ImageUrl = value; }
  12: } 

The property ImageUrl just gives public access to the private field m_ImageUrl. Having (again) the client side developer in mind we provide the XML comments summary, value and remarks and additionally set the description attribute, which allows Visual Studio to display details in the properties window.

Delegates and events are one of the most powerful tools that come with .NET Framework and we can use them to let the client developer attach his/her custom long running operation as custom event handler code. Even more then one method can be attached so that all together are the “one long running task” that is processed while the graphic is displayed. Separated nicely from the infrastructure code (the control we are currently writing). Here is our event code:

   1: public delegate void ProcessHandler(object sender, EventArgs e);
   3: public event ProcessHandler Process;

The OnProcess method triggers the event and invokes the attached custom long running operations.

Now that the ability is given that custom code can be executed using the event/delegate we need to actually raise the event and before that ensure that the response buffer is set to true and the graphic is send to the client. The load event of out control does this job for us.

   1: /// <summary>
   2: /// Triggers the Load event.
   3: /// </summary>
   4: protected override void OnLoad(EventArgs e)
   5: {
   6:     base.OnLoad(e);
   8:     Page.Response.Buffer = true;
  10:     #region Show splash screen
  11:     Page.Response.Write("...");
  12:     #endregion
  14:     Page.Response.Flush();
  16:     OnProcess();
  18:     Page.Response.Flush();
  20:     #region Hide splash screen
  21:     Page.Response.Write("...");
  22:     #endregion
  23: }

After OnProcess is called we call Flush like we did in classic ASP and send the piece of client side script to the client that will hide the div-layer containing the graphic indicating the user that something is happening.

On the client side the control can now be used as follows in the *.aspx-Page:

   1: <%@ Page Language="C#" AutoEventWireup="true"  CodeFile="Default.aspx.cs" Inherits="_Default" %>
   2: <%@ Register Assembly="devcoach.Web.WaitScreen" Namespace="devcoach.Web.UI.Controls" TagPrefix="ctrl" %>
   3: <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "">
   5: <html xmlns="" >
   6: <head runat="server">
   7:     <title>Untitled Page</title>
   8: </head>
  10: <body>
  11:     <form id="form1" runat="server">
  12:     <div>
  14:     <ctrl:WaitScreen ID="WaitScreen" runat="server" OnProcess="WaitScreen_Process" />
  16:     </div>
  17:     </form>
  18: </body>
  19: </html>

And the associated code behind page:

   1: using System;
   2: using System.Threading;
   3: using System.Web.UI;
   5: public partial class _Default : Page 
   6: {
   7:     protected void WaitScreen_Process(object sender, EventArgs e)
   8:     {
   9:         // Put long running operation in here...
  10:         Thread.Sleep(4000);
  11:     }
  12: }

Design-time isn’t as easy as I would wish and therefore the functionality in this specific area is often kept short or even missing. One of the biggest problems is the missing HttpContext. Without it we cannot ask for example for the path of the current page, web environment variables, URL-parameters and so on…

Our control is really a good example to show how we can add this kind of functionality because it is quite simple. The goal is to show the image set by the control default or the client side developer in the designer of Visual Web Developer.

So we add a new class to our project and name it “WaitScreenDesigner.cs”. To access the built-in designer functionality we need to add reference to the “System.Design” assembly and a using declaration to the System.Web.UI.Design namespace on top of our control class. We also need to derive it from the ControlDesigner-class:

   1: /// <summary>
   2: /// The designer for the wait screen control.
   3: /// </summary>
   4: public class WaitScreenDesigner
   5:     : ControlDesigner
   6: {
   7:     /// <summary>
   8:     /// Retrieves the HTML markup that is used to represent 
   9:     /// the control at design time.
  10:     /// </summary>
  11:     /// <returns>
  12:     /// The HTML markup used to represent the control at design time.
  13:     /// </returns>
  14:     public override string GetDesignTimeHtml()
  15:     {
  16:         ...
  17:     }
  18: }

The designer of Visual Web Developer calls the method GetDesgnTimeHtml on the base class to generate the Html to display – If we override that method we can control what the designer shoes to the client side developer of our controls.

To display the defined image we need to access the property of our control from the designer’s class’s method. Visual Studio’s extensibility API’s enable us to cast form the Component property of the designer to our control and easily read values of properties. The following example gets the value of the control’s class property ImageUrl:

   1: string imageUrl = ((WaitScreen)this.Component).ImageUrl;

Now that we have the set path to an image we should be able to display the image in an <img>-Tag except the URL contains the useful „~/“, which tells ASP.NET that the path starts with the root path of the current application. To translate such an URL to a physical path that we can use we need an instance of the web application – as the control this can be achieved though casting with the Component property of the designer.

   1: IWebApplication webApp = 
   2:     (IWebApplication)Component.Site.GetService(
   3:         typeof(IWebApplication));

The next step is to find the image as project item of our web project. The IWebApplication interface provides the method GetProjectItemFromUrl, which fits our needs.

   1: IProjectItem item = webApp.GetProjectItemFromUrl(imageUrl);

On the interface IProjectItem we will now find a property called PhysicalPath that we can use as value for the src attribute of an <img>-Tag, which will be returned form the GetDesignTimeHtml method:

   1: return string.Concat(
   2:     "<img src=\"",
   3:     item.PhysicalPath,
   4:     "\" alt=\"Please wait...\" />");

Now its time to tell the Designer that our control should be rendered with our designer class we just created. This is done by assigning the DesignerAttribute to the control class.

   1: [Designer(typeof(WaitScreenDesigner))]
   2: public class WaitScreen : WebControl
   3: {
   4:     ...
   5: }

If you now look at the designer the page should look like the following screenshot:

To make it even more comfortable the control can be add to the Toolbox. But if you don’t want your control to be displayed as the “default 3rd party control grind” we still have one job to do.

Add a 16x16 pixel icon file names WaitScreen.ico to the root of the project as embedded resource. And add the following attribute to the control class:

   1: [ToolboxBitmap(
   2:     typeof(WaitScreen), 
   3:     "devcoach.Web.UI.Controls.WaitScreen.ico")]
   4: public class WaitScreen : WebControl
   5: {
   6:      ...
   7: }

The long name is in the format {default_namespace_of_project}.{filename_and_ext} just in case you wonder where the devcoach.Web.UI.Controls.” comes from.

Now you’ll see your icon instead of the “default 3rd party control grind”.


Comments (17) -

  • Great app.  Do you by any chance have any more samples that might show how to do a staged update.  That is, lets you show different messages as the messages moves through, (like putting up a progress bar in pieces).

  • Thanks for the article - well done.

    I'm glad to see these step by step tutorials on controls that use javascript
  • Hey,

    Does this control support masterpages? I would like for this control to show up in a certian cell of my masterpage. What I think is happening is that this control is executed before the masterpage is processed therefor when the control shows up, it does not include any of the masterpage that make it up. (It gives a white page with only the image. Maybe Im rookie, but I would appreciate the
  • Nice control, but if you try it in a page with some contents before and after, you will notice that something doesn't work as expected.

    The problem is that all the code that manage the process is put inside the load event.
    At this point the other's page contents is not ready to be rendered and the Response.Flush doesn't affect what we view on the browser.

    I've tried to move the code from the Loa
  • Since your code is executing inside an instance of a control, you can replace:

    new Control().ResolveUrl(m_ImageUrl)


  • Hi, this is an excellant control, just what I was looking for for a sometimes slowly responding application, where the operators often pressed buttons multiple times not sure if they had successfully pressed the first time.

    I found one small issue with the use of the div tag, was that by only "hiding" the visiibility it pushed all of my subseqeunt page down by the size of the busy.gif.
  • Much appreciated control, Daniel.

    What if I would like to use it with Visual Studio .NET 2003 (as far as I can see in the download zip there is a VS 2005 Solution)?

    Thank you in advance.
  • Thanks for all the great feedback!

    @Pure Krome

    I dont like the idea of throwing an exception inside the designer since the control is displayed as an error in this cases.
    My solution is:

    IWebApplication webApp =
    if (webApp == null)
        return "[WaitScreen]";

    @Omar Armenteros

    Yes Smile

    @Luca mauri

    Don't yo
  • Hi. I love this control. Two questions: Are there limits on how many threads can be used on the server? I'd read somewhere that the limit was 25.

    Also, what component are you using to generate the robot-preventer when users are adding comments? I've been looking for one of these and have had no luck finding one.
  • Nice control, but it has an issue when "long processing" time is greater than server or client timeout. The better sollution could be using multithreading on a server-side and refresh meta tag on a client. I have not done this by myself, but Windows SharePoint Services admin tool probably use this approach.
  • Jon
    Thanks for this code.

    I tried to port it to VB.NET (Framework 1.1), but have one major problem: the HTML code is sent to the browser correctly (unfortunately before any body or head tags, which might cause problems with some non-IE-browsers, but for my project this isn't necessary, so i don't care about this one).

    But: the image isn't displayed until the whole page is sent to the client. It seem
  • I would suspect the image would not be shown since usually images in an html document are seperate requests to the server.
  • AC
    Great control... thanks!

    How hard would it be to actually add subcontrols to the WaitScreen? For example (replacing brackets to get the comment to post):
    put some stuff here

    Then if the WaitScreen was empty or had no contents, it would show the busy image... otherwise it would show the contents you added? This would make it much more dynamic.

  • Another problem I had is when an exception is thrown I do a Respone.Redirect. If an exception is thrown while processing the 'long' procedure, I get an exception that a redirect is not allowed because the buffer is flushed...
  • What would be a good approach to add the capability to cancel during the wait process?
  • i like ajax for update process notification even more... but this implementation is pretty interesting !
  • I am not certain where you're getting your info, but good topic. I needs to spend a while learning more or figuring out more. Thank you for excellent information I was looking for this info for my mission.

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