SiteBannerManager - Event Raiser

First we will write the control that will raise the event cummunicating that image and navigation URLs have changed.

• The geenral definitation of an event handler is as follows.

  public void MyEventHandler(object sender, MyEventArgs args);
  												

  The second argument of the event handler encapsulates the data specific to the event that is being raised. So the first step is to decide what information needs to be passed with the event. In our case, we want to notify all the client of new image and navigation URL. We create BannerChangeEventArgs class, derived from System.EventArgs class. The class has two properties storing the values of two URL strings.

  public class BannerChangeEventArgs : EventArgs
  												

• Then provide the definition of event handler.

  public delegate void BannerChangeEventHandler(object sender, BannerChangeEventArgs e);
  												

• Here is the important part. Add an instance of this event handler in SiteBannerManager class. This instance will provide the connection point for the consumers where they will register their event handlers. And also provide the function that will raise the event.

  public abstract class SiteBannerManager : System.Web.UI.UserControl
  {
  	.
  	.
  	public event BannerChangeEventHandler BannerChange;
  	protected void OnChangeBannerImg(object sender, System.EventArgs e)
  	{
  		BannerChangeEventArgs args = new BannerChangeEventArgs();
  		args.ImageURL = wndImgURLTxt.Text;
  		args.LinkURL = wndLinkURLTxt.Text;
  		OnBannerChange(args);
  	}
  	protected virtual void OnBannerChange(BannerChangeEventArgs args)
  	{
  		if (BannerChange != null)
  		{
  			BannerChange(this, args);
  		}
  	}											
  }
  												

  The above code provides the overview of code that needs to be provided for raising the event. OnChangeBannerImg is the event handler for Click event of the button on the control. When user clicks on the button, this event is handled. We extract the values of new URL string s from two asp:Textbox controls and stuff them into BannerChangeEventArgs object. Then the control raises ther event for all the delegates that are registered for handling the event.

SiteBanner - Event Consumer

Event raiser alone cannot accomplish anything unless there is a consumer that will act upon the event raised by it. If there is no event handler registered with SiteBannerManager object's event handler, the changes to the URLs amde in that control will not make any effect on the page.

So we need to write a consumer that will handle BannerChange events. This is done in SiteBanner class. Following steps are followed to implement event handling.

• The foremost responsibility of the event consumer is to provide the event handling function. This function has to have the same signatures as the event handler defined by the event raiser. Here is the implementation of the event handler in SiteBanner class.

  public void ChangeBannerImage(object sender, BannerChangeEventArgs args)
  {
  	if (args.ImageURL.Length != 0 &&
  		args.LinkURL.Length != 0)
  	{
  		BannerHyperLink.ImageUrl = args.ImageURL;
  		BannerHyperLink.NavigateUrl = args.LinkURL;
  	}
  }												
  												

  The arguments of this event handler function are the same as delegate defined in event raiser class SiteBannerManager.

Putting It All Together

Now we have a class that raises the event and a class that will handle the raised events. Lets see the whole thing gets wired up for real action.

We have put together a very simple web form, BannerAds.aspx. It does nothing fancy. It provides a place holder for adding the two controls that we have created. As discussed in previous articles, first we add the reference to two UserControls at the top of the page.

<%@ Page language="c#" Codebehind="BannerAds.aspx.cs"
	AutoEventWireup="false" Inherits="ASPNet_App.BannerAds" %>
<%@ Reference Control="./Controls/SiteBanner.ascx"%>
<%@ Reference Control="./Controls/SiteBannerControl.ascx"%>
										

Then we provide a place holder for adding the controls.

<form id="BannerAds" method="post" runat="server">
	<asp:Panel Runat="server" ID="BannerDiv"></asp:Panel>
</form>										
										

Notice that we have put the asp:Panel placeholder control inside form tag. It is essential to do so because we have asp:Textbox and asp:Button controls in the SiteBannerManager control. To handle the events raised by these controls it is necessary that they are defined inside form tag. If you forget to do so, don't worry. You will get a run time exception, reminding you to do so.

Now comes the real wiring up of all the pieces. First we load two controls using LoadControl method in Page_Load method of BannerAds class. And then we register ChangeBannerImage method as the event handler with BannerChange event of SiteBannerManager class. Following is the implementation of Page_Load in BannerAds class, showing how this wiring up of both controls take place.

Control bannerControl;
Control banner;
try
{
	bannerControl = LoadControl("Controls/SiteBannerControl.ascx");
	banner = LoadControl("Controls/SiteBanner.ascx");
	if (bannerControl != null)
	{
		((SiteBannerControl)bannerControl).BannerChange +=
			 new BannerChangeEventHandler(((SiteBanner)banner).ChangeBannerImage);
	}
	CreateBannerControlTable(bannerControl, banner);
}
									
										

And now we are ready to see this whole thing in action. Bring up the BannerAds.aspx page. It will show you two text boxes and a button on left side and a banner on right side. Type in new URL for new image and navigation and click on "Change Banner" button. You will see that banner image changes on right side, provided the URL points to valid image file.