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Custom Web Controls Demystified, Part 2 : Page 9

In Part 1 you learned how to build a custom rendered control. In Part 2 you'll learn how to build a composite control.


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Designers. By not applying a designer class to your controls you inherently use the default designer. A designer controls the way the control renders in Visual Studio during design of a Web Form. Some controls, such as custom grids for example, may need to display themselves in a certain way during design time and a different way at run time (to display sample data perhaps). A custom designer comes into play here. Complex Properties. These are properties that are object types which contain other properties. They display in the property browser as an expandable line and serve the purpose of grouping properties together and can be reused in many controls. The Style object is an example of a complex property.

Type Converters. These are classes that can be applied to properties using attributes. They change the way a property displays itself in the property browser. Think of displaying Yes or No choices for a Boolean instead of True or False. Templates. Templates are incredibly powerful and allow you as the control developer to set aside areas within your control whose content will be defined later by the developer using the control. In a DataGrid, for example, you can define a column as a templated-column. As the developer using the DataGrid control, you can either drag out or programmatically add other controls inside this column.

Event Bubbling. Typically used in conjunction with templates, event bubbling allows the handling of events fired from controls that another developer inserts into templates defined in your controls. Client-side Scripting. In the ConfirmationButton control I demonstrated some simple client-side scripting by adding JavaScript code to the button's onclick event. This just touched the surface on what you can do with client-side scripting. In fact, custom Web controls are a great way to expose client-side scripting functionality to Web Forms.



Custom Style Objects. You can define your classes that inherit from the Style object and add further properties or functionality. An example of this is the TableStyle object which adds a couple more properties that apply to Tables; including CellSpacing and Cellpadding. Data Binding. This can be a broad topic depending on exactly what kind of data binding you have in mind. The infrastructure that the Control class provides for you supports the basics of data binding; that is, the properties, methods, and events involved. How you use them is up to you, the control developer. Depending on their intent, custom Web controls can provide general all-purpose data binding such as that which is compatible with any object that supports the IEnumerable interface, or they can have very business-specific data binding.

Control Builders. Custom control builders allow you to redefine how a control renders on an ASPX page, in regards to white space, and nested tags within the main one. Validation. There are different types of validation in the context of Web controls. Client-side validation enables your custom controls to be compatible with the ASP.NET validation controls that .NET provides. Server-side validation is provided by implementing the IValidator interface and works in conjunction with the IsValid check performed on the Page object. The last type of validation is the act of actually developing a validation control, such as the validators that Microsoft already provides. This is accomplished by deriving a control from the BaseClassValidator base class.

Custom Verbs. These are the little links you might have seen at the bottom of the property browser for a given control. Drop a DataGrid on a Web Form and look at its properties in the property browser. You'll see a couple of links at the bottom. These usually provide some sort of wizards to assist you in the configuration of the control. Special formatting functionality is also commonly seen in this fashion. Other Attributes
I've demonstrated a couple of different attributes you can apply to control classes or properties, but there are many others I did not have the room to talk about here. Some are used to apply the technologies I'm covering in this list.

Cross-Platform Compatibility
Web controls have the ability to contain their own platform checks-this includes browser checks and mobile platform checks. You can then choose different rendering methods depending on what platform is using them. ASP.NET 2.0 will expose a much more robust framework for determining client information and building more robust controls that will work with not only different browsers, but with any client. ASP.NET 2.0
The newest release of ASP.NET adds a few extra gifts to the world of Web controls. For starters, there are claims that ViewState has been decreased in size by as much as 47 percent in some cases. This in itself is a great achievement since ViewState growth is always something to keep an eye on when designing Web pages. Also, there is a new StateBag called ControlState. You access it using its own set of three methods (as in ViewState) only. As a control developer, you decide what properties are important enough to store here. This eliminates the all-or-nothing method of controlling state you have today.

ASP.NET 2.0 also includes many new Web controls out of the box, including a new grid called GridView. In my last article I described a few of these in detail so I won't repeat it here. Some of these new controls support a new feature called script callbacks. This allows for a control to obtain information form the server without a "complete page postback." The details are beyond this article, but this method of calling the server-side code does not refresh the entire page, thus eliminating the page-flicker caused by a postback. The technology behind script-callbacks is based on XMLHTTP and is similar to what today is known as AJAX. ASP.NET 2.0's new controls, including the DetailsView, FormView, and an assortment of authentication-based controls, are a perfect example of how you can use Web controls not only for simple visual objects, but also for larger more complex solutions, including complete forms. ASP.NET 2.0's new data source controls demonstrate how a Web control may not even have a user interface, but instead can be used to service other controls. Programming Web Forms in this manner is known as Declarative Programming.

Declarative Programming
Custom Web controls do not only have to serve their cause as reusable components, they can be used as an approach to ASP.NET development. This can serve many purposes including better encapsulation, further breakdown of visual components, and full object-orientation for the UI. Of course reusability is always a welcome side-effect. Moving site objects and functionality to Web controls is a style called Declarative Programming. I urge you to refer to the March/April 2005 issue of CoDe Magazine for a full explanation of this approach to ASP.NET development. True object-oriented programming has been given to us with the release of .NET, and Web controls have allowed us to bring it to the UI level, giving us a level of encapsulation and reusability that exceeds anything we have used in the past. Many of you are already familiar with them and are already developing your own, but it amazed me to speak to many other ASP.NET developers after over three years since the release of .NET, and discover that they never went beyond dragging pre-packaged Web controls onto Web Forms. I understand that the .NET Framework is massive and I'm still learning new things every single day, and I hope that reading this article gets you interested in learning more about these elements of reusability that have once again revolutionized the way Web applications are designed and developed.

The complete code for the three controls I described in this article is available for download at: http://www.infotekcg.com/downloads.aspx. Much more functionality is in these controls that this article had room for, so enjoy. I also post other controls I develop so keep at a lookout for some pretty cool stuff. If you've developed some cool Web controls you'd like to let me know about, please contact me via the contact info on the last page of this article. Also, feel free to give me any feedback on what you think of these controls or how this article has helped you. And don't forget to check out my blog.



Miguel A. Castro is President of InfoTek Consulting Group, Inc., a professional consulting firm that specializes in architecting, designing, and developing solutions using Microsoft .NET technologies. His VB background goes all the way back to 1.0. Miguel's focus for the last couple of years has been the .NET Framework and languages; he is fluent in both VB .NET and C#. His programming experience goes back 20 years when he started on TRS-80s, Apple IIs, and Atari 800 computers. Miguel lives in Lincoln Park, New Jersey with his wife Elena and his daughter Victoria.
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