Friday, July 31, 2009

Dragging and dropping with ASP.NET 2.0 and Atlas


This tutorial is intended to help readers understand how certain aspects of Microsoft's new AJAX Extensions technology works. AJAX Extensions is intended to simplify the development of AJAX-style functionality. As with all technologies, however, to use a tool well, it is important to understand the underlying technology that Atlas abstracts. One of the key ASP.NET AJAX abstractions is the new XML markup syntax developed to make coding with AJAX easier (originally included with the core Atlas files, XML markup is now a part of the CTP called AJAX Futures). With XML markup, developers can modify their code declaratively. However, there are times when a developer may want to be able to change her code programmatically, and in order to accomplish this, she will need to understand that underneath the markup abstraction, she is actually dealing with good 'ol JavaScript and some custom JavaScript libraries developed by Microsoft. In order to demonstrate the relationship between the Atlas declarative model and the programmatic model, I will go through a series of examples in which the same task will be accomplished both declaratively and programmatically. I will be demonstrating how to use the PreviewDragDrop library file to perform basic drag-drop operations as well as setting up drop zones.


As I write this, Microsoft has made some important changes to ASP.NET AJAX for the Beta 2 that have the unfortunate side-effect of breaking most of the original Atlas implementation, and has required a bit of rework of the original samples. These revised examples apply to the Beta 2 of ASP.NET AJAX. Future releases of AJAX Extensions may affect the accuracy of this tutorial. I will attempt to update the code as new versions of AJAX Extensions become available. AJAX Extensions works with .NET 2.0, and will work with Orcas when it is released.

I. Declarative Drag Drop

The first task is to use XML markup to add drag-drop behavior to a div tag. By drag and drop, I just mean the ability to drag an object and then have it stay wherever you place it. The more complicated behavior of making an object actually do something when it is dropped on a specified drop target will be addressed later in this tutorial. To configure your webpage to use ASP.NET AJAX, you will need to install the Microsoft.Web.Extensions.dll into your Global Assembly Cache. You will also need a reference to the library Microsoft.Web.Preview.dll. Finally, you will need to configure your web.config file with the following entry:

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