Dojo's Industrial-Strength Grid Widget
In almost any application that's complex enough to require a database for persistent storage, there is a need to render data in a tabular format for display or interaction. Although a seemingly simple task, it quickly becomes quite involved once you introduce the need to sort columns, reorder columns, account for editing the contents of cells and managing pagination. As you might expect, this is one of those problems that has been solved so many times that commoditized solutions, such as Dojo's grid widget, now are available for use. After all, you'd much rather concentrate on actually building out the interesting parts of your app than creating and maintaining the infrastructure that holds it all together, right?
If you've been following along with my previous LJ articles on Dojo (see Resources), you should have a good idea of how to get Dojo up and running with minimal hassle using AOL's Content Delivery Network (or if you prefer, get it from the official Dojo Web site). As a reminder, the minimal page template is basically an HTML page with a script tag and a dojo.addOnLoad block that waits until all cross-domain loading has completed before it executes in order to guarantee that dependencies inside the code block have been met before it executes. You may recall that the skeleton for this page looks something like this:
Prior to the Dojo 1.2 release, the grid was under heavy development and largely in a state of flux. Although its API still is subject to change, the 1.2 release substantially firmed it up, and aside from accessibility considerations, it is expected to remain intact. Bottom line: don't let the dojox namespace or previous experiences with the grid prior to 1.2 scare you off; it's ready for prime time. Although at the time of this writing, Dojo version 1.3 is just about to be released, the code examples reference version 1.2 and work just fine. There should be minimal, if any, changes necessary to update the examples to version 1.3.
Now I'm going to move to putting the grid widget to work, so consult a reference such as Dojo: The Definitive Guide (O'Reilly, June 2008) or on-line documentation at the Dojo Campus if you need a quick refresher.
The grid widget is necessarily data-centric and builds directly upon the abstractions offered by the toolkit's robust data APIs, so a very brief introduction is helpful for setting that context. In Dojo parlance, the abstraction that the data APIs offer is a store that contains items, where a store can implement a particular subset of the four dojo.data APIs:
Read: APIs for querying items in a store.
Identify: APIs for uniquely identifying items in a store.
Write: APIs for creating, modifying and deleting items in a store.
Notification: APIs for triggering event handlers when items in a store are created, modified or deleted.
The toolkit comes stocked with two handy store implementations that are bundled into the dojo.data module: the ItemFileReadStore and the ItemFileWriteStore. The ItemFileReadStore implements the Read and Identity APIs, while the ItemFileWriteStore implements all four of these APIs. Be aware, however, that the dojox.data module contains a plethora of additional data modules for common tasks that you will want to leverage to your advantage; interfacing with comma-separated value (CSV) files, Flickr, Amazon's S3 service, OPML files and Atom content are just a few of the handy implementations available (Figures 1 and 2).
In general, you provide data to an ItemFileReadStore or ItemFileWriteStore in one of three ways:
By feeding it a file containing JSON data that meets a particular structure.
By programmatically creating new items for the store.
Let's briefly consider examples that illustrate these operations. The grid widget simply reflects whatever data is in the store that backs it, so understanding how to manipulate data stores is essential to controlling what ultimately appears in a grid widget (Listing 1).
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