The Mobile Developer

HDML/WML: HyperCard For Wireless Devices

Our industry is quite adept at dressing old concepts in new clothes and presenting them as the "hot new thing". Remember Java and its vaunted cross-platform portability? The way the press fauned over it, you'd think they'd never heard about Pascal's P-code. (Don't get me wrong, I'm a big Java fan, I just found the initial fuss kind of amusing.) When you come across some "new" technology, take a close look underneath and you're likely to discover it's not so new. You may be suprised to learn that both HDML and WML are really just reincarnations of HyperCard.

If you're unfamiliar with HyperCard, here's a quick overview: a HyperCard application is a stack of cards. Each card can display text, images, input controls, etc., and the card can have various actions associated with it. As the application runs it switches from card to card -- the "topmost" card being the one the user interacts with.

Originally released by Apple in the middle 80's for the early Macintosh computers, HyperCard is still a shipping product. You can read about it at www.apple.com/hypercard. The product's been considerably enhanced over the years to include support for advanced multimedia. And it spawned a number of imitators for other platforms. I think I still have a copy of one, CanDo (for the Amiga), in my basement somewhere (along with my Amiga!).

The basic concepts in HyperCard -- the stack/deck metaphor, the ability to navigate between cards programatically and in response to user input -- are central to both HDML and WML, except of course HyperCard wraps it all in a nice GUI interface. This is one area where HDML/WML are different from HTML. There's no concept of grouping pages of information together in HTML -- each HTML file is basically a complete, independent page. But in HDML/WML the page is split into a series of cards, each containing a piece of the larger "virtual" page. The closest you can come to this with HTML is by using frames, but frames are really just a way of embedding independent pages within another page -- not really the same thing.

You can even think of HyperCard as the ultimate predecessor of the "wizards" that are found in so much software today. And when you're writing at HDML/WML application, you're basically writing a wizard.

Isn't it neat how yesterday's technology can gain new relevance?

Eric Giguère is the author of Palm Database Programming: The Complete Developer's Guide and an upcoming book on the Java 2 Micro Edition. He works as a developer in Sybase's Mobile & Embedded Computing division. Visit his website at www.ericgiguere.com or send him mail at ericgiguere@ericgiguere.com.