Chapter 23: Drag and Drop

Of all the direct-manipulation idioms characteristic of the GUI, nothing defines it more than the drag-and-drop operation, clicking and holding the button while moving some object across the screen. Surprisingly, drag-and-drop isn't used as widely as we imagine, and it certainly hasn't lived up to its full potential. In particular, the popularity of the Web and the myth that Web-like behavior is synonymous with superior ease of use have set back the development of drag-and-drop on the desktop, as developers mistakenly emulated the crippled interactions of Web browsers in other, far less appropriate contexts.

Defining Drag-and-Drop

Any mouse action is very efficient because it combines two command components in a single user action: a geographical location and a specific function. Drag-and-drop is doubly efficient because, in a single, smooth action, it adds a second geographical location. Although drag-and-drop was accepted immediately as a cornerstone of the modern GUI, it is remarkable that drag-and-drop is found so rarely outside of programs that specialize in drawing and painting. Thankfully, this seems to be changing, as more programs make use of this idiom.

We might define drag-and-drop as "clicking on an object and moving it to a new location," although that definition is somewhat narrow in scope for such a broad idiom. A more accurate description of drag-and-drop is "clicking on some object and moving it to imply a transformation."

The Macintosh was the first successful system to offer drag-and-drop. It raised a lot of expectations with the idiom that were never fully realized for two simple reasons. First, drag-and-drop wasn't a system-wide facility, but rather an artifact of the Finder, a single program. Second, as the Mac was at the time a single-tasking computer, the concept of drag-and-drop between applications didn't surface as an issue for many years.

To Apple's credit, they described drag-and-drop in its first user interface standards guide. On the other side of the fence, Microsoft not only failed to put drag-and-drop aids in its early releases of Windows, but didn't even describe the procedure in its programmer documentation. However, Microsoft eventually caught up and even pioneered some novel uses of the idiom, such as movable toolbars and dockable palettes.