A Specification Language for Direct-Manipulation User Interfaces

A direct-manipulation user interface presents a set of visual representations on a display and a repertoire of manipulations that can be performed on any of them. Such representations might include screen buttons, scroll bars, spreadsheet cells, or flowchart boxes. Interaction techniques of this kind were first seen in interactive graphics systems; they are now proving effective in user interfaces for applications that are not inherently graphical. Although they are often easy to learn and use, these interfaces are also typically difficult to specify and program clearly. Examination of direct-manipulation interfaces reveals that they have a coroutine-like structure and, despite their surface appearance, a peculiar, highly moded dialogue. This paper introduces a specification technique for direct-manipulation interfaces based on these observations. In it, each locus of dialogue is described as a separate object with a single-thread state diagram, which can be suspended and resumed, but retains state. The objects are then combined to define the overall user interface as a set of coroutines, rather than inappropriately as a single highly regular state transition diagram. An inheritance mechanism for the interaction objects is provided to avoid repetitiveness in the specifications. A prototype implementation of a user-interface management system based on this approach is described, and example specifications are given.