ABSTRACT
The widespread use of advanced user interfaces has raised the expectations of users to new levels. Highly graphical, interactive programs with easy-to-use, intuitive interfaces, such as those available for the Apple Macintosh, have become the goal, if not the reality, in personal and workstation computing. These interfaces are usually based on three concepts: accurate and appealing visual metaphors, interaction by direct manipulation, and immediate reflection of changes in the data. Programs based on these concepts are called reactive, after Alan Kay's Reactive Engine [Kay 69].
