The design and construction of usable interactive systems requires a user-centered approach to system development. Such an approach requires tools and representations reflecting a behavioral view of the interface-a view centered on user activities and the system activities and states that the user can perceive. We present a model of these behavioral phenomena well suited for defining, extending, and analyzing behavioral representations. Our model is based on the timetree, a novel tree-based structure representing tasks, user actions, system activity, and system and interface state, all within a framework of branching sequential timelines. We introduce the timetree model by relating it to well-known interactive behaviors. We present a formal definition of timetrees and some of the operations they support, and we show some ways in which the model has contributed to our understanding of behavioral descriptions.

The Artefact framework is a tool for building collaborative applications that deliver = representations of art objectoriented appXcation space to standard browsers.We present some aspects of Artefact's implementation, including_ enhancements to support synchronous collaboration, the de-coup~ng of input and output in the interaction protocol, a lighhveight gened-purpose Java appleqand tie Wera~en~ that bridge the gap behveen a browser and an appficahon.We describe some of the characteristics that m&e it easy to create multi-user applications witi ArtefacL and illustrate WISwith a simple example application.Finally, we compare Artefact to some existing distributed application platforms.