Extending the statechart formalism

Abstract

Statecharts are extended to deal with events when no applicable transition is available, and to resolve conflicts relative to event scheduling and response that can arise whenever multiple states can be active simultaneously. "Event closure" and event scheduling are achieved without having to clutter up a basic statechart. The extensions are effected by means of declarative event disposition rules. These rules, together with the statechart topology, determine the contents of one or more disposition matrices. These matrices are combined with the statechart state to determine the response of the event dispatcher to incoming events. The operation of the event dispatcher is also described. A detailed example illustrates these concepts, which are further characterized, for the benefit of working programmers, in the form of a behavioral design pattern. A tool called StateCraft embodies these notions.