An object-oriented implementation of concurrent and hierarchical state machines

ContextState machine diagrams are a powerful means to describe the behavior of reactive systems. Unfortunately, the implementation of state machines is difficult, because state machine concepts, like states, events and transitions, are not directly supported in commonly used programming languages. Most of the implementation approaches known so far have one or more serious drawbacks: they are difficult to understand and maintain, lack in performance, depend on the properties of a specific programming language or do not implement the more advanced state machine features like hierarchy, concurrency or history.ObjectiveThis paper proposes and examines an approach to implement state machines, where both states and events are objects. Because the reaction of the state machine depends on two objects (state and event), a method known as double-dispatch is used to invoke the transition between the states. The aim of this work is to explore this approach in detail.MethodTo prove the usefulness of the proposed approach, an example was implemented with the proposed approach as well as with other commonly known approaches. The implementation strategies are then compared with each other with respect to run-time, code size, maintainability and portability.ResultsThe presented approach executes fast but needs slightly more memory than other approaches. It supports hierarchy, concurrency and history, is human authorable, easy to understand and easy to modify. Because of its pure object-oriented nature depending only on inheritance and late binding, it is extensible and can be implemented with a wide variety of programming languages.ConclusionThe results show that the presented approach is a useful way to implement state machines, even on small micro-controllers.