States, transitions and events

A statechart is a graphical model which describes the behavior of some thing or entity. It describes how such an entity reacts to changes and events in its environment. Due to it’s graphical nature, lucid concepts, and well defined semantics it provides a comprehensive and executable basis for system specification and implementation which can be shared between stakeholders without demanding too many technical skills.

The most basic element of a statechart are states, transitions, and events. To introduce these basic concepts we take a look at the simple light switch example below. States represent the modes of a system or entity and in the case of this very simple light switch two states Off and On exist. States have a name, are visualized as rounded rectangles. At any point of time states are either active or inactive. Active state are highlighted using a yellow background color while inactive states have a blue background color.

If something happens then the statechart may change its state. Each state can have outgoing transitions. These transitions connect two states and have a direction which is indicated by the arrow head. Our light switch states are mutually connected by transitions. These transitions are directed and define how state changes (state transitions) can occur under which conditions. Here, the light switch simply defines that Off and On are toggled whenever a switch event occurs. As the state machine only reacts in response to events, it is called event driven.

Events typically occur in the environment of the statechart’s entity. To simulate this environment and to play with the light switch you can interactively trigger events e.g. by clicking on interactive elements like switch.

The light switch declares the switch event as:

	in event switch 

State machines as a whole can be active or inactive. If a state machine is inactive then no state is active and it won’t react on any event. If a state machine is active then is has at least one active state and it is ready to react on events. Entering a state machine activates it exiting deactivates it. If you enter the light switch state machine then Off is the first active state. The first active state is called initial state and is defined by the entry node whose single outgoing transition points to Off.