Behavioural State Machines

Example State Machine Diagram

A state machine represents the internal states of a single object and how it transitions between those states in response to events.

• States: Different conditions or modes an object can be in (e.g., “new patient,” “current patient”).
• Transitions: The movement from one state to another, often triggered by an event.
• Events: Triggers for state transitions (e.g., “patient check-in”).

Usage

• Typically used for complex objects that go through multiple states.
• Focuses on how an object’s internal attributes change over time, such as a patient’s status moving from new to treated.

Components

• States
  • Initial State: Represented by a filled black circle, it indicates where the state machine begins.
  • Final State: Depicted as a filled black circle inside a larger circle, it signifies the completion of the state machine’s operation.
  • Stable States: Intermediate states that an object can occupy throughout its lifecycle.
• Transitions
  • Arrows connecting states, indicating the movement from one state to another.
  • May include guard conditions that determine whether the transition can occur based on specific criteria.
• Events
  • Triggers that cause a transition to occur, often labeled on the transition arrow.
  • Examples include user actions, system events, or time-based triggers.
• Actions
  • Behaviors that occur as a result of a transition.
  • Actions can be associated with transitions or can happen on entering or exiting a state.

Guidelines for Creating State Machines:

• Ensure all states are reachable.
• Avoid “black holes” (states from which objects can’t exit) and “miracles” (unexplained transitions).
• Transitions should be clearly tied to specific messages or operations.