State Machine Diagram (UML)

A state machine diagram is used to:

• Model the possible states of a system or object
• Show how the state transitions occur as a consequence of events
• Show what behaviour the system or object exhibits in each state

There are multiple components to a state machine diagram:

• State (UML State Machine)

  State is the nodes of the state diagram.

  • When a state is active, the object is in that state.
  • All internal activities specified in the current state can be executed.
  • An activity can consist of multiple actions.

  Activity specifications:

  • entry / Activity(...): executed when the object enters the state
  • exit / Activity(...): executed when the object exits the state
  • do / Activity(...): executed while the object remains in this state

  

  

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• Transition (UML State Machine)

  A transition changes from one state to another.

  

  The syntax used is as follows:

  1. Event (trigger): exogenous stimulus, can trigger state transition
  2. Guard (condition): boolean expression
     • if the event occurs, guard is checked
     • if guard is true:
       • any activities in current state are terminated
       • relevant exit activity is executed
       • transition takes place
     • if guard is false:
       • no state transition takes place, event is discarded
  3. Activity (effect): sequence of actions executed during the state transition

  There are two types of transition:

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• Event (UML State Machine)

  There are different types of events we need to handle in a state diagram:

  • Signal event: receipt of a signal e.g. rightmousedown, sendSMS(message)

  • Call event: operation call e.g. occupy(user, lectureHall), register(exam)

  • Time event: time-based state transition e.g. after(5 seconds) or when(time==16:00)

  • Any receive event: occurs when any event occurs that does not trigger another transition from the active state e.g. keyword = all

  • Completion event: generated automatically when everything to be done in the current state is completed

  • Change event: permanently checking whether a condition becomes true e.g. when(x,y), after(90min)

    Below is a comparison between a change event and a guard:

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• Initial State (UML State Machine)

  The initial state is the start of a state machine diagram.

  • It is a pseudo-state.
  • No incoming edges.
  • If there is more than 1 outgoing edge, guards must be mutually exclusive and cover all possible cases to ensure that exactly one target state is reached.
  • If initial state becomes active, the object immediately switches to the next state.
  • No events allowed on the outgoing edges.

  

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• Final State (UML State Machine)

  The final state is a real state and marks the end of the sequence of states. Object can remain in a final state forever.

  

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• Terminate Node (UML State Machine)

  The terminate node is a pseudo-state that terminates the state machine. The modelled object ceases to exist.

  

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• Decision Node (UML State Machine)

  A decision node is a pseudo-state and is used to model alternative transitions.

  

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• Parallelisation Node (UML State Machine)

  A parallelisation node is a pseudo-state that splits control flow into multiple concurrent flows. There is one incoming edge and one or more outgoing.

  

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• Synchronisation Node (UML State Machine)

  A synchronisation node is a pseudo-state that merges multiple concurrent flows. There is one or more incoming edge and one outgoing edge.

  

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