State Machine Diagrams - Part 1

Introduction §

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Every object takes a finite number of different stages during its life. State machine diagrams are used as follows:

• To model the possible states of a system or object.
• To show how state transitions occur as a consequence of events.
• To show what behaviour the system or object exhibits in each state.

For example: high-level description of the behaviour of a lecture hall:

Where occupy() and release() are the transitions and “free = true”/“free = false” are the states.

Another example would be a Digital Clock:

States §

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They are nodes of the state machine:

When a state is active:

• The object is in that state.
• All internal activities specified in this state can be executed, meaning an activity can consist of multiple actions.

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.

Transitions §

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Syntax §

Change from one state to another:

• Event (trigger)

  • Exogenous (has an external cause or origin) stimulus.
  • Can trigger a state transition.

• Guard (condition)

  • Boolean expression.
  • If the event occurs, the guard is checked.
  • If the guard is true:
    • All activities in the current state are terminated.
    • Any relevant exit activity is executed.
    • The transition takes place.
  • If the guard is false:
    • No state transition takes place, the event is discarded.

• Activity (effect)

  • Sequence of actions executed during the state transition.

Types §

• Internal:

• External:

When do the following transitions take place? §

Sequence of Activity Executions §

Assuming $S1$ is active, what is the value of $x$ after $e$ occurred?

• $S1$ becomes active, $x$ is set to the value 4.
• $e$ occurs, the guard is checked and evaluates to true.
• $S1$ is left, $x$ is set to 5.
• The transition takes place, $x$ is set to 10.
• $S2$ is entered, $x$ is set to 11.

Registration status of an exam example:

Types of Events §

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Signal Event §

Receipt of a sinal.

• e.g.: rightmousedown, sendSMS(message).

Call Event §

Operation call.

• e.g.: occupy(user, lectureHall), register(exam).

Time Event §

Time-based state transition:

• Relative: based on the time of the occurrence of the event.
  • e.g.: after(5 seconds).
• Absolute
  • e.g.: when(time==16:00), when(data==20150101).

Any Receive Event §

Occurs when any event occurs that does not trigger another transition from the active state:

• Keyword all.

Completion Event §

Generated automatically when everything to be done in the current state is complete.

Change Event §

Permanently checking whether a condition becomes true.

E.g.: when(x > y), after(90min).

Change Event vs Guard §

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Types of States §

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Initial State §

It is a pseudo-state, meaning it is transient (system cannot remain in that state). It is more of a control structure than a real state.

No incoming edges.

• If > 1 outgoing edges:

  • 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 (exception: new()).

Final State and Terminate Node §

• Final State

  • Real state.
  • Marks the end of the sequence of state.
  • Object can remain in a final state forever.
  • 

• Terminate Node

  • Pseudo-state.
  • Terminates the state machine.
  • The modelled object ceases to exist (= is deleted).
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Decision Node §

It is a pseudo-state and is used to model alternative transitions.

Example §

Parallelisation and Synchronisation Node §

• Parallelisation node

  • Pseudo-state.
  • Splits the control flow into multiple concurrent flows.
  • 1 incoming edge.

  • 1 outgoing edges.

  • 

• Synchronisation node

  • Pseudo-state.
  • Merges multiple concurrent flows.

  • 1 incoming edges.

  • 1 outgoing edge.
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Summary §

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Part 2 here. §