State Machine and Activity Diagrams

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State Machine and Activity Diagrams

• University of Sunderland

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Purpose

• Sequence and collaboration diagrams are often
  insufficient to describe the full behavior of a
  class or subsystem. State machine and activity
  diagrams may also be required. These are based on
  Petri Nets.
• State machine diagrams show the behavior of the
  class/subsystem in response to external stimuli.
• Activity diagrams show the behavior of the the
  class/subsystem in response to internal
  processing.

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State Machine Diagram Notation

• Describes the states an object can have during
  its lifecycle, the behaviour in those states, and
  the events that can cause the state to change.
• Statesrepresenting the distinct behaviors of the
  class or subsystem. Often represented by state
  objects.
• Transitionsthe processes by which the class or
  subsystem changes behavior.

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Technique for Construction

• Specify the class or subsystem
• Identify the distinct behaviors of the
  subsystem/class/object
• Identify the circumstances under which the
  behavior may change
• Define the types of transitions between behavior.
• Iterate

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Sample State Machine Diagram
Splitting of Control
Initial State
State1
State3
State
State2
Transition
Final State
Joining of Control
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States

• May be active or inactive
• May have an internal state machine
• May be composed of multiple state machines.
• May be initial
• May be final
• May have a memory (history)
• May generalize other states

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Transitions

• Are paths relating two states or a state and a
  splitting/joining of control.
• May iterate, returning to the same state.
• May be associated with logical conditions
• May be periodic or may have an associated time.
• May be compound
• May be protected by guard conditions.

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Splitting/Joining of Control

• Represent when multiple concurrent threads are
  created from one or when threads are
  synchronized.
• You can also send messages between state machines.

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Events

• Classes that define triggers of state changes
• For internal state transitions
• May be entry events, associated with the entry
  into a state.
• May be exit events, associated with exit from a
  state.
• May call other state machines (do events)
• Events (signal and call events) may invoke
  operations (actions).
• May reflect condition changes (condition events)
  or times (time events).

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Activity Diagram Notation

• Similar to state machine diagrams
• Contain swimlanes associated with individual
  subsystems, classes or objects that are
  interacting in the activity.
• Elements include
• Swimlanes
• Action states
• Action flows
• Object flows

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Purposes of Activity Diagrams

• To capture workflow
• To show how related actions can be performed,
  affecting objects around them
• To show how a use case can be performed in terms
  of actions and object state changes
• To model a business or organization

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Technique for Construction

• Identify the subsystem or class
• Model swimlanes
• Model action states
• Model action flows
• Model object flows
• Iterate

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Sample Activity Diagram
Class1
Class2
Class3
Responsible Object
Action Flow
State1
State2
Action State
State3
OClass2
Swimlane Border
Object Flow
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Swimlanes

• Used to associate various actions with the
  responsible objects
• Contain activities and action states

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Action States

• Atomic actions or operations
• Contents may be a statechart
• Need not be unique

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Flows

• Action flows are associations between action
  states
• Object flows show the transfer of objects between
  action states.
• Can include decisions
• Can use pins to model inputs and outputs.