State Machine Diagrams Chapter 10

1
State Machine DiagramsChapter 10

• Ragnhild Van Der Straeten

2
State Diagrams

• State diagrams describe all the possible states
  an object can get into and how the objects state
  changes as a result of events that reach the
  object
• State diagrams are drawn for a single class to
  show the lifetime behavior of a single object
• Use state diagrams for those classes that exhibit
  interesting behavior only
• UI and control objects are popular candidates

3
States

• State is a condition or situation during the life
  of an object during which it
• satisfies some condition,
• performs some activity or
• waits for some event.
• Object remains in a state for a finite amount of
  time.

4
States

• Different parts of a state
• Name textual string a state may be anonymous
  meaning that it has no name.
• Internal activities states react to events
  without transition.
• Entry/exit activities activities executed on
  entering and exiting the state resp.
• Composite states nested structure of a state,
  involving disjoint (sequentially active) or
  orthogonal (concurrently active) substates

5
Initial and Final States

• Initial state indicates the default starting
  place for the state machine or composite
  state.(filled black circle)
• Final state indicates that the execution of the
  state machine or the enclosing state has been
  completed(filled black circle surrounded by an
  unfilled circle)
• Initial and final states are pseudo-states.
  Neither may have the usual parts of a normal
  state, except for a name.

6
Transitions

• Transition is a relationship between two states
  indicating that
• an object in the first state will perform certain
  activities and
• enter the second state when a specified event
  occurs and specified conditions are satisfied.
• On such a change of state, the transition is said
  to fire.

7
Transitions

• Transition has 5 parts
• Source state the state affected by the
  transition if an object is in the source state,
  an outgoing transition may fire when the object
  receives the trigger event of the transtion and
  if the guard condition is satisfied
• Target state the state that is active after the
  completion of the transition

8
Transitions

• Trigger-signature Guard / Activity
• Trigger-signature an event whose reception by
  the object in the source state makes the
  transition legible to fire, providing the guard
  condition is satisfied.
• Guard a boolean expression, evaluated when the
  transition is triggered byt the reception of the
  event trigger if the expression evaluates True,
  the transition is legible to fire, if the
  expression evaluates to False, the transition
  does not fire and if there is no other transition
  that could be triggered by the same event, the
  event is lost.
• Activity some behaviour executed during the
  transition

9
Trigger-signature

• Event-name ( comma-separated-parameter-list )
• parameter-name type-expression
• Elapsed-time eventafter(5 seconds)after(10
  seconds since exit from state A)
• Condition becoming true is shown with the keyword
  when followed by a boolean expression.
  (continuous test for the condition until it is
  true)
• Transitions without a Trigger-signature within
  their label occur as soon as the activity
  associated with the state is finished

10
Guard Condition

• Boolean expression enclosed in square brackets
  and placed after the trigger event.
• Evaluated only after the trigger event for its
  transition occurs.
• It is possible to have multiple transitions from
  the same source state and with the same event
  trigger, as long as those conditions do not
  overlap.
• Within the boolean expression, it is possible to
  include conditions about the state of an object.

11
Activity

• Activity is executed if and when the transition
  fires
• Activities may include
• operation calls to the object that owns the state
  diagram or to other visible objects),
• attributes and links of the context object,
• parameters of the triggering event
• activity sequence

12
Ok / New, PutGreen, Open
Enter
idle Do/display welcome mess
checking Do/Checks Do/display wait mess
not Ok
refusing-access Do/display refusal mess
after 30 sec
13
(No Transcript)
14
AccountOk / New, PutGreen, Open
Enter / CardOk?
idle
checking-card
CardOk / AccountOk?
after 30 sec
checking-account
not CardOk
refusing-access
not AccountOk
15
Advanced States and Transitions

• UMLs state diagrams have some advanced features
  helping to
• Manage complex behavioral models
• Reduce the number of states and transitions
• Codify a number of common and somewhat complex
  idioms.
• Internal Activities

16
Entry and Exit Activities

• Dispatch the same activity whenever you enter a
  state
• In the symbol for the state, include an entry
  activity marked by the keyword event entry
• Dispatch the same activities whenever you leave a
  state
• In the symbol for the state, include an exit
  activity marked by the keyword event exit
• Entry and exit activities may not have arguments
  or guard conditions.

17
Internal Activities

• Events occuring inside a state but handled
  without leaving the state.
• Difference with self-transitions!!
• Self-transition event triggers the transition,
  state is left, an activity (if any) is
  dispatched, same state is reentered. A
  self-transition dispatches the states exit
  activity and dispatches the states entry
  activity.
• Internal activity if event is triggered, the
  corresponding activity is dispatched WITHOUT
  leaving and then reentering the state!!
• Internal activities may have events with
  parameters and guard conditions, internal
  activities are essentially interrupts.

18
Activities

• Ongoing activity while the object is in a state,
  waiting for an event to occur, i.e., while in a
  state, the object does some work that will
  continue until it is interrupted by an event.
• Do transition specifying the work that is to be
  done inside a state after the entry activity is
  dispatched.
• The activity of a do activity might
• name another state machine,
• specify a sequence of activities.
• Regular activities cannot be interrupted, while
  do-activities can take finite time and can be
  interrupted.

19
Sensorwarning / Blockmotor
Stop-bell
ringing
silent
opening Do/activate-motor-till-closed Entry/Ring-b
ell Exit/Stop-bell
Ring-bell
Open
closed
open
closing Do/Activate-motor-till-open SensorWarning/
Blockmotor Entry/Ring-bell Exit/Stop-bell
Close
20
Composite States

• Composite states can be introduced to group a
  number of states. All substates inherit any
  transition on the composite state. The use of
  composite states makes diagrams often more
  readable
• Orthogonal state diagrams combine different
  independent behaviours of a given object. An
  object can be in different states, each from an
  orthogonal region in the diagram

21
Sequential Composite States

• Sequential composite states composite states can
  be introduced to group a number of states.
• If the object is in a composite state, it is in
  only one of its substates at a time. (substates
  are disjoint)
• Transition leading out of a composite state may
  have its source the composite state or a
  substate. In either case, control first leaves
  the nested state, then it leaves the composite
  state.
• From a source outside a composite state, a
  transition may target the composite state or a
  substate.

22
Sequential Composite States (continued)

• If the target is the composite state, this must
  include an initial state to which control passes
  after entering the composite state and after
  dispatching its entry activity (if any).
• It the target is a substate, control passes to
  the substate after dispatching the entry activity
  (if any) of the composite state and then the
  entry activity (if any) of the substate.

23
AccountOk / New, PutGreen, Open
Cancel
Enter / CardOk?
idle
Cancel
checking-card
CardOk / AccountOk?
Cancel
after 30 sec
checking-account
not CardOk
refusing-access
not AccountOk
24
AccountOk / New, PutGreen, Open
Cancel
idle
Enter / CardOk?
active
checking-card
CardOk / AccountOk?
after 30 sec
checking-account
not CardOk
refusing-access
not AccountOk
25
Stop-bell
ringing
silent
opening Do/activate-motor-till-closed Entry/Ring-b
ell Exit/Stop-bell
Ring-bell
Open
closed
open
closing Do/Activate-motor-till-open Entry/Ring-bel
l Exit/Stop-bell
Close
26
Orthogonal Composite States

• An orthogonal composite state lets you specify
  two or more state machines that execute in
  parallel in the context of the enclosing object.
• If one orthogonal region reaches its final state
  before the other, control in that region waits at
  its final state. When both nested regions reach
  their final state, control from the two
  orthogonal regions joins back in one flow.

27
/ New, PutGreen, Open
idle
Cancel
Enter
checking
CardOk?
checking-card
CardOk
AccountOk?
checking-account
AccountOk
28
Orthogonal Composite States (continued)

• Transition to a composite state decomposed into
  orthogonal regions control forks into as many
  orthogonal flows as there are orthogonal regions.
• Transition from a composite state decomposed into
  orthogonal regions control joins back into one
  flow.
• If all orthogonal regions reach their final
  state, or if there is an explicit transition out
  of the enclosing orthogonal composite state,
  control joins back into one flow.

29
Well-structured state machine (1/2)

• One state machine represents the dynamic aspects
  of an individual object, representing, e.g., an
  instance of a class or the system as a whole
  across several use cases.
• Is simple and therefore should not contain any
  superfluous states or transitions.
• Has a clear context and therefore may have access
  to all the objects visible to its enclosing
  object.
• Is efficient and therefore should carry out its
  behaviour with an optimal balance of time and
  resources required by the activities it
  dispatches.

30
Well-structured state machine (2/2)

• Is understandable and therefore should name its
  states and transitions from the vocabulary of the
  system.
• Is not nested too deeply.
• Uses orthogonal composite states sparingly.

31
Exercises!!!