Software engineering methods Requirements

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Software Engineering Methods
State Diagrams
With additions by Zvika Gutterman and Adam Carmi
Written By
Zvi Avidor
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State Diagram

• A Type of a finite state machine.
• Best for modeling one class whose behavior
  depends on its state.
• Models the states of an object and how it moves
  from state to state for its entire lifetime.
• A class should have its own state diagram if it
  has interesting dynamic behavior.
• Controllers
• Devices (e.g. modem dialing, sending,
  receiving)
• Objects that change state or role (e.g. a Book
  may be borrowed, overdue or in Library)

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Example
Starting Point
Event
State
Transition
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Events

• An event is something that happens and that may
  cause some action.
• Stimuli that affects an object
• Primary means of communication between objects.
• Drive objects from one state to another.
• Corresponds to
• Operations
• Modification of attribute values.
• Passage of a designated period of time.
• The source of the event is not specified.

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States
All compartments are optional
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State Variables

• The composite values of all attributes and links
  held by an object represent the objects
  identity.
• Ignore attributes that do not affect the objects
  behavior.
• No two distinct states of a single object can
  have identical responses to all events.
• Lump together sets of values that have the same
  response to events.
• Temporary attributes (that trigger events) may
  also be represented.

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Activities Actions

• Activity
• Takes time
• Associated only with states.
• May be interrupted by some event.
• Can often be modeled as a nested state diagram.
• Example
• Downloading a file.

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Activities Actions (cont.)

• Action
• Have short duration
• Associated with states and transitions
• Atomic
• Examples
• Beep
• Display a menu
• set a flag
• Sending a message (firing an event)

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Activities Actions (cont.)

• Actions can be spawned on entry and exit to
  states.
• entry/ entry-action(s)
• exit/ exit-action(s)
• Activities may be indicated
• do/ activity(ies)
• Internal events may be indicated (instead of self
  directed transitions)
• internal-event/ action(s)

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Activities Actions (cont.)

• Example

Actions/Activities should be specified as
operation signatures whenever possible
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Transitions

• A transition is a relationship between a source
  state and a target state indicating that an
  object having the source state will change its
  state to the target state, when a specified set
  of events occur and/or a set of conditions are
  satisfied.
• A self transition is a transition whose source
  and target states are the same.
• Actions may be attached to transitions.
• Transitions from the same state must be mutually
  exclusive. Otherwise, objects state is undefined.

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Transitions (cont.)

• Represented as a directed edge between two
  states.
• Transition (general) syntax
• event (attributes) condition(s) / action(s)
  send-clause
• All components are optional.
• Transition semantics
• IF the event occurs AND all specified conditions
  are true THEN spawn the specified actions and
  change object state to the target state.
• If the triggering event is not specified, the
  transition will be attempted when all the state
  activities complete.

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Transitions (cont.)

• A send-clause is used to send a message (event)
  to other objects (or to the same object).
• Syntax destination-expression .
  destination-event-name (arguments)
• Example

fall asleep() myFriends.tell("be quiet")
awake
asleep
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Examples
It would be better to add the guard condition
!isHungry
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Sub-states

• A sub-state is a state that is contained in
  another state (a composite state).
• Sub-states may be nested to any level.
• Example

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Transitions and sub-states

• If a transition is to a sub-state the sub-state
  is entered after any entry action for the
  enclosing composite state is executed.
• If a transition is from a sub-state within the
  composing state, any exit action for the
  sub-state is executed followed by any exit action
  for the enclosing composite state.
• A transition from the composite state may occur
  from any of the sub-states and takes precedence
  over any of the transitions of the current
  sub-state.

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Sub-states (cont.)

• or sub-state
• Sub-states can occur only one at the time.

Driving
Forward
Backward
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Sub-states (cont.)

• and sub-states
• Some of the sub-states can exist simultaneously

Driving
Forward
Backward
Low speed
High speed
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History indicator

• Used to memorize internal states
• usually used in cases we want to return to an
  interrupted state.

Cook
Prepare ingredients
Prepare kitchen
Prepare food
H
Answer Phone