3.2 Requirements Modeling

• State Representations:

  • There are 2 different characterizations of states: the state of every class as the system is preforming a function
  • The sate of the system observed from outside while the system is performing its function
  • The sate of a class can be active or passive
  • Passive sate: the current status of all attributes of an object
  • Active state: current status of the object while it is continuing transformation or processing.
• The states of a system:

  1. State: a set of observable circumstances that characterize the behavior of a system at a given time
  2. state transition: moving from one state to another
  3. event: an occurrence that causes the system to exhibit some predictable behavior
  4. action: process that occurs as a consequence of making a transition
• State Diagram Basics

  • The UML notation for describing a finite amount of states
  • States are round corner rectangles with the state name
  • Transitions are solid line arrows with 1 or more transition strings
  • Transition strings: describe triggers and the resulting actions
  • Initial (pseudo) state is a solid circle
  • Final state is stopping and is optional
• State Diagram Elements

  • Transition: the relationship between states. It can have a trigger, guard condition or effect
  • Self-Transition: a transition that returns to itself. Useful for effects that are associated with a transition
  • Transition String Format: event-signature [guard]/action-expression
  • Event Signature: empty string or event name followed by a list of event-parameters in parentheses. You can chose to leave out parameter type.
  • Guard: Boolean expression in square brackets
  • Action Expression: is optional. A description of a computation done when the transition occurs
• Optional Parts of a Transition in State Diagram

  • Trigger: the cause of the transition. Can be a signal, an event, a change in some condition, or time passing
  • Guard condition: a condition that must be true for a trigger to cause the transition
  • Effect: an action that happens directly on the object that owns the state as a result of the transition
• State Diagram Execution

  1. There is always a current state
  2. Execution starts in initial state
  3. Event occurs
  4. If a transition has an action-expression, the action occurs when the transition takes place
  5. Machine stops when final state becomes current state
• What happens when an event occurs ?

  • If event matches an event-signature on a transition from the current state and the guard is absent or true: the transition occurs and the current state becomes the target state.
  • If it does not match an event-signature or the guard is false: no change of state occurs
• If the computation occurring in the current state completes, andthere is a transition from the current state with no event-signature:

  • Guard is absent or true: the target state becomes the current state.
  • Guard is false: no change of state occurs.
• Internal Transitions

  • Entry: execute the action-expression when you enter the state
  • Exit: execute the (exit) action-expression while leaving state
  • Do: execute the (do an activity) action-expression when you enter the sate up until you exit
  • Include: action-expression names a finite process. This process is a placeholder for a nested state diagram
• Sequence Diagram Notation

  • Objects are rectangles with names underlined and at the top
  • message are solid line arrows
  • Time is vertical
  • Lifelines are dashed lines extending down from objects
  • Activation is: rectangle on each lifeline. It represents an operation that the object does. The length is how long it takes to do the operation.
  • Truncated lifeline: ' X '
  • Solid line with triangle arrow: sync msg
  • Solid line with ->: Async msg
  • --->: sync msg return/ create instance
• Interaction Fragments

  • Show types of message flow like loops, branches, parallel execution
  • ALT: does 1/2 actions depending on state
  • OPT: Executes if given condition is true
  • LOOP: executes multiple times based on guard
  • SD: surrounds entire sequence diagram
  • BREAK: checks if guard is true and does what's inside. If guard is false, break skips