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Software Engineering, CPSC-4360-01, CPSC-5360-01, Lecture 7

Software Engineering, CPSC-4360-01, CPSC-5360-01, Lecture 7. Review of Last Lecture. Class Diagram Association Class, Reification N-Ary Association Qualified Association Interface Interactions diagrams Collaborations, classifier and association roles

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Software Engineering, CPSC-4360-01, CPSC-5360-01, Lecture 7

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  1. Software Engineering, CPSC-4360-01, CPSC-5360-01,Lecture 7 CPSC-4360-01, CPSC-5360-01, Lecture 7

  2. Review of Last Lecture • Class Diagram • Association Class, Reification • N-Ary Association • Qualified Association • Interface • Interactions diagrams • Collaborations, classifier and association roles • Interaction diagrams, object creation and destruction • Role multiplicity and iterated messages • Multi-objects • Conditional messages, messages to self CPSC-4360-01, CPSC-5360-01, Lecture 7

  3. Overview of This Lecture • Statechart • State-dependent behaviors, events, transitions • Initial and final states, guard conditions • Actions, activities, composite states, history states CPSC-4360-01, CPSC-5360-01, Lecture 7

  4. Requirement Analysis Design Implement Test Where are we now? • Topic Covered: • Detailed Class Diagram • Object Diagram • Collaboration Diagram • Sequence Diagram CPSC-4360-01, CPSC-5360-01, Lecture 7

  5. Object Behavior • The behavior of an object is defined by its reaction to messages at any point in execution. • Some object behaviors can be studied by: • Object Diagram, OR • Interaction Diagram. • Not sufficient: • Cannot model all possible scenarios, e.g., only specific sequence of messages are studied. • Only model some legal (possible) states - show how an object behaves in particular interactions. • We need to know about illegal or impossible states to plan for them. CPSC-4360-01, CPSC-5360-01, Lecture 7

  6. Specifying Behaviour in UML • A different notation is needed to summarize the overall behaviour of objects. • UML defines a statechart for this purpose. • Complement interaction diagrams for understanding the dynamic behaviour of the system: • Interaction Diagram: • Models some inter-object messages with well defined order during a short duration. • Statechart: • Models the entire lifetime of a single object, specifying all possible sequences of messages and responses. CPSC-4360-01, CPSC-5360-01, Lecture 7

  7. State-dependent Behaviour • Objects respond differently to the same stimulus/events at different times. • Can be modelled by defining a state machine: • It has a set of states: • an object can be in one state at any time; • the state it is in determines how it responds to events. • It has a set of transitions: • an event can cause the object to move from one state to another. CPSC-4360-01, CPSC-5360-01, Lecture 7

  8. CD Player: An Example • Behaviour of a simple CD player: • A drawer to hold the CD; • Control interface with 3 buttons: • Load button: the drawer will open if it was shut and will shut if it was open; • Stop button: the player will stop playing. If there is no CD, there is no effect; • Play button: the CD will be played. If the drawer is open, the drawer shuts before playing starts. CPSC-4360-01, CPSC-5360-01, Lecture 7

  9. UML Statechart Semantics • A statechart defines the behaviour of instances of a given class. • It shows: • The possible states of an object; • The events it can detect; • Its response to those events. • An object is in oneactive state at a time: • Events may be received at any time, which can trigger a transition to the next active state; • If an Event only causes a loop on a state (i.e., no change of an active state), the transition is known as self-transition. CPSC-4360-01, CPSC-5360-01, Lecture 7

  10. Statechart: Deciding States • Identifying separate states: • Informal principle: • States S1 and S2 are separate if an object in state S1 responds differently to at least one event from the way it responds to that event in state S2. • Syntax: • States: rounded rectangles with the name of the state ( ). • Example: • There are 3 states for the CD player: • Closed, Playing, Open. StateName CPSC-4360-01, CPSC-5360-01, Lecture 7

  11. trigger Statechart: Identifying Events • Events are usually external stimulus: • E.g., messages that can be sent to an object; • Internal stimulus will be covered later in the lecture. • Events usually cause an object to change state: • Moving from one state to another is known as transition; • Events that causes transition are known as trigger; • Events can optionally carry data (similar to message parameter). • Syntax: • An arrow with the trigger attached ( ). • Example: • There are three events for the CD Player: load, play, stop. CPSC-4360-01, CPSC-5360-01, Lecture 7

  12. Statechart CD Player: Statechart Ver. 1 Transition: From Closed state to Open state triggered by load event. State: The Open state. CPSC-4360-01, CPSC-5360-01, Lecture 7

  13. Initial and Final States • To specify the first active state when an object is created/initialized: • Use Initial State: • A transition leading from the initial state indicates the first active state. • No event should be written on a transition from an initial state. • Syntax: a black disc ( ). • To model the destruction of an object: • Use Final State: • Represent the end of flow, i.e., the object no longer exists. • May correspond to actual destruction for software object. • Syntax: a circled black disc ( ). CPSC-4360-01, CPSC-5360-01, Lecture 7

  14. Statechart CD Player: Statechart Ver. 2 Initial State:CD Player switched on Final State: Switched Off the CD Player CPSC-4360-01, CPSC-5360-01, Lecture 7

  15. Non-Deterministic System • When two or more transitions leading from a state share the same trigger: • No way to distinguish between them; • Randomly choose one of them; • The same history of events may end up in a different state; • Known as non-deterministic system. • Can be removed by adding information to distinguish between transitions with the same trigger. CPSC-4360-01, CPSC-5360-01, Lecture 7

  16. CD Player: Non Deterministic Example • In real-life, a CD player will not play if there is no CD inside. • Statechart Ver. 2 does not describe this. • Correct Behaviour: • When the play event is detected, the CD Player should remain in the Closed state if there is no CD inside. • Resulting Statechart (partial): This transition should be used when there is a CD inside. This transition should be for no CD inside. Statechart CPSC-4360-01, CPSC-5360-01, Lecture 7

  17. Guard Condition • Although the two transitions are meant for different scenarios, the statechart does not distinguish among the two, resulting in a non-deterministic behavior. • The non-determinism can be removed if we can specify the condition that determine the correct transition to use. • Syntax: trigger [guard_condition] • Semantics: • If a transition has a guard condition, it can only fire if that transition is evaluated to true; • If all guard conditions are false and there is no unguarded transition, the event will be ignored. • Usually, only one condition is true (determinism). CPSC-4360-01, CPSC-5360-01, Lecture 7

  18. Statechart CD Player: Statechart Ver. 3 Guard Condition • Note that guard conditions are also added to transitions leading from the Open state. What do they represent? CPSC-4360-01, CPSC-5360-01, Lecture 7

  19. Actions • A state can have actions triggered. • There are three ways an action can be triggered: • By an event; • By entering a state; • By leaving a state. • An action triggered by an event takes place in response to that event: • Syntax: Trigger [Guard_Condition] /action • Example: • The CD drawer will be closed when play is detected in the Open state. CPSC-4360-01, CPSC-5360-01, Lecture 7

  20. Statechart CD Player: Statechart Ver. 4 Action triggered by Event CPSC-4360-01, CPSC-5360-01, Lecture 7

  21. Action: Entering/Exiting a State • An action can be triggered as soon as the state is entered. • Useful to capture actions that must be performed regardless the transition used to arrive at the state. • Syntax: • Write in a compartment in the state box: entry/action • Similarly, an action can be performed just before leaving a state. • Syntax: • Write in a compartment in the state box: exit/action CPSC-4360-01, CPSC-5360-01, Lecture 7

  22. CD Player: Statechart (partial) Action triggered by entering a state. Action triggered by leaving a state. Statechart Question: What happened when the play buttonis pressed during CD playing? CPSC-4360-01, CPSC-5360-01, Lecture 7

  23. Action and Activity • Properties of Actions: • Short, self-contained processing; • Finished “instantaneously”; • Cannot be interrupted by events. • An “action” that does not conform to the above is termed as activity instead. • Property of Activities: • Performed during a state; • Carry out for an extended period of time; • Can be interrupted by events. • Syntax: • Write in a compartment of the state box: do/activity CPSC-4360-01, CPSC-5360-01, Lecture 7

  24. Statechart (Partial) CD Player: Activity Example • Question: When the play track activity is being performed, what happen if the stop button is pressed? Activity performed during the Playing state. CPSC-4360-01, CPSC-5360-01, Lecture 7

  25. Completion Transition • As well as being interrupted by events, some activities will come to an end of their own accord. • If an activity completes uninterrupted, then it can trigger a completion transition. • these are transitions without event labels. • Multiple completion transitions can be distinguished by guard conditions. • Example: • When the play track activity is completed: • If it is the last track, go to Closed state; • If it is not the last track, play the next track. CPSC-4360-01, CPSC-5360-01, Lecture 7

  26. Statechart (Partial) CD Player: Completion Transition Example Completion Transition:No more track. Stop playing Completion Transition:Play the next track CPSC-4360-01, CPSC-5360-01, Lecture 7

  27. Internal Transition • There are transitions that leave the object in the same state but does not trigger the entry and exit actions: • A self-transition is not appropriate. Why? • Use an Internal Transition which does not trigger the entry and exit actions. • Syntax: • Write in a compartment of the state box: event/action • Example: • Suppose we add an info button for the CD Player to display the remaining playing time for current track. CPSC-4360-01, CPSC-5360-01, Lecture 7

  28. CD Player: Internal Transition • Compare the two different ways to model the info button. Info /display time Statechart – Self- transition(Partial) Statechart – Internal Transition (Partial) CPSC-4360-01, CPSC-5360-01, Lecture 7

  29. Composite State • States that share similar behavior can be grouped into a Composite State to simplify the statechart. • Composite State: • Syntax similar to simple state; • Contains a number of substates; • When a composite state is active, exactly one of the substate must be active; • Transitions can lead away from a composite state as well as any of its substates. • Example: • CD Player: the response to the play event is similar during the Open or Closed states. CPSC-4360-01, CPSC-5360-01, Lecture 7

  30. Statechart CD Player: Composite State Composite State Transition shared by all substates Sub-State Transition specific to one substate CPSC-4360-01, CPSC-5360-01, Lecture 7

  31. Composite State: Additional Property • A composite state is just like a simple state: • Can have Entry/Exit actions; • Can have extended activity. • A composite state is also like a mini-statechart: • Can have an Initial State to indicate the default substate if a transition terminates at boundary of a composite state; • Can have a Final State, which is triggered when ongoing activity within the state has finished. • Transitions: • Leading away from a composite state apply to all substates; • Arrive at composite state go to the default state; • Can cross composite state boundaries. CPSC-4360-01, CPSC-5360-01, Lecture 7

  32. CD Player: pause button • Pressing the pause button causes playing to be interrupted; • When the button is pressed again, playing continues from the position where it was paused. Only triggers if the Composite State is entered. As there is no entry action, reentering the Playing substate will not restart the track. Statechart CPSC-4360-01, CPSC-5360-01, Lecture 7

  33. History State • When a composite state is entered, it begins at the initial state or directly transits into one of the substates. • Sometimes, it is useful to re-enter a composite state at a point at which it was left. • Re-enter the last active substate. • Indicate this using the History State. • Transitions arriving at the History State activate the last active substate. • If there is no last active substate, a default state can be indicated by an unlabelled transition from History State. • Syntax: • A circled ‘H’ ( ) H CPSC-4360-01, CPSC-5360-01, Lecture 7

  34. CD Player: History State • Suppose re-pressing the play button: • During Playing: restart and play the current track; • During Paused: restart the current track but remain paused. Make use of History State to model the behavior correctly. Statechart CPSC-4360-01, CPSC-5360-01, Lecture 7

  35. CD Player: Final Version Statechart Statechart CPSC-4360-01, CPSC-5360-01, Lecture 7

  36. Entry /action Exit /action Do /activity Event /action State Name Statechart Notation Summary Event(param) [condition] /action Transition and Trigger State H Initial State Final State HistoryState CPSC-4360-01, CPSC-5360-01, Lecture 7

  37. Steps for Constructing Statechart • Identify the objects that have complex behavior; • Determine the initial and final states of the object; • Identify the events that affect the entity; • Working from the initial state, trace the impact of events and identify the intermediate states; • Identify any entry and exit actions on the states; • Expand states into a composite state if necessary; • Check which actions in the state are supported by operation (method); • Refine the class if necessary. • Another example of constructing Statechart: “Ticket Machine”. CPSC-4360-01, CPSC-5360-01, Lecture 7

  38. Creating a Statechart • How information from interaction diagrams can be used to derive statecharts? • It can be hard to identify all necessary states. • Statecharts can be developed incrementally: • consider individual sequences of events received by an object; • these might be specified on interaction diagrams; • start with a statechart for one interaction; • add states as required by additional interactions. CPSC-4360-01, CPSC-5360-01, Lecture 7

  39. Ticket Machine • Consider a ticket machine with two events: • select a ticket type; • enter a coin. • Basic interaction is to select a ticket and then enter coins. • model this as a ‘linear’ statechart. • Problems: • It defines only one transaction, whereas the ticket machine is able to carry out repeated transactions; • It shows only 3 coins, but this number should be arbitrary. CPSC-4360-01, CPSC-5360-01, Lecture 7

  40. Refining the Statechart • This can be improved by adding ‘loops’: • the number of coins entered will vary: entry will continue until the ticket is paid for; • the whole transaction can be repeated. • State ‘Idle’: no transaction is in progress. CPSC-4360-01, CPSC-5360-01, Lecture 7

  41. Adding Another Interaction • Suppose the requirements allow the user to enter a coin before selecting a ticket. • A ‘coin’ transition from the ‘Idle’ state is needed to handle this event. • this transition can’t go to the ‘Paying for Ticket’ state as the ticket is not yet selected. • so a new state ‘Inserting Coins’ is required. • The statechart is thus built up step-by-step. CPSC-4360-01, CPSC-5360-01, Lecture 7

  42. Adding a Second Interaction • If all coins are entered before ticket selected: CPSC-4360-01, CPSC-5360-01, Lecture 7

  43. Integrating the Interactions • Suppose the requirements allow the user to enter some coins before selecting a ticket and the rest after that. • In fact, events can occur in any sequence: CPSC-4360-01, CPSC-5360-01, Lecture 7

  44. Time Events • Suppose the ticket machine times out after 30 seconds. • We need to fire a transition that is not triggered by a user-generated event; • UML defines time events to handle these cases. • Example: a transition will fire 30 seconds after the ‘no ticket selected’ state is entered. CPSC-4360-01, CPSC-5360-01, Lecture 7

  45. Activity States • Both ‘No Ticket Selected’ and ‘Ticket Selected’ should check if the machine is able to return any change that is required. • Efficient solution: activity states! • Activity states define periods of time when the object is carrying out internal processing. • unlike normal activities, these cannot be interrupted by external events; • only completion transitions leading from them; • useful for simplifying the structure of complex statecharts. CPSC-4360-01, CPSC-5360-01, Lecture 7

  46. Returning Change • Note that activity as a property of state (slide 23) is not the same as activity states. • Use an activity state to calculate change. CPSC-4360-01, CPSC-5360-01, Lecture 7

  47. Ticket Machine Statechart • It incorporates ‘Pressing Cancel’ in the middle of a transaction. • The composite state ‘Transaction’ reduces the number of statechart transitions. • Still room for refining: When ‘cancel’ event occurs, the machine should give the money back  Statechart CPSC-4360-01, CPSC-5360-01, Lecture 7

  48. Comparison between statecharts and activity diagrams • There are many similarities between state diagrams and activity diagrams (Chapter 3 of [Bimlesh, Andrei, Soo; 2007]), as activity diagrams are state diagrams extended with some extra notation. • The main difference between activity diagrams and state diagrams is that activity diagrams do not normally include events. • Moreover, an activity is intended to proceed without getting stuck, whereas for state diagrams it is perfectly acceptable for an object never to reach some of its potential states. CPSC-4360-01, CPSC-5360-01, Lecture 7

  49. Requirement Analysis Design Implement Test Where are we now? • Topic Covered: • Detailed Class Diagram • Object Diagram • Collaboration Diagram • Sequence Diagram • Statechart CPSC-4360-01, CPSC-5360-01, Lecture 7

  50. Summary • Statechart • State-dependent behaviors, events, transitions • Initial and final states, guard conditions • Actions, activities, composite states, history states CPSC-4360-01, CPSC-5360-01, Lecture 7

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