Modelling dynamic information

1. n.a.shulver@staffs.ac.uk Modelling dynamic information • So far we have seen: • Use Case Diagrams – requirements capture, interface modelling • Use Case Scripts – more detailed and with alternate, less common cases noted • Class Diagrams – description of the structure of a class and possibly its relationship with other classes • These are static descriptions of software

2. n.a.shulver@staffs.ac.uk Dynamic Modelling • In order to model the dynamic (run-time) relationships we need different tools • The UML provides us with State Diagrams and Object Sequence Diagrams (OSD)‏ • There are others...

3. n.a.shulver@staffs.ac.uk State Diagrams • What do state diagrams describe? • When things happen • The life-cycle of an instance (object)‏ • How useful are they? • Often but not always used • Logically links a Class Diagram to a State Diagram – events are operations on classes in the Class Diagram • Clarifies the internal state and external behaviour of an object

4. n.a.shulver@staffs.ac.uk State Diagram Elements start point states end point(s) transitions (labeled with events)‏

5. n.a.shulver@staffs.ac.uk State Diagram examples A chess game class • White takes the first turn, then turns alternate • After any move “mate” or “stalemate” may occur

6. n.a.shulver@staffs.ac.uk State Change Events • A state describes a particular condition of an object for a period of time – it waits for an event to change state or to “fire” • Events can be of the following types: • Change – occurs when a condition becomes true i.e. a boolean • Call/Signal – occurs when an object receives a call for one of its operations (from another object or from itself)‏ • Time trigger

7. n.a.shulver@staffs.ac.uk Guards • Known as a guard condition • It guards the transition i.e. the transition can only take place if the guard condition evaluates to true • Guards are constraints on behaviour • Example in Visio

8. n.a.shulver@staffs.ac.uk Event Types • Kinds of events • receipt of a call on an operation - a message • receipt of an explicit signal from another object - a message • a condition becoming true • passage of a designated period of time • Basic semantics about event labelled transitions • events are triggers that activate state transitions • if an event occurs and guard is false then the event is ignored (events are not stored)‏

9. n.a.shulver@staffs.ac.uk Visio Support – State Chart

10. n.a.shulver@staffs.ac.uk Inside States • Three kinds of internal content for a state • These represent the internal activities or state activities that are executed in that state • Entry actions • Exit actions • Internal transitions - do not cause a state change • All the above represent the operations for the class that is being modelled

11. n.a.shulver@staffs.ac.uk How to find States • A state chart captures all the possible responses of a single object to all the Use Cases in which it is involved • So a cross reference of which of the Use Cases a class is involved in will give you possible states for an object of that class • The state chart can be seen as a description of all the possible life-cycles for an object • Provides a dynamic view of a class

12. n.a.shulver@staffs.ac.uk Object Sequence Diagrams • OSDs depict a Use Case Script visually but also show the classes (objects) used in the script • They show the sequence of object interaction • Read the Visio tutorials for detailed information on the Sequence Chart • The next slides show example Sequence Charts

13. n.a.shulver@staffs.ac.uk Simple Example

14. n.a.shulver@staffs.ac.uk

15. n.a.shulver@staffs.ac.uk Sequence Diagrams • Purpose • Captures the behaviour of a single use case - how messages are sent within a use case • Shows how objects interact with each other - show how they send and receive messages • Focuses on time • Syntax • Object • Time • message

16. n.a.shulver@staffs.ac.uk Sequence Diagram • Example

17. n.a.shulver@staffs.ac.uk Sequence Diagram • Types of messages • simple • flat flow of control - control is passed without describing any details • synchronous • nested flow of control - operation call • handling of the message is completed before the caller resumes execution • asynchronous • there is no explicit return to the caller • the sender continues to execute after sending the message without waiting for it to be handled

18. n.a.shulver@staffs.ac.uk Sequence Diagram • Activation • Time during which object performs an action either directly or through another object (subcontractor)‏ • Return • Implicit • Explicit

19. n.a.shulver@staffs.ac.uk Drawing an OSD • Select a Use Case for which you have a Script • Make a list of classes which appear on that script (you may have done this already if you have created a Class Diagram)‏ • Use Visio to create a new Sequence Diagram • Do the simple things first and build it up • Place the objects (classes) on the diagram • Think about the order that you put them in

20. n.a.shulver@staffs.ac.uk Drawing an OSD • When creating messages, for the first draft just use the simple message call shape • Refine to synchronous or asynchronous at the 2nd draft • Finally, put the rectangle duration box (operation life) overlay onto the timeline for each object • Then review it and do a 2nd draft

21. n.a.shulver@staffs.ac.uk Self-paced tutorial • Create a State Diagram in Visio for the third version of the example Soup Menu application • Hints: states include ingredient choice (three types) and action choice • What kind of transitions are there? - write down a list • Hints: remember that each transition is a flow from one state to another • Finally, what events/actions are there for the transitions in your list?