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OOM-UseCaseScripts

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									          Use Case Scripts
          What is a Use Case Script?
• The text to describe a particular Use Case
  interaction
• Takes the form of a 2-way dialogue between
  the actor and the system
• Provides the supporting detail for the Use
  Case diagram
• Do not start until the diagram is complete or
  nearly complete
• Also known as Use Case Descriptions

Originated by K.Ingram, J.Westlake.   Edited by N.A.Shulver
          Use Case Scripts
          What to describe
• Describe the most common/normal form of
  interaction first - the basic course
• Describe possible variations separately - the
  alternative courses
• The script should be in a conversational style:
    –   actor requests….
    –   System responds by….
    –   Actor does…..
    –   Etc..

Originated by K.Ingram, J.Westlake.   Edited by N.A.Shulver
          Use Case Scripts
          Example of a Use Case Script
             DVD rental shop interaction may be:
Actor Actions - Shop Assistant          System Response - Rent DVD
1. Customer tenders DVD(s) to be
rented and membership card
2. Counter assistant enters             3. System provides member details
    member and number into                  of: status of loans and fines
    system

4. Assistant enters identification of   5. System accepts ids and gives fee
    eachDVD to be rented                    payable

6. Assistant requests payment,
    takes money and enters              7. System logs payment details
    payment made
Originated by K.Ingram, J.Westlake.                       Edited by N.A.Shulver
          Use Case Scripts
          Guidelines
• Include a series of numbered sections or steps
  which describe noteworthy events and possibly
  related context, constraints and business rules
• Steps may alternate between actor and system
• May be a series of consecutive steps taken by
  either of them
• Written from the user’s point of view
• Consists of user’s vocabulary


Originated by K.Ingram, J.Westlake.     Edited by N.A.Shulver
          Use Case Scripts
          Conversational Style
• This conversational style script (as if for a
  theatre play) is a good compromise between
  the advantages and disadvantages of other
  methods:
    – it is quick and easy to write (important for capturing
      early, outline information)
    – it is quick and easy to read and to understand
    – it encourages conciseness
    – it identifies the required sequence of actions
    – it highlights causes and effects
Originated by K.Ingram, J.Westlake.           Edited by N.A.Shulver
          Use Case Scripts
          Styles of Description
• In addition to the conversational style script,
  there are other ways of describing the
  interactions e.g.
    –   unstructured narrative
    –   structured English
    –   decision tree
    –   decision table
• Out of interest let us just pause for thought
  about the first two….

Originated by K.Ingram, J.Westlake.    Edited by N.A.Shulver
          Use Case Scripts
          Unstructured Narrative
• A text description of what happens in
  standard English sentences.
• Advantages:
    – easy to write
• Disadvantages:
    –   easy to include ambiguity,
    –   lengthy both to read and to check,
    –   does not highlight cause and effect
    –   does not highlight sequence of actions

Originated by K.Ingram, J.Westlake.          Edited by N.A.Shulver
           Use Case Scripts
           Example of Narrative Description
How long does it take to understand this narrative?
A DVD shop primarily rents DVDs to customers.
Customers can only borrow DVDs if they are registered
members, and need to produce their membership card
each time they borrow a DVD.
Customers come into the shop and once they have
chosen the DVDs that they wish to borrow they take them
to the checkout and hand them to the shop assistant.




 Originated by K.Ingram, J.Westlake.      Edited by N.A.Shulver
          Use Case Scripts
          Example of Narrative Description
   How long does it take to understand this narrative?
   The shop assistant then enters the membership
     number into the system. This will produce on screen
     the member’s personal details, and whether any
     other DVDs are currently on loan. Customers
     cannot borrow more DVDs if there is an overdue
     balance owing.
   Providing the customer does not owe money, the
     shop assistant enters the DVD code which shows
     the rental period and the rental amount. This is
     repeated for each DVD. The shop assistant then
     asks the customer for the amount of money.
     Customers can pay by cash or credit card.
Originated by K.Ingram, J.Westlake.        Edited by N.A.Shulver
             Use Case Scripts
             Structured English
     A text description of what happens but
      using a limited range of phrases.
     Advantages: concise, eliminates
      ambiguity, highlights sequence of actions
     Disadvantages: does not highlight cause
      and effect, may use phrases which are
      unfamiliar to some users


    Originated by K.Ingram, J.Westlake.   Edited by N.A.Shulver
          Use Case Scripts
          “Essential” Use Cases
• These are used during the feasibility and
  analysis stages of the project.
• The aim is to be free of implementation detail
  to show the essence of the business
  requirements (the conceptual model).
• Enables analysts, developers, users and
  clients to understand the scope of the
  problem and the processes required


Originated by K.Ingram, J.Westlake.   Edited by N.A.Shulver
          Use Case Scripts
          “Real” Use Cases
• Now the Essential Use Cases will be used as the
  basis for lateral, creative thinking with the
  opportunity for new ideas on how create the
  system.
• Real Use Cases are used to document the
  design of the project i.e. how it will work in reality.
• For a user interface it may include prototype
  screen shots, print layouts, form layouts, menus.
• For a system interface it may include file layouts.

Originated by K.Ingram, J.Westlake.         Edited by N.A.Shulver
          Use Case Scripts
          Template Sections
• Use Case - its identifier/name
• Actors - list of actors involved. Show which
  one initiates the use case and any other
  actors involved – can be more than one
• Overview - short outline description
  summarising the use case
• Type - category of the use case – see next
  slide
• Cross References - use case relationships
Originated by K.Ingram, J.Westlake.   Edited by N.A.Shulver
          Use Case Scripts
          Categories of Use Cases
• A category is useful as a check that the main
   processes have been identified
    – The category is loosely allocated – it is an attempt
      to think about whether the use case (process) is a
      major process or a minor one or optional process
1. Primary - major common process e.g. Rent Video
2. Secondary - minor or rare processes e.g. Request to
   supply unstocked New Video
3. Optional - processes that may or may not be used


Originated by K.Ingram, J.Westlake.          Edited by N.A.Shulver
          Use Case Scripts
          Alternative Courses
• Alternative courses – very important
    – Remember Use Case Diagrams can show this
          • Getting down to the detail of how this use case script operates
• Can describe alternative events to the typical story.
  These are the less common, the exceptional or error
  cases.




Originated by K.Ingram, J.Westlake.                        Edited by N.A.Shulver
          Use Case Scripts
          Alternative Courses
• Place all the alternatives after all the typical course of
  events
• Example:

7. Customer pays clerk by cash or credit

Alternative Courses
7. Customer has unpaid late charges and will not pay them.
   Collect payment or cancel rental transaction



Originated by K.Ingram, J.Westlake.            Edited by N.A.Shulver
          Use Case Scripts
          Use Case Summary
• Use Case descriptions supply the detail of system
  requirements
    – Very useful and augment Use Case Diagrams
    – Use Case Diagrams have what we call a granularity problem
      i.e. the level of detail is limited
          • Use Case Scripts deal with this limitation
• Conversational scripts are used to describe the
  interactions between actors and use cases
• The basic course may be followed by 1 or more
  alternative courses
• Essential Use Cases are used during Analysis, Real
  Use Cases are used during Design
Originated by K.Ingram, J.Westlake.                      Edited by N.A.Shulver
          Use Case Scripts
          Lecture recap questions
1.    What is an <<extends>> relationship between Use
      Cases? (“is” or “has”?)
2.    What is an <<includes>> relationship? (“is” or
      “has”?)‫‏‬
3.    Would “returning a DVD to the shelf” be likely to be
      a Use Case? …….. Why?
4.    Why is the Use Case description written as a script
      like a play?
5.    How are <<extends>> relationships shown on a
      Use Case Script?
6.    What is meant by “an essential Use Case”?
Originated by K.Ingram, J.Westlake.           Edited by N.A.Shulver

								
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