use-cases. outline introduction use case diagrams writing use cases
TRANSCRIPT
Use-Cases
Outline
• Introduction
• Use Case Diagrams
• Writing Use Cases
Illustration
Use Cases
• A use case is a contract of an interaction between the system and an actor.
• A full use-case model comprise of:– A diagram, describing relations between use-cases
and actors.
– A document describing the use case in details
Register User
Use case in diagram Use Case in script
admin
Use Case Diagram Objective
1. Create a semi-formal model of the functional requirements
2. Analyze and define:– Scope– External interfaces– Scenarios and reactions
Outline
• Introduction
• Use Case Diagrams
• Writing Use Cases
• Guidelines for Effective Use Cases
What’s in a Use Case Model?Use Case Diagram
Actor Use Case
ActorSomeone or something outside the system acting in a role that interacts with the system. Could be a human or another system.
Use CaseDefines a sequence of actionsperformed by a system that yields an observable result of value to the actor
Example
A Simple Example
Handle Message
Cellular Phone
Customer
Bill Management
Handle CallExternal Phone
Company
ActorsUse CaseSystem
boundaryAssociation
Project Accounting Consultant
System
Project Manager
Submit Timesheet
Complete Timesheets
Use Case Model: Identify
Use Case NameThe name should be:•Brief•Unique•Written as an active verb•Meaningful
You should be able to tell what the actor wants to achieve in the use case from its name.
Each Actor and Use Case needs a brief description.
ActorBriefly state its role, goals and responsibilities in relation to the system.
Use CaseBriefly summarise what takes place in the use case, giving an indication of its scope.
Example actor:
Name: Consultant
Brief Description: The consultant updates their daily timesheet and expense information and gets status reports.
Example use case:
Name: Submit Timesheet
Brief Description: The consultant enters details of the number of hours worked and the activities worked on, then confirms and submits the timesheet.
Linking Use-Cases
• Three mechanism are used:– Include– Extend– Inheritance
Use-Case Levels
User goals
Perform Sale
Choose Products
Fill-in billing info
Sub-functionality
Base Use Case: Used directly by the user
Alistair Cockburn “Writing Effective Use Cases”
The “Include” Construct
• Include is used when:– Decomposing complicated behavior– Centralizing common behavior
• The base use case explicitly incorporates the behavior
of another use case at a location specified in the base.
Perform Sale
Fill-in billing info
<<include>>
Example
Extend – Graphical Representation
• The base use case can incorporate another use case at certain points, called extension points.
• Note the direction of the arrow– The base use-case does not know which use-case
extends it
Perform SaleAfter checkout
Gift wrap Products
<<extend>>Product is a gift
Example
Example: Amazon
Product Page
Review Writing
Shopping Cart
Example – cont’d
Search Product
Navigate Deals
Checkout
Handle Order Status
Login Register
View Product Details Write
Review
Rank Supplier«include»
«include»
«include»
«include»
«extend»user is not a member
«extend»
«extend»
After page generation
Add to cart
«extend»
Customer
Outline
• Introduction
• Use Case Diagrams
• Writing Use Cases
• Guidelines for Effective Use Cases
Structure of a Use Case SpecificationName
Actors
Preconditions
Post conditions
Success Scenario
Alternatives flows
Trigger
Triggers
• What starts the use-case?
• Examples:– Customer reports a claim– Customer inserts card– System clock is 10:00pm
Preconditions
• What the system needs to be true before running the use-case.
• Examples– User account exists– User has enough money in her account– There is enough disk space
Post-Conditions
• A post-condition is the outcome of the use-case.
• Examples– Money was transferred to the user account– User is logged in– The file is saved to the hard-disk
Success Scenario
• The success scenario is the main story-line of the use-case
• It is written under the assumption that everything is okay, no errors or problems occur, and it leads directly to the desired outcome of the use-case
• It is composed of a sequence of action steps • Example:
1. Administrator enters course name, code and description
2. System validates course code
3. System adds the course to the db and shows a confirmation message
Interaction step
Validation Step
Internal Change Step
(plus) Interaction Step
Use-Cases – Common Mistakes
• Complex diagram• No system• No actor• Too many user interface details
– “User types ID and password, clicks OK or hits Enter”• Very low goal details
– User provides name– User provides address– User provides telephone number– …
Alternative Flows
Success Scenario
ShortcutsExceptions
Endpoints
• Used to describe exceptional functionality
• Examples:– Errors– Unusual or rare
cases– Failures– Starting points– Endpoints– Shortcuts
Starting points
Alternative Flows - Example
• Errors:– “Case did not eject properly”– “Any network error occurred during steps 4-7”– “Any type of error occurred”
• Unusual or rare cases– “Credit card is defined as stolen”– “User selects to add a new word to the dictionary”
• Endpoints– “The system detects no more open issues”
• Shortcuts:– “The user can leave the use-case by clicking on the “esc”
key
Exercise• I am the manager of a theatre.• I want to create an automated movie ticket machine.• You are analysts who need to describe
what the customer wants as a set of use cases• Simplifying assumptions:
– One movie showing at a time– Movie time is same every day, only one time, same price– Only manager can change/add movie– Customer can only buy tickets
• Who or what are the actors?• What are the use cases (goals of actors)?
Use case diagramfor Movie Ticket Machine
• Why are there three Actors?• Why three use cases for Customer?• Which use cases look easy to write?
Use cases for ManagerUse case: Set titleActors: Manager, Machine1. Manager requests a change of movie title2. Machine asks manager for new movie title3. Manager enters movie title
Use case: Set priceActors: Manager, Machine1. Manager requests a change of ticket price2. Machine asks manager for new price for movie title3. Manager enters ticket priceAlternatives: Invalid priceIf manager enters price below $1 or greater than $103a. Machine asks manager to reenter price
Use case: Set seatsActors: Manager, Machine1. Manager requests a change in number of seats2. Machine asks manager for number of seats in theatre3. Manager enters number of seatsAlternatives: Invalid number of seatsIf manager enters number less than 20 or greater than 9993a. Machine asks manager to reenter number of seats
Use cases for Customer
Use case: # of ticketsActors: Customer, Machine1. Customer enters number of tickets2. Machine displays total balance dueAlternative: Customer wants zero ticketsAt step 1, customer enters zero tickets1a. Display thank you message1b. Set balance to $0.0
Use case: Return change to customerActors: Customer, Machine1. Customer requests change2. Machine dispenses money3. Machine updates customer balance
Use case: Buy ticketsUse case: Buy ticketsActors: Customer, MachineActors: Customer, Machine1. Customer requests tickets1. Customer requests tickets2. Machine tells customer to put 2. Machine tells customer to put balance due in money slot balance due in money slot3. Customer enters money in money slot3. Customer enters money in money slot4. Machine updates customer balance4. Machine updates customer balance5. Customer requests tickets5. Customer requests tickets6. Machine prints tickets6. Machine prints tickets7. Machine updates number of seats7. Machine updates number of seatsAlternative: Insufficient seatsAlternative: Insufficient seatsAt step 1, if number of tickets requested At step 1, if number of tickets requested is less than available seats, is less than available seats, 1a. Display message and end use case1a. Display message and end use caseAlternative: Insufficient fundsAlternative: Insufficient fundsAt step 5, if money entered < total cost,At step 5, if money entered < total cost,5a. Display insufficient amount entered 5a. Display insufficient amount entered 5b. Go to step 35b. Go to step 3