The Use of Cases: Use Case Scenarios College of Alameda Copyright © 2007 Patrick McDermott Sometimes a Word is worth a thousand Pictures

Why I Recommend Scenario-Driven Can become Test Cases One Case—user did test cases B4 we started –Wonderful! –But never worked in another case Can become User Manual Better yet, can come FROM user Manual: –“Write the user manual, then write the code.”

 Words & Pictures  Eventually Pictures fail, Words are Needed Pictorial diagrams become too complex (e.g., trying to show every decision point and path in an event-driven interface), and don’t illustrate desired system functions Flowchart is no better than code –Once you grasp it, you no longer need it Access Query no Better than SQL Code Like a storyteller with unnecessary detail –It was 5 years ago, Or maybe 6. No, wait, it was 4½ years ago, or was it?

A Use Case has Scenarios Use Case Scenario Class Requirements Program Spec Class Diagram

What’s a Use Case Scenario? A use case scenario is a specific example—an instance of a use case. The sunny-day scenario is the normal case: What happens if the user does exactly what is expected and completes the transaction? The rainy-day scenarios (of which there are usually many) are variations on the use case (e.g., the user tries to perform a task for which she isn’t authorized). Use Case Scenarios are a powerful tool to specify requirements, and can also serve as test case scenarios to assure those requirements are met. They are often used as design or coding specs. They can even evolve into the User Manual. Use brainstorming techniques to help come up with Use Cases and Use Cases Scenarios to better understand the problem.

Scenarios A complete path through a use case, from the first step to the last, is called a Scenario. Most use cases have several scenarios, but they always share the same user goal.

3 Audiences Write your use cases in a way that makes sense to you, your boss, and your customers.— H1 st OOA&D, p. 151

 Check the Weather   UC Main Path ☼Sunny Day  Alternate Path  Rainy Day –Optional Path

UC Formats Simple, Step-based format Focus on Interaction When … Then Can be Paragraph Table Steps Numbers/Subnumbers Alternate too

Use Case Scenario Note that one use case is a collection of several use case scenarios. A use case might be Register Vehicle. Four scenarios: Register New Vehicle, Register Out- of-State Vehicle, Re-Register Existing Vehicle, Register Stolen Vehicle. We’ll use “step” to refer to the lower level items: they are called steps on our diagrams and are portrayed as boxes, but the individual workers often call them tasks, or activities.

A Scenario

Instance Use Case Instance The execution of a sequence of actions in a specified use case. An instance of a use case.

Loops in UC “Lots of times a use case has a set of steps that need to be repeated, but there’s not a standard way to show that in the cases. So we just made one up!” —H1 st OOA&D They put “the system repeats steps a-x until complete.” In reality, not a problem

Include & Extend It’s “easy to spend a lot of time arguing over whether a use case extends this use case, or includes that one…but it’s really not that big of a deal, and those keywords should never distract from the overall design process.” —H1 st OOA&D