CRC

1. CRC • Before you can build an object-oriented system, you have to define the classes (objects) that represent the problem to be solved, how the classes relate and interact, the inner workings of objects (attributes and operations), and communication mechanisms (messages) that allow them to work together • Object-Oriented Analysis begins with a description of use-cases. Class-Responsibility-Collaborator (CRC) modeling transforms the information contained in use-cases into a representation of classes and their collaborations with other classes. The characteristics of classes are then modeled using UML

2. CRC Cards

3. Responsibility • What the class knows or does • Student: have names, address, phone number, enroll courses

4. An Example, the IIKH • Imagine you are the chief software architect in a major computing firm • One day your boss rushes into your office with a specification for the next PC-based product. It is drawn on the back of a used dinner napkin, in handwriting that appears to be your boss’s • Briefly, the Intelligent Interactive Kitchen Helper will replace the box of index cards of recipes in the average kitchen

5. Your Job • Your job is to develop the software that will implement the IIKH

6. Abilities of the IIKH • Here are some of the things a user can do with the IIKH [identified as a result of a use-case analysis]: • Browse a database of recipes • Add a new recipe to the database • Edit or annotate an existing recipe • Plan a meal consisting of several courses • Scale a recipe for some number of users • Plan a longer period, say a week • Generate a grocery list that includes all the items in all the menus for a period

7. Scenarios • A scenario is an instance of a use case • It expresses a specific occurrence of the use case • A specific actor ... • At a specific time ... • With specific data • Can be part of the Description field of a use case

8. CRC Cards • Responsibilities • Express responsibilities as short verb phrases containing active rather than passive verbs • Define what should be done, not how to do it (declarative) • Include only essential information, not a great deal of detail • Collaborators • A collaborator for a class is another class with which the first class collaborates to carry out its responsibilities • The sender of a message usually is not one of the collaborators of the receiver

9. CRC Cards • Identify an initial set of classes. Create one card for each • Identify an initial set of responsibilities and collaborators for each card. This may lead to the identification of other classes • Spread the cards out on a desk or attach them to a wall • For a group project, assign one or more cards to each person in the group • Run through usage scenarios; as the flow of control passes to a class, bring that card to a prominent spot • As the scenarios are played out, gaps in the design are filled in. This is a form of iterative refinement

10. The Greeter • Let us return to the development of the IIKH. The first class your team defines is the Greeter. When the application is started, the Greeter puts an informative and friendly welcome window (the greeting) on the screen

11. The Greeter • Offer the user the choice of several different actions • Casually browse the database of recipes • Add a new recipe • Edit or annotate a recipe • Review a plan for several meals • Create a plan of meals • Many of the details concerning exactly how this is to be done can be ignored for the moment

12. A Component, The Greeter • CRC card?

13. The Recipe Database • Ignoring the planning of meals for the moment, your team elects to next explore the recipe database • Must Maintain the Database of recipes • Must Allow the user to browse the database • Must permit the user to edit or annotate an existing recipe • Must permit the user to add a new recipe

14. Postponing Decisions • Many decisions, such as the method of browsing, can be ignored for the moment, as they are entirely encapsulated within the recipe database, and do not effect other components • Scroll bars and windows? • A virtual “book” with thumb-holes and flipping pages? • Keywords and phrases? • Only need to note that somehow the user can manipulate the database to select a specific recipe

15. The Who/What Cycle • As we walk through scenarios, we go through cycles of identifying a what, followed by a who • What action needs to be performed at this moment, • Who is the class charged with performing the action • Every what must have a who, otherwise it simply will not happen. Sometimes the who might not be obvious at first, i.e., who should be in charge of editing a recipe?

16. Responsibilities of a Recipe • We make the recipe itself into an active data structure. It maintains information, but also performs tasks • Maintains the list of ingredients and transformation algorithm • Must know how to edit these data values • Must know how to interactively display itself on the output device • Must know how to print itself • We will add other actions later (ability to scale itself, produce integrate ingredients into a grocery list, and so on)

17. The Planner • Returning to the greeter, we start a different scenario. This leads to the description of the Planner • Permits the user to select a sequence of dates for planning • Permits the user to edit an existing plan • Associates with Date object

18. The Date • The Date class holds a sequence of meals for an individual date • User can edit specific meals • User can annotate information about dates (“Bob's Birthday”, “Christmas Dinner”, and so on) • Can print out grocery list for entire set of meals

19. The Meal • The Meal class holds information about a single meal • Allows user to interact with the recipe database to select individual recipes for meals • User sets number of people to be present at meal, recipes are automatically scaled • Can produce grocery list for entire meal, by combining grocery lists from individual scaled recipes