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Object Design Examples with GRASP (Ch. 18)

Object Design Examples with GRASP (Ch. 18). Use Case Realization. Use Case Realization – how a particular use case is realized within the design model in terms of collaborating objects The use case suggests the system operations that are shown in SSD’s

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Object Design Examples with GRASP (Ch. 18)

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  1. Object Design Examples with GRASP (Ch. 18)

  2. Use Case Realization Use Case Realization – how a particular use case is realized within the design model in terms of collaborating objects • The use case suggests the system operations that are shown in SSD’s • System operations become starting messages entering the Controllers for domain layer interaction diagrams • Domain layer interaction diagrams illustrate how objects interact to fulfill the required tasks

  3. Examples from POS The GRASP design principles will be illustrated by looking at the system operations: • makeNewSale • enterItem • endSale • makePayment We will also look at the following design considerations: • Calculating Sale total • Designing Start Up

  4. Use Case Realization Key point: System operations in the SSDs are the starting messages into the domain layer controller objects

  5. Fig. 18.3

  6. Use Case Realization • Operation contracts are useful for generating interaction diagrams • Conceptual classes inspire design classes

  7. How to Design makeNewSale? Recall that makeNewSale system operation occurs when a cashier requests to start a new sale, after the customer has arrived with goods to buy Operation Contract CO1: makeNewSale -

  8. Contract CO1: makeNewSale Operation:makeNewSale() Cross references: Process Sale use case Preconditions: None Postconditions: - A Sale instance s was created (instance creation) - s was associated with a Register (association formed) - Attributes of s were initialized

  9. How to Design makeNewSale? First need to choose a controller for message makeNewSale Choices for controller: • Object that represents overall system (a façade controller): e.g. Store or Register • Object that represents use case scenario: e.g. ProcessSaleHandler

  10. Fig. 9.17

  11. How to Design makeNewSale? Here we choose the façade controllerRegister Remember Register is now a software object in the design model. It isn’t a physical register.

  12. Fig. 18.5

  13. How to Design makeNewSale? Using Controller and Creator principles we get the sequence diagram:

  14. How to Design makeNewSale? Summary: Register creates the Sale which in turn creates an empty collection of SalesLineItems

  15. How to Design enterItem? enterItem occurs when cashier enters the itemID and the quantity of something to be purchased Operation Contract CO2: enterItem -

  16. Contract CO2: enterItem Operation:enterItem(itemID : ItemID, quantity : integer) Cross references: Process Sale use case Preconditions: There is a sale underway Postconditions: - A SalesLineItem instance sli was created (instance creation) - sli was associated with the current Sale (association formed) - sli.quantity became quantity (attribute modification) - sli was associated with a ProductDescription, based on itemID match (association formed)

  17. How to Design enterItem? First – who has responsibility for system event enterItem? by Controller principle it will be Register Note: use case states that description and price are displayed But by Model-View Separation non-GUI objects should not get involved in output

  18. How to Design enterItem? Need to create and initialize a SalesLineItem and associate it with a Sale • From domain model a Sale contains SalesLineItem objects • By Creator principle a Sale is given responsibility for creating a SalesLineItem

  19. How to Design enterItem? Next we need to find a ProductDescription that matches the ItemID Who should be responsible for knowing a ProductDescription based on an ItemID match? Apply Information Expert principle - From domain model => ProductCatalog knows all ProductDescriptions So assign responsibility to ProductCatalog => operation: getProductDesc(id)

  20. Fig. 9.17

  21. How to Design enterItem? Who sends getProductDesc message? Register is reasonable only if Register can see ProductCatalog Visibility – ability of one object to “see” another object, required in order for an object to send a message to another object - we’ll return to this issue later

  22. Fig. 18.7

  23. How to Design enterItem? Partial DCD related to enterItem design:

  24. How to Design endSale? endSale event occurs when cashier pushes a button indicating end of a sale Contract CO3:endSale Operation:endSale() Cross references: Process Sale use case Preconditions: There is a sale underway Postconditions: - Sale.isComplete became true (attribute modification)

  25. How to Design endSale? First, who is responsible for system event endSale()? => using Controller we make it Register Then, who should be responsible for setting the attribute isComplete to true?=> using Information Expert it should be Sale

  26. Fig. 18.9

  27. How to Calculate Sale Total? From the Process Sale use case: Main Success Scenario: • Customer arrives … • Cashier tells System to create a new sale • Cashier enters item identifier • System records sale line item and … Cashier repeats steps 3-4 until done. • System presents total with tax Total is needed in step 5not concerned with presentation but want to determine who knows total (who has responsibility)

  28. How to Calculate Sale Total? Responsibility: Who should know the sale total? (review domain model) What information is required? • Sale total is sum of subtotals of all sales line-items • Can calculate sales line-item subtotal using line-item quantity and product description price

  29. How to Calculate Sale Total? Summarize information:

  30. Fig. 18.10

  31. Fig. 18.11 & 18.12

  32. How to Design Start Up Start Up use case – Designing interactions involved in system initialization Key: do initialization design last Create an initial domain object - responsible for creating other domain objects

  33. How to Design Start Up POS system – Review initialization requirements based on previous interaction diagrams: • Store, Register, ProductCatalog, ProductDescriptions need to be created • ProductCatalog needs to be associated with ProductDescriptions • Store needs to be associated with ProductCatalog • Store needs to be associated with Register • Register needs to be associated with ProductCatalog

  34. How to Design Start Up POS system – Choose Store as initial domain object Guideline: Choose as an initial domain object a class at or near the root of the containment or aggregation hierarchy of domain objects. This may be a façade controller, such as Register, or some other object considered to contain all or most other objects, such as Store.

  35. Fig. 18.20

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