Connie's Convenience Store - About Peter Coad

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62 Connie’s Convenience Store Working Out Problem-Domain Dynamics with Scenarios Here are some scenario view conventions and notes: – The “recognizer” object is in the left-most column. – The first service in the scenario is listed immediately below the “class with objects” symbol, in that first column. – A sending service is the service name that precedes the tail of a message arrow. Within an object symbol, when you need to explicitly identify which service is the sending service, use square brackets, identifying the sending service. – The argument list consists of inputs, followed by a semicolon, then outputs. Now expand this scenario. What does it take for a sale to calculate its subtotal? Remember the pattern at work here. Sale–sale line item is an instance of the transaction–transaction line item pattern. A sale tells each of its items to calculate its total. Does a sale line item know enough to calculate its total? No. What else does it need? It needs the item’s price. Who knows the price? An item does (indirectly; it needs to work with its price objects). What do you really want an item to do for you? Instead of giving you a value, what answer or what result do you need? It’s this: how much for quantity and date. Now we’re getting somewhere. That’s one of the system features. Yes! Work out the scenario view, this time for sale, sale line item, and item (Figure 1–40): Sale calcTotal calcSubtotal calcTax calcTotal calcSubtotal [calcTotal] calcTax n SaleLineItem calcSubtotal calcSubtotal howMuchForQty getPriceForDate Figure 1–40: The “sale calculates its total” scenario view (round II). Here’s how to read it: A sale object is told to calculate its total. The “calcTotal” service invokes the “calcSubtotal” service. Item howMuchForQty getPriceForDate Scenario: sale calculates its total. Scenario constraint: 0.1 seconds, 9 out of 10 times, 0.5 seconds max. sale.calcTotal ( ; total) sale.calcSubtotal ( ; subtotal) saleLineItem.calcSubtotal ( ; subtotal) item.howMuchForQty (qty, date ; amt) item.getPriceForDate (date ; price) sale.calcTax ( ; totalTax)
Working Out Problem-Domain Dynamics with Scenarios Connie’s Convenience Store 63 The “calcSubtotal” sends a “calcSubtotal” message to a sale line item object (it repeats this for all of its sale line items). A sale line item is told to calculate its subtotal. The “calcSubtotal” service sends a “howMuchForQty” message to an item object, giving it a quantity and a date. 4 An item object is told to calculate its amount for quantity. The “howMuchForQty” service invokes the “getPriceForDate” service, giving it a date. The “getPriceForDate” service returns a price. The “howMuchForQty” returns an amount. The “calcSubtotal” service returns a subtotal. The “calcSubtotal” service returns “subtotal.” The “calcTotal” service invokes the “calcTax” service. The “calcTax” service returns “totalTax.” The “calcTotal” service returns “total.” Here’s another scenario view convention: – The “n” on a message arrow indicates that a message may be sent to some number of objects (Figure 1–40). So when you see (Figure 1–41): Sale SaleLineItem n Figure 1–41: Sending a message to some number of objects. 4Note that “how much for qty” requires that the quantity be passed to it, as an argument. One might cry out, “just ask for the value and reduce the argument coupling.” But wait; argument coupling is not the full story here. Consider the following. Would it be generally useful for an item to be able to calculate “how much for qty?” Yes. And as the business rules become more complicated, what object will eventually know enough so it can calculate quantity discounts for a particular item? An item object (line items never need to know about detail like that). Very cool.
Page 1 and 2:
GETTING STARTED 1 Connie’s Conven
Page 3 and 4:
Identifying System Purpose and Feat
Page 5 and 6:
Selecting Objects Connie’s Conven
Page 7 and 8:
Selecting Problem-Domain Objects Co
Page 9 and 10:
Selecting Problem-Domain Objects Co
Page 11 and 12: Selecting Problem-Domain Objects Co
Page 19 and 20: Applying Patterns: Select and Organ
Page 29 and 30: Establishing Responsibilities for P
Page 55 and 56: Applying Patterns: Establish Proble
Page 61: Working Out Problem-Domain Dynamics
Page 65 and 66: Scenario: sale calculates its total
Page 67 and 68: Selecting Human-Interaction objects
Page 69 and 70: Establishing Human Interaction Resp
Page 71 and 72: Establishing Human Interaction Resp
Page 73 and 74: Working Out Human-Interaction Dynam
Page 75 and 76: Scenario: actual cashier logs on. S
Page 77 and 78: Scenario: sell item. Receipt SaleLi
Page 79 and 80: Scenario: get payment by check Rece
Page 81 and 82: Selecting System-Interaction Object
Page 83 and 84: Establishing System-Interaction Res
Page 85 and 86: Working Out System-Interaction Dyna
Page 87 and 88: Selecting Data-Management Objects a
Page 89 and 90: Working Out Data-Management Dynamic
Page 91 and 92: Progress at This Point Connie’s C
Page 93 and 94: Progress at This Point Connie’s C
Page 95: Wrap Up Connie’s Convenience Stor
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Connie's Convenience Store - About Peter Coad

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