Connie's Convenience Store - About Peter Coad

Working Out Problem-Domain Dynamics with Scenarios Connie’s Convenience Store 61
Scenario: sale calculates its total
Okay. So check it out.
(Pete) How do you calculate the total for a sale?
(Connie) Well, it is not too hard. You have to calculate the subtotal of all of the
line items and add to it the total taxes due on that amount.
A scenario view shows:
– the objects participating in the scenario (across the top of the view)
followed by a time sequence (going down the page) with the following:
– sending service, a message arrow, a receiving service, and arguments
– sending service, a message arrow, a receiving service, and arguments
– sending service, a message arrow, a receiving service, and arguments.
You get the idea.
Please note that there is a leveling of detail here. Scenario views fully expose the
object interactions. Service descriptions present the details of a specific service. So, it’s
time to build a scenario view.
Here’s a good way to do it. Begin with the services within the “recognizer.” Then
expand the scenario, to include interacting objects, too. Consider sale and its messaging
(hey, it’s not very exciting, but it’s a start). Develop a scenario view (Figure 1–39):
Sale
calcTotal
calcSubtotal
calcTax
calcTotal
calcSubtotal
[calcTotal]
calcTax
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)
sale.calcTax ( ; totalTax)
Figure 1–39: The “sale calculates its total” scenario view (round I).
Here’s how to read it:
A specific sale object is told to calculate its total.
The “calcTotal” service invokes the “calcSubtotal” service.
The “calcSubtotal” service returns “subtotal.”
The “calcTotal” service invokes the “calcTax” service.
The “calcTax” service returns “totalTax.”
The “calcTotal” service returns “total.”