1. BTS530: Major Project Planning and Design
Basic Design Pattern Concepts
All References and Material unless otherwise specified From: Applying UML and Patterns, 3rd Edition, chapter 25

2. Basic Patterns/Concepts
Controller
Use Case / Session
Façade
For protecting from variations
For simplicity
Polymorphism

3. Use Case / Session Controller
What first object beyond the UI controls (receives and coordinates) a system operation?
A “controller” object
Uses use case logic to determine how a message from the UI should be handled
Can be considered a special type of Facade
Design Patterns, p. 302

4. Use Case / Session Controller

5. Facade Knows which subsystem classes are responsible for a request
Delegates subsystem requests to appropriate subsystem objects
“Protects” and/or “Simplifies”
Design Patterns, p. 461

6. Façade-protect from variationsUI
Business Logic
Data Services

7. Façade-simplify The home tv example:
“client” talks to one object—the HomeTheatreFacade
Much simpler than talking to:
Amplifier
Tuner
Dvd player
Etc.
One function “watch movie” initiates all required functions from other classes
Each individual component can be accessed if required
Adapted from Head First Design Patterns, by Freeman, Freeman, Sierra, Bates, p258

8. Subsystem

9. Façade-simplify Provides a simple interface to a complex subsystem
Still have access to each component in the subsystem

10. Polymorphism The Problem:
How do you handle alternatives based on type? How do you create pluggable software components?
if/then/else or case conditional logic is difficult to maintain
difficult to extend a program with new variations
how can you replace one server component with another without affecting the client?

11. Polymorphism A Suggestion/Solution:
When alternatives/behaviours vary by type (class):
assign responsibilities for the behaviours using polymorphic operations to the types for which the behaviour varies.
polymorphic => same name is given to services (methods) of different objects—the services might vary.
usually implement a common interface or are in a hierarchy with a common superclass (this is language dependent)

12. Problem1: how to support 3rd party tax calculators?
Fig. 25.1

13. Problem2: in the game Monopoly a different action is initiated when players land on different squares—for example landing on GO initiates “receive $200”. What is the behaviour that varies? Why don’t we want to test for the type of an object and use conditional logic to handle this, all in one class?
why?
why?
Fig. 25.2

14. What this looks like in a sequence diagram.
why?
why?
Fig. 25.3

15. For individual squares:
Fig

16. For individual squares:
Fig