Using Software Design Patterns

1. Using Software Design Patterns Bill Anderson

2. About me • Fox developer since 1987 • Program Director, Los Angeles Visual Foxpro Developers Group (LAFox) • Independent Consultant since 1991 • SoCal speaker and author

3. Design Patterns • What are patterns? • A solution to a problem in a context. • Similarities to architectural design issues.

4. Design Patterns • Four elements to a design pattern • Name • When to apply pattern • Description of elements and responsibilities • Consequences of usage • You’re familiar with patterns!

5. Bridge Pattern • Definition: Decouple an abstraction from the implementation so the two can vary independently. • Fundamental design pattern • Three types of bridges • Reference bridges • Multiple bridges • Aggregation bridges

6. Decorator Pattern • Definition: Attach additional responsibilities to an object dynamically. • Heuristic: Subclass all “black box” objects (VFP base classes, ActiveX controls, etc.) • Subclassed a control and changed its behavior? Then you know the pattern.

7. Adapter Pattern • Definition: Convert the interface of a class into another interface clients expect. • Adapters modify interfaces to an object (unlike Decorators, which modify behavior of an object). Makes an object look like another.

8. Template Method • Definition: Define the skeleton of an algorithm in an operation. • Used to define a common sequence of events • Template methods contain • Abstract operation methods • Hook operations

9. Iterator Pattern • Definition: Provide a way to access elements of an aggregate object. • Means of traversal handled by iterator object. • Should be a container. • VFP collection class is ideal. • Similarities to traversing an array.

10. Observer Pattern • Definition: Define dependency between objects so that if state change, all dependents are notified. • Two types of Observers • Active (Voyeur) • Passive (Publish and Subscribe)

11. Mediator Pattern • Definition: Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling via one communication point. • Used to handle event notification. • Mediator is a form of a Passive Observer.

12. Memento Pattern • Definition: Capture an object’s internal state so that the object can be restored. • Use this pattern for VFP housekeeping chores – SET settings, ON settings, work areas, etc.

13. Chain of Responsibility • Definition: Avoid coupling request sender to the receiver by giving more than one object a chance to handle the request. Chain the receiving objects. • Pattern often used in hierarchies – Error handling, processing import data, multi-tier applications, etc.

14. Chain of Responsibility

15. Abstract Factory Pattern • Definition: Provide an interface for creating families of dependent objects without specifying their concrete classes. • Avoid NewObject and CreateObject methods – let Factory handle it. • VFP data access performance is ideal for implementing this pattern.

16. Summary • Use Software Patterns to keep an application as flexible as possible. • Think in Patterns • You’re already familiar with most patterns. • Use pattern heuristics as a guideline. • Understand pattern usage. • Don’t get lost in formal definitions. • Fox Wiki is a great resource • http://fox.wikis.com

17. Thank you! Remember to fill out your session evaluation. Session slides, white paper, and code samples will not be updated.