Using Software Design Using Software Design PatternsPatterns

Bill AndersonBill Anderson

About meAbout me

Fox developer since 1987Fox developer since 1987 Program Director, Los Angeles Visual Program Director, Los Angeles Visual

Foxpro Developers Group (LAFox)Foxpro Developers Group (LAFox) Independent Consultant since 1991Independent Consultant since 1991 SoCal speaker and authorSoCal speaker and author

Design PatternsDesign Patterns

What are patterns? What are patterns? A solution to a problem in a context. A solution to a problem in a context. Similarities to architectural design Similarities to architectural design

issues.issues.

Design Patterns Design Patterns

Four elements to a design patternFour elements to a design pattern NameName When to apply patternWhen to apply pattern Description of elements and Description of elements and

responsibilitiesresponsibilities Consequences of usageConsequences of usage

You’re familiar with patterns!You’re familiar with patterns!

Bridge PatternBridge Pattern

Definition: Decouple an abstraction from Definition: Decouple an abstraction from the implementation so the two can vary the implementation so the two can vary independently.independently.

Fundamental design patternFundamental design pattern Three types of bridgesThree types of bridges

Reference bridgesReference bridges Multiple bridgesMultiple bridges Aggregation bridgesAggregation bridges

Decorator PatternDecorator Pattern

Definition: Attach additional Definition: Attach additional responsibilities to an object responsibilities to an object dynamically.dynamically.

Heuristic: Subclass all “black box” Heuristic: Subclass all “black box” objects (VFP base classes, ActiveX objects (VFP base classes, ActiveX controls, etc.)controls, etc.)

Subclassed a control and changed its Subclassed a control and changed its behavior? Then you know the pattern.behavior? Then you know the pattern.

Adapter PatternAdapter Pattern

Definition: Convert the interface of a Definition: Convert the interface of a class into another interface clients class into another interface clients expect.expect.

Adapters modify interfaces to an Adapters modify interfaces to an object (unlike Decorators, which object (unlike Decorators, which modify behavior of an object). Makes modify behavior of an object). Makes an object look like another.an object look like another.

Template MethodTemplate Method

Definition: Define the skeleton of an Definition: Define the skeleton of an algorithm in an operation.algorithm in an operation.

Used to define a common sequence Used to define a common sequence of eventsof events

Template methods containTemplate methods contain Abstract operation methodsAbstract operation methods Hook operationsHook operations

Iterator PatternIterator Pattern

Definition: Provide a way to access Definition: Provide a way to access elements of an aggregate object. elements of an aggregate object.

Means of traversal handled by Means of traversal handled by iterator object.iterator object.

Should be a container.Should be a container. VFP collection class is ideal.VFP collection class is ideal. Similarities to traversing an array. Similarities to traversing an array.

Aggregate6.Execute()

Iterator.Execute()Aggregate2.Execute()Aggregate3.Execute()Aggregate4.Execute()Aggregate5.Execute()

Aggregate1.Execute()

Observer PatternObserver Pattern

Definition: Define dependency Definition: Define dependency between objects so that if state between objects so that if state change, all dependents are notified.change, all dependents are notified.

Two types of ObserversTwo types of Observers Active (Voyeur)Active (Voyeur) Passive (Publish and Subscribe)Passive (Publish and Subscribe)

Mediator PatternMediator Pattern

Definition: Define an object that Definition: Define an object that encapsulates how a set of objects encapsulates how a set of objects interact. Mediator promotes loose interact. Mediator promotes loose coupling via one communication coupling via one communication point.point.

Used to handle event notification.Used to handle event notification. Mediator is a form of a Passive Mediator is a form of a Passive

Observer.Observer.

Memento PatternMemento Pattern

Definition: Capture an object’s Definition: Capture an object’s internal state so that the object can internal state so that the object can be restored.be restored.

Use this pattern for VFP Use this pattern for VFP housekeeping chores – SET settings, housekeeping chores – SET settings, ON settings, work areas, etc.ON settings, work areas, etc.

Chain of ResponsibilityChain of Responsibility

Definition: Avoid coupling request Definition: Avoid coupling request sender to the receiver by giving sender to the receiver by giving more than one object a chance to more than one object a chance to handle the request. Chain the handle the request. Chain the receiving objects.receiving objects.

Pattern often used in hierarchies – Pattern often used in hierarchies – Error handling, processing import Error handling, processing import data, multi-tier applications, etc.data, multi-tier applications, etc.

Chain of ResponsibilityChain of Responsibility

Business Layer

Data Layer

Presentation Layer

oPresentObj.Delete()

oBizObj.Delete() .StartTransAction()

oCursor.Delete()IF NOT DELETED() DELETEENDIF NOT DELETED()RETURN DELETED()

.EndTransAction()

Abstract Factory PatternAbstract Factory Pattern

Definition: Provide an interface for Definition: Provide an interface for creating families of dependent creating families of dependent objects without specifying their objects without specifying their concrete classes.concrete classes.

Avoid NewObject and CreateObject Avoid NewObject and CreateObject methods – let Factory handle it.methods – let Factory handle it.

VFP data access performance is ideal VFP data access performance is ideal for implementing this pattern.for implementing this pattern.

SummarySummary

Use Software Patterns to keep an Use Software Patterns to keep an application as flexible as possible.application as flexible as possible.

Think in Patterns Think in Patterns You’re already familiar with most patterns.You’re already familiar with most patterns. Use pattern heuristics as a guideline.Use pattern heuristics as a guideline. Understand pattern usage.Understand pattern usage. Don’t get lost in formal definitions.Don’t get lost in formal definitions. Fox Wiki is a great resourceFox Wiki is a great resource

http://fox.wikis.comhttp://fox.wikis.com

Thank you!Thank you!

Remember to fill out your sessionRemember to fill out your session

evaluation. evaluation.

Session slides, white paper, and code Session slides, white paper, and code samples will not be updated.samples will not be updated.