Introduction to Design Patterns as a Best Practice

8/14/2019 Introduction to Design Patterns as a Best Practice

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Mark Godfrey

Introduction to Design Patterns as a Best Practice

Kevin Thomas - 8/11/2009


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Objectives

Introduce the concept of design patterns as

originally defined by the Gang of Four

Define and explain several commonly-used GoFpatterns

Provide a working .NET solution (code samples)

demonstrating these patterns in a somewhat

real-world situation Lay the groundwork for exploring Test-Driven

Development, which requires the use of design

patterns to be successful


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Agenda

Provide a somewhat real-world set of user requirements

Define terms Best Practice

Design Pattern Briefly discuss a possible ugly code solution

Explore several design patterns Pattern definition

Example using metaphor

UML Explain how the design patterns can be used in a

solution

Code samples from a working Visual Studio 2008 .NETsolution


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User Requirements

Assignment:

2 users. Both want:

Extract files from an archive

Validate the archive before extraction

Report the results back to the UI

User #1 wants:

Windows (console)

application Only Zip files for now

1 MB archive file size limit

User #2 wants:

Web application

Zip or Tar files, maybeothers

20 KB archive file size limit

Archive must contain a

Readme.txt file


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Solution Overview

This talk will demonstrate the best practice of

using design patterns to solve this problem and

meet all of the user requirements for both users.

The result will be easily maintained, flexible,

reusable, and testable code.

Sample code: C#.NET using Visual Studio 2008


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Definition: Best Practice

A Best Practice is a solution

that when compared to other

solutions for the same

problem offers improved

ease-of-implementation,

maintainability, performance,

and/or security


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Definition: Design Pattern

A Design Pattern describes a general solution to

a design problem that recurs repeatedly.

1. Pattern Name labels a

common solution2. Problem

What the pattern solves

When to apply the pattern

3. Solution

Elements that comprise thedesign

A template, not copy and paste

code

4. Consequences the results

and trade-offs of applying the

pattern

4 fundamental components to any Design Pattern:


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Possible Solution - Ugly Code

Windows App:

1. Get archive file

2. Validation

1. Check archive file extension(code block)

2. Check archive file size (codeblock)

3. Extraction

1. Unzip the archive file

4. Display the results

Web App:

1. Get archive file

2. Validation

1. Check archive file extension(copy/paste from Windows App)

2. Check archive file size(copy/paste from Windows App)

3. Check archive contents

3. Extraction

1. If .zip

(copy/paste from Windows App)2. Else

Extract from the Tar file

4. Display the results

You might be tempted to do something like this:


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Issues - Ugly Code

This ugly code solution is hard to maintain, noteasily reused, and difficult to test

Lots of copying and pasting code from one

solution to the other Difficult to modify if add a new validation, drop a

validation, or need to change the order Add new if/then/else blocks

Reorder chunks of code Not easily tested

Big chunks of code

Difficult to run code in test environment


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Design Patterns Explained

Following slides will define several Gang of Four

design patterns

For each DesignPattern, will provide:

Definition

Metaphoricalexample

UML


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Adapter Pattern - Definition

Adapter Pattern

Convert the interface of a class into

another interface clients expect.Adapter lets classes work together

that couldn't otherwise because of

incompatible interfaces.


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Adapter Pattern - Example (appliance)

USA Appliance

Intended

consumer code

framework(120 V)

Class I want to

use

Class is designed to work with a particular framework

My framework

(240 V)

Make the assembly work with my code using an adapter

USA Appliance

Add a custom

Adapter


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Adapter Pattern - UML


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Strategy Pattern - Example (game)

Strategies can be plugged in to existing code - the

existing code doesnt care which strategy is used

Go to building

Open

Door

Enter building

Turn knob

Kick down door

Battering ram

Grenade

Door-opening Strategies

Strategy

interface


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Strategy Pattern - UML


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Decorator Pattern - Definition

Decorator Pattern

Attach additional responsibilities to an

object dynamically. Decoratorsprovide a flexible alternative to

subclassing for extending

functionality.


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Strategy Pattern - Example (budget)

Decorators are added for each employee depending on the

employees title, job description, etc. Call code doesnt care which

decorator(s), if any, have been applied

CalculateRent

Calculate

Salary

Calculate

Travel

total += rent

for each employee e in employees

total += e.GetPay()

total += travel

StaffOvertime (decorator)

GetPay():

Return component.GetPay() + overtime

StaffPension (decorator)

GetPay():

Return component.GetPay() - pension

Employee

GetPay(): Return hours * rate


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Decorator Pattern - UML


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Faade Pattern - Definition

Faade Pattern

Provide a unified interface to a set of

interfaces in a subsystem withouthiding the subsystem. Faade defines

a higher-level interface that makes

the subsystem easier to use.


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Faade Pattern - Example (hardware store)

Faade provides an easy-to-use interface to a library and

provides the most common features

Get dog

Put dog outside

Hardware Store

Get dog house

Preassembled dog house

easy to use

most common features

Custom dog house

harder to use - must determine

correct tools, sequence, etc

can customize features


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Faade Pattern - UML


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Other Patterns

Template pattern

Define the skeleton of an algorithm in an operation,deferring some steps to subclasses. Template Methodlets subclasses redefine certain steps of an algorithmwithout changing the algorithm's structure.

Singleton pattern

Ensure a class has only one instance and provide a

global point of access to it.

http://www.dofactory.com/Patterns/Patterns.aspx for moredetails


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Revisit the User Requirements

Extraction

Extract from ZIP

Extract from TAR

Extract from other

Suggests Strategy pattern

User Interface

Need Console and Web

UI

Want to reuse code Want to make it easy for

client to use

Suggests Faade pattern to

minimize code in UI andmake easy to use

Also suggests Adapter

pattern because different

UIs will supply methods

Validation

Validate size and/orcontent and/or extension

and/or other

Suggests Decoratorpattern


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Citations

Data & Object Factory, LLC . Design Patterns.

Dofactory.com. 2009.

Fowler, Martin. Patterns of Enterprise Application

Architecture. Massachusetts: Addison-Wesley,

2002.

Gamma, Helm, Johnson, and Vlissides. Elements of

Reusable Object-Oriented Software. Massachusetts:

Addison-Wesley, 1995. (Gang of Four)


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UI CODE

COMPARISON


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UI Code Comparison

Web Application Console Application


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UI Code Comparison


Setup is similar:

name upload file

save/copy upload file

instantiate a directory manager


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UI Code Comparison


Different extract strategies:

Web = zip, tar

Console = zip


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UI Code Comparison


Different validation decorators: Web = extension + size + content

Console = extension + size


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UI Code Comparison


Both use the faade to get

a status message for display:


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CODE SAMPLES

VS 2008

Introduction to Design Patterns as a Best Practice

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