Design Patterns

Ric Holt & Sarah Nadi

U Waterloo, March 2010

What is a Design Pattern

• A Design Pattern systematically names, explains, and implements an important recurring design.

• These define well-engineered design solutions that practitioners can apply when crafting their applications.

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Why Design Patterns?

• Good designers do not solve every problem from first principles. They reuse solutions.

• Practitioners do not do a good job of recording experience in software design for others to use. Patterns help solve this problem.

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Four Example Patterns

1. MVC (Model View Controller)

2. Adapter

3. Facade

4. Factory

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1. MVC Pattern(Model View Controller)

• There is a model (subject) which can be thought of as the application

• There are one or more views of the object (by observers)

• There is a controller that mediates between the model and the view(s).

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Example MVC

a b ca

bc

[Mancoridis picture]

Views

Controller

Modela b c

503020

a = 50%b = 30%c = 20%

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Example MVC

a b ca

bc

[Mancoridis picture]

Views

Controller

Modela b c

503020

a = 50%b = 30%c = 20%

See

Interact

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Multiple View Problem

• Need to keep all the views consistent

• If user (or one of users) changes a view, all other views should be updated

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Information Hiding:Insulating Model from Presentation

• The model is the “back end” or “business logic”, or “application”

• To minimize complexity, the model should know as little as possible about– How users interact with system– How many users are there– Any other models

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Implementing MVC

• There are many choices– Where is list of views (observers) kept?– How is notification of change transmitted?– How manage web interface?– Should a view ask for (or should it be told of)

details about changes?

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What Kind of Pattern is MVC?

• Design pattern proper?– Use it for structuring modest configurations

• Architectural pattern?– Use it for overall structure of a software

system, defining interactions among services, browsers etc.

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2. Adapter Pattern

• You have an existing client (application) that uses an old interface to an existing support package.

• You are given a new interface to a new support package

• You need to produce an adapter so that:– The client can use the new interface instead of

the old one (without changing the client)

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Illustration of Adapter Pattern

ClientOld

package

Old interfac

e

Client Adapter

Old interfac

e New package

New interfac

e

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Implementing Adapter Pattern Using Object Composition

ClientOld Interface

Request()

New Interface

SpecificRequest()

Adapter

Request() SpecificRequest()

adapteeinherit

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Advantages of Adapter Pattern

• Can change behavior of existing software without changing its source code

• Can be used for legacy software

• Do not need access to old source code

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3. Facade Pattern

• A facade provides a single simplified interface to a larger part of the system (a subsystem)

• The facade makes this subsystem more readable and easier to understand

• Having facades minimizes communication and dependencies between subsystems

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Structure of the Facade Pattern

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Subsystem Classes

Facade

[Mancoridis picture]

Adapter vs. Facade

• An Adapter is used when the wrapper must respect a particular interface and must support a polymorphic behavior. On the other hand, a facade is used when one wants an easier or simpler interface to work with

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4. Factory Pattern

• A creational pattern dealing with creating objects without knowing their concrete class

• It defines an interface for creating an object, but let the subclasses decide which class to instantiate

• Useful when concrete class of object should be decided at runtime according to parameters

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How does it work

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<abstract>

Product

<Implementation Class>

ProductA<Implementation Class>

ProductB

<static>ProductFactory

createProduct(in param) : Product

Produces different subclasses of Product according to parameters

inherits