Design Patterns

Ric Holt

U Waterloo, March 2009

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.

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.

Two Example Patterns

1. MVC (Model View Controller)

2. Adapter

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).

Example MVC

a b ca

bc

[Mancoridis picture]

Views

Controller

Modela b c

503020

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

Example MVC

a b ca

bc

[Mancoridis picture]

Views

Controller

Modela b c

503020

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

See

Interact

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

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

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?

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.

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)

Illustration of Adapter Pattern

ClientOld

package

Old interfac

e

Client Adapter

Old interfac

e New package

New interfac

e

Implementing Adapter Pattern Using Object Composition

ClientOld Interface

Request()

New Interface

SpecificRequest()

Adapter

Request() SpecificRequest()

adapteeinherit