SOFTWARE DESIGN AND ARCHITECTURE

LECTURE 29

Review

• Design pattern classifications• Structural Design Pattern– Adapter Pattern

Outline

• Structural Design Patterns– Façade Pattern

Structural Patterns

• deal with the composition of classes or objects.

• Structural class patterns:– use inheritance to compose classes

• Structural object patterns:– describe ways to assemble objects.

FAÇADE PATTERN

Façade Pattern

• Problem– There may be undesirable coupling to many things

in the subsystem, or the implementation of the subsystem may change.

– What to do?

Façade – Example (Home Theater)

Façade - Example• Starting the HomeTheater• Put the screen down• Turn on the projector• Dim the lights• Set the projector input to

DVD• Turn the amplifier on• Set the amplifier to DVD

input• Turn the DVD player on• Start the DVD Player playing

• Turning everything off at the end• Complex task to listen music• When upgraded or new things added, you would have to

learn new steps

Lights, Camera, Façade!

• A Façade is what you need.• With the façade pattern, you can take a

complex subsystem and make it easier to use by implementing a façade class that provides a reasonable set of interfaces.

Watch a movie with ease

Façade - Definition

• Provides a unified interface to a set of interfaces in a sub-system.

• Defines a higher level interface that makes the subsystem easier to use.

Facade

• Problem– There may be undesirable coupling to many things in the

subsystem, or the implementation of the subsystem may change. What to do?

• Solution– Define a single point of contact to the subsystem facade

object that wraps the subsystem. – This facade object presents a single unified interface and is

responsible for collaborating with the subsystem components

Facade

Facade

Façade - Consequences

• It shields clients from subsystem components, thereby reducing the number of objects that clients deal with and making the subsystem easier to use

• It promotes weak coupling between the subsystem and its clients. Weak coupling lets you vary the components of the subsystem without affecting its clients

• It doesn't prevent applications from using subsystem classes if they need to.

Façade vs. Adapter• With both the Facade and Adapter pattern, we have

preexisting classes.• With the Facade, however, we do not have an interface we

must design to, as we do in the Adapter pattern.• We are not interested in polymorphic behavior with the

Facade; whereas with the Adapter, we probably are. • In the case of the Facade pattern, the motivation is to simplify

the interface. With the Adapter, although simpler is better, we try to design to an existing interface and cannot simplify things even if a simpler interface were otherwise possible.

Façade Pattern - Example

Individual Classes

Façade Class

*

Summary

• Structural Design Patterns– Façade Pattern