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Facade Pattern

Facade : Interface .

The Facade design pattern is a structuralpattern. A structural pattern is "concerned

with how classes and objects are composed to

form larger structures.

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Facade Pattern

Intent

Provide a unified interface to a set of interfaces in a subsystem. Facade

defines a higher-level interface that makes the subsystem easier to use.

Applicability

You want to layer your subsystem. Use Facade to define an entry point to

each subsystem level.

Introduce a Facade to decouple subsystems from clients and other

subsystems, thereby promoting subsystem independence and portability.

A Facade can produce a simple default view of the subsystem.

Collaborations

Clients communicate with the subsystem by sending requests to Facade,

which forwards them to the appropriate subsystem object. Clients that

use facade do not have to access subsystem objects directly.

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Facade Pattern Key Features

Intent You want to simplify how to use an existing system. Youneed to define your own interface.

MotivationStructuring a system into subsystems helps reducecomplexity. A common design goal is to minimize

the communication and dependencies between

subsystems.

Problem You need to use only a subset of a complex system. Or

you need to interact with the system in a particularway.

Solution The Facade presents a new interface for the client ofthe existing system to use.

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CS 406: Design Patterns 4

Facade Pattern: Problem

Client Classes

Subsystem classes

Need to communicate

with

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CS 406: Design Patterns 5

Facade Pattern: Solution

Client Classes

Subsystem classes

Facade

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CS 406: Design Patterns 6

Facade Pattern: Why and What?

Need to provide a simple interface to many, often small,classes. But not necessarily to all classes of the subsystem.

Facade provides a simple default view good enough for most

clients.

Facade decouples a subsystem from its clients.

Subsystems often get complex as they evolve.

A facade can be a single entry point to each subsystem level.

This allows layering.

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CS 406: Design Patterns 7

Facade Pattern: Participants and

Communication

Clients communicate with subsystem classes bysending requests to facade.

Facade forwards requests to the appropriate

subsystem classes.

Clients do not have direct access to subsystemclasses.

Participants: Facade and subsystem classes

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Facade Design Pattern Structure

not exposed

P

not exposed

Faade

exposed

This call

replaced

by 1 & 2;(Client

cant refer

to P)

Client1

2

not exposed not exposed

not exposed

not exposed

Adapted from Software Engineering: An Object-Oriented Perspective by Eric J. Braude (Wiley 2001), with permission.

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Structure

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CS 406: Design Patterns 10

Facade Pattern: Benefits

Promotes weak coupling between subsystem and its

clients.

Helps in layering the system. Helps eliminate circular

dependencies.

Shields clients from subsystem classes; reduces

the number of objects that clients deal with.

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Facade Real World Example

Consumers encounter a Facade when ordering

from a catalogue. The consumer calls one number and speaks

with a customer service representative.

The customer service representative acts as aFacade, providing an interface to the orderfulfilment department, the billing department,

and the shipping department.

Another example : Web-services.

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CS 406: Design Patterns 13

Example: A compiler

StackMachineCodegeneratorRISCCodegenerator

Stream

BytecodeStream

CodeGenerator

Scanner Token

Parser Symbol

PnodeBuilder Pnode

ExpressionNodeStatementNode

Compiler

Compile()Invocations

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Related Patterns

Abstract Factory : The Abstract Factory designpattern can be used with the Facade design pattern tocreate subsystem objects in a subsystem-independentway. It is also possible to replace a Facade with anAbstract Factory to perform the same task.

Mediator : The Facade and Mediator are very

closely related except the Mediator abstractsarbitrary communication between colleague objects,while the Facade abstracts functionality of existing

classes.

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Related Patterns-cont.

Adapter : both use the indirection patterns ,both hide complexity and reduce coupling but

adapter hides differences among interfaces to

similar things while the facade hide identities

of classes in a subsystem.