Object-Oriented Programming

Design Pattern: Facade

Michael L. Collard, Ph.D.

Department of Computer Science, The University of Akron

Facade

  • Structural Pattern
  • 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

Facade: Motivation

Facade: Motivation

Facade

  • Converts client requests into target class requests
  • Allows a current class to be used by client code expecting a different interface
  • Often the first step towards replacement/major changes to a class with the “wrong” interface
  • May add functionality missing in the target class

Facade: Applicability I

Use the Facade pattern when you want to provide a simple interface to a complex subsystem:

  • Subsystems often get more complicated as they evolve
  • Most patterns result in more and smaller classes
  • Subsystems are more reusable and easier to customize, but also harder to use for clients that don’t need customization
  • Facade provides a simple default view of the subsystem that is good enough for most clients
  • Only clients needing more customizability look beyond the facade for more functionality

Facade: Applicability II

  • There are many dependencies between clients and the implementation classes of an abstraction
    • Decouple the subsystem from clients and other subsystems, thereby promoting subsystem independence and portability
  • You want to layer your subsystems
    • Define a Facade entry point to each subsystem level
    • Simplify dependencies between subsystems by communicating solely through facades

Facade: Structure

Facade: Participants

  • Facade (e.g., Compiler)
    • Knows which subsystem classes are responsible for a request and delegates client requests to appropriate subsystem objects
  • subsystem classes (e.g., Scanner, Parser, ProgramNode, etc.)
    • Handle work assigned by the Facade object
    • Do not know about the Facade, and keep no references or pointers to the Facade
    • Are not marked as part of the pattern

Facade: Collaborations

  • Clients communicate with the subsystem by sending requests to Facade, which forwards them to the appropriate subsystem objects
  • Subsystem objects perform the actual work; the facade may have to work to translate the Facade interface to subsystem interfaces
  • Clients that use the Facade don’t have to access its subsystem objects directly, but can if needed

Facade: Consequences (Benefits)

  • Shields clients from subsystem components reducing the number of objects that clients deal with and making the subsystem easier to use
  • Promotes weak coupling between the client and the subsystems
  • Reduces need for recompilation
  • Simplifies porting to other platforms
  • Allows a choice between ease of use and generality as clients can use the subsystems directly

Implementation

Known Uses: Choices Operating System

Related Patterns

  • Abstract Factory
    • Used with Facade to provide an interface for creating subsystem objects
    • An alternative to Facade to hide platform-specific classes
  • Mediator
    • A facade merely abstracts the interface to subsystem objects to make them easier to use
  • Singleton
    • Usually only one Facade object is required, and Facade objects are often Singletons