Facade

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

• Structural Pattern

Facade: Motivation

Facade: Motivation

Facade: Structure

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: 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 the 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