The facade pattern (also spelled ''façade'') is a

software design pattern In software engineering, a software design pattern or design pattern is a general, reusable solution to a commonly occurring problem in many contexts in software design. A design pattern is not a rigid structure to be transplanted directly into s ...

commonly used in

object-oriented programming Object-oriented programming (OOP) is a programming paradigm based on the concept of '' objects''. Objects can contain data (called fields, attributes or properties) and have actions they can perform (called procedures or methods and impl ...

. Analogous to a

façade A façade or facade (; ) is generally the front part or exterior of a building. It is a loanword from the French language, French (), which means "frontage" or "face". In architecture, the façade of a building is often the most important asp ...

in architecture, it is an

object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Object (abstract), an object which does not exist at any particular time or place ** Physical object, an identifiable collection of matter * Goal, an a ...

that serves as a front-facing interface masking more complex underlying or structural code. A facade can: * improve the readability and usability of a

software library In computing, a library is a collection of resources that can be leveraged during software development to implement a computer program. Commonly, a library consists of executable code such as compiled functions and classes, or a library can ...

by masking interaction with more complex components behind a single (and often simplified)

application programming interface An application programming interface (API) is a connection between computers or between computer programs. It is a type of software Interface (computing), interface, offering a service to other pieces of software. A document or standard that des ...

(API) * provide a context-specific interface to more generic functionality (complete with context-specific input validation) * serve as a launching point for a broader refactor of

monolithic A monolith is a monument or natural feature consisting of a single massive stone or rock. Monolith or monolithic may also refer to: Architecture * Monolithic architecture, a style of construction in which a building is carved, cast or excavated f ...

or tightly-coupled systems in favor of more loosely-coupled code Developers often use the facade design pattern when a system is very complex or difficult to understand because the system has many interdependent classes or because its source code is unavailable. This pattern hides the complexities of the larger system and provides a simpler interface to the client. It typically involves a single

wrapper class In object-oriented programming, a wrapper class is a class that encapsulates types, so that those types can be used to create object instances and methods in another class that needs those types. So a primitive wrapper class is a wrapper class th ...

that contains a set of members required by the client. These members access the system on behalf of the facade client and hide the implementation details.

Overview

The Facade design pattern is one of the twenty-three well-known '' GoF design patterns'' that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse. What problems can the Facade design pattern solve? * To make a complex subsystem easier to use, a simple interface should be provided for a set of interfaces in the subsystem. * The dependencies on a subsystem should be minimized. Clients that access a complex subsystem directly refer to (depend on) many different objects having different interfaces (tight coupling), which makes the clients hard to implement, change, test, and reuse. What solution does the Facade design pattern describe? Define a Facade object that * implements a simple interface in terms of (by delegating to) the interfaces in the subsystem and * may perform additional functionality before/after forwarding a request. This enables to work through a Facade object to minimize the dependencies on a subsystem.
See also the UML class and sequence diagram below.

Usage

A Facade is used when an easier or simpler interface to an underlying object is desired. Alternatively, an

adapter An adapter or adaptor is a device that converts attributes of one electrical device or system to those of an otherwise incompatible device or system. Some modify power or signal attributes, while others merely adapt the physical form of one co ...

can be used when the wrapper must respect a particular interface and must support polymorphic behavior. A decorator makes it possible to add or alter behavior of an interface at run-time. The facade pattern is typically used when * a simple interface is required to access a complex system, * a system is very complex or difficult to understand, * an entry point is needed to each level of layered software, or * the abstractions and implementations of a subsystem are tightly coupled.

Structure

UML class and sequence diagram

In this UML

class diagram In software engineering, a class diagram in the Unified Modeling Language (UML) is a type of static structure diagram that describes the structure of a system by showing the system's classes, their attributes, operations (or methods), and the re ...

, the Client class doesn't access the subsystem classes directly. Instead, the Client works through a Facade class that implements a simple interface in terms of (by delegating to) the subsystem classes (Class1, Class2, and Class3). The Client depends only on the simple Facade interface and is independent of the complex subsystem. The sequence diagram shows the run-time interactions: The Client object works through a Facade object that delegates the request to the Class1, Class2, and Class3 instances that perform the request.

UML class diagram

; Facade : The facade class abstracts Packages 1, 2, and 3 from the rest of the application. ; Clients : The objects are using the facade pattern to access resources from the packages.

Example

This is an abstract example of how a client ("you") interacts with a facade (the "computer") to a complex system (internal computer parts, like CPU and HardDrive).

C++

struct CPU ; struct HardDrive ; struct Memory ; class ComputerFacade ; int main()

References

External links

Description from the Portland Pattern Repository
{{Design Patterns Patterns Software design patterns Articles with example C Sharp code Articles with example Java code Articles with example Ruby code