The facade pattern (also spelled façade) is a software design pattern commonly used in object-oriented programming. Analogous to a façade in architecture, it is an object that serves as a front-facing interface masking more complex underlying or structural code. A facade can: 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 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? 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 object that This enables to work through a 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 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

Structure

UML class and sequence diagram

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

UML class diagram

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).

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