The facade pattern (also spelled façade) is a software-design pattern commonly used in object-oriented programming. Analogous to a facade in architecture, a facade is an object 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 by masking interaction with more complex components behind a single (and often simplified) 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 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 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[edit]

The Facade^[1]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?^[2]

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.

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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[edit]

A Facade is used when an easier or simpler interface to an underlying object is desired.^[3] 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.

Pattern	Intent
Adapter	Converts one interface to another so that it matches what the client is expecting
Decorator	Dynamically adds responsibility to the interface by wrapping the original code
Facade	Provides a simplified interface

The facade pattern is typically used when Icloud photo library not uploading mac sierra update.

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[edit]

UML class and sequence diagram[edit]

A sample UML class and sequence diagram for the Facade design pattern.

In this UMLclass diagram, 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 interfaceand is independent of the complex subsystem.^[4]

The sequence diagram shows the run-time interactions: The Client object works through a Facade object that delegates the request tothe Class1, Class2, and Class3instances that perform the request.

UML class diagram[edit]

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[edit]

Facade Game Not Loading Mac

Facade Game Not Loading Machine

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++[edit]

References[edit]

^Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides (1994). Design Patterns: Elements of Reusable Object-Oriented Software. Addison Wesley. pp. 185ff. ISBN0-201-63361-2.CS1 maint: multiple names: authors list (link)
^'The Facade design pattern - Problem, Solution, and Applicability'. w3sDesign.com. Retrieved 2017-08-12.
^Freeman, Eric; Freeman, Elisabeth; Sierra, Kathy; Bates, Bert (2004). Hendrickson, Mike; Loukides, Mike (eds.). Head First Design Patterns(paperback). 1. O'Reilly. pp. 243, 252, 258, 260. ISBN978-0-596-00712-6. Retrieved 2012-07-02.
^'The Facade design pattern - Structure and Collaboration'. w3sDesign.com. Retrieved 2017-08-12.

External links[edit]

The Wikibook Computer Science Design Patterns has a page on the topic of: Facade implementations in various languages

Wikimedia Commons has media related to Facade