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.
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]
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
This enables to work through a Facade object to minimize the dependencies on a subsystem.
See also the UML class and sequence diagram below.
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.
The facade pattern is typically used when
In this UML class 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 interface
and 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 to
the Class1, Class2, and Class3instances that perform the request.
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).
struct CPU { void Freeze(); void Jump(long position); void Execute();};struct HardDrive { char* Read(long lba, int size);};struct Memory { void Load(long position, char* data);};class ComputerFacade { public: void Start() { cpu_.Freeze(); memory_.Load(kBootAddress, hard_drive_.Read(kBootSector, kSectorSize)); cpu_.Jump(kBootAddress); cpu_.Execute(); } private: CPU cpu_; Memory memory_; HardDrive hard_drive_;};int main() { ComputerFacade computer; computer.Start();}{{cite book}}: CS1 maint: multiple names: authors list (link)
