Thefacade pattern (also spelledfaçade) is asoftware design pattern commonly used inobject-oriented programming. Analogous to afaçade in architecture, it is anobject 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 singlewrapper 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-knownGoF 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 aFacade object that
This enables to work through aFacade 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, anadapter can be used when the wrapper must respect a particular interface and must supportpolymorphic behavior. Adecorator 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
In thisUMLclass diagram, theClient class doesn't access the subsystem classes directly.Instead, theClient works through aFacade class that implements a simple interface in terms of (by delegating to) the subsystem classes (Class1,Class2, andClass3).TheClient depends only on the simpleFacade interfaceand is independent of the complex subsystem.[4]
The sequence diagram shows the run-time interactions: TheClient object works through aFacade object that delegates the request totheClass1,Class2, andClass3instances 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).
structCPU{voidFreeze();voidJump(longposition);voidExecute();};structHardDrive{char*Read(longlba,intsize);};structMemory{voidLoad(longposition,char*data);};classComputerFacade{public:voidStart(){cpu_.Freeze();memory_.Load(kBootAddress,hard_drive_.Read(kBootSector,kSectorSize));cpu_.Jump(kBootAddress);cpu_.Execute();}private:CPUcpu_;Memorymemory_;HardDrivehard_drive_;};intmain(){ComputerFacadecomputer;computer.Start();}
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