Question

What are the pattern elements for the Facade Design Pattern?

Answer 1

Pattern elements for a Facade design pattern are listed below :-

• Intent : Facade is a structural design pattern that wraps a complicated subsystem with a simpler interface. It basically adds an interface to existing system to hide its complexities.   
• Motivation : A common goal is to minimize the communication and dependencies between subsystems. To achieve this goal we can introduce a facade that provides a single, simplified interface to the facilities of a subsystem.   
• Problem & Solution : Please refer to the below attached image for better understanding.

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• Applicability : We should use facade subsystem as an entry point to each subsystem level for clients and other subsystems. If subsystems are dependent, then we can simplify the dependencies between them by making them communicate with each other through their facades.
• Facade structure : Please refer to the below attached image for Facade structure.
• Participants : 1) Facade - it has all the information about the responsible subsystem classes for a client's request and appoints them to appropriate subsystem objects. 2) Subsystem classes - they implement their functionality and also handle the work assigned by the facade object.
• Collaborations : Clients communicate with the subsystem by a single unified interface(Facade). And Facade forwards the requests to respective subsystems objects.
• Consequences : 1) Facade reduces the number of objects that clients deal with by providing a unified interface and make the usage of Subsystem very easy for the client. 2) Facade doesn't prevent applications from using subsystem classes if they need to. So we can choose between ease of use and generality.
• Implementation : We are creating a MobileShope interface and concrete classes implementing the MobileShope interface. A facade class ShopeKeeperFacade is defined in the next step. ShopekeeperFacade class uses the concrete classes to pass user calls to these classes FacadeClient, our client class will use ShopekeeperFacade class to show the results. Please refer to the image below :
• Related Patterns : 1) Abstract factory pattern :- We can use this with facade to provide an interface for developing subsystem objects in a subsystem independent way. 2) Mediator :- it is similar to facade, it abstracts functionality of existing classes. It's main function is to abstract arbitrary communication between colleague objects.