The bigger and more complex a system based on subsystems gets, the more difficult it is for a client to make requests and get what is expected. The Facade acts as an interface to simplify communication with the elements that make up the subsystem and the client. So instead of communicating with all of the components of the subsystem, the client interacts with a single component, the Facade.
The class diagram for the Façade looks a good deal different from most other design patterns. The classes that make up the subsystem are intentionally vague to represent any subsystem. The problem, though, is clearly illustrated by the network of possible connections between the classes in the subsystem and how a client might run into problems when it needs to interact with them all. Figure 1 shows a modified Façade class diagram. In the original GoF diagram, the client is not listed as a participant—only the Façade and the subsystem classes. However, following the Freemans’ diagram (with some slight adjustments of my own to make it closer to the GoF diagram) I included a client but made it implicit.
Figure 1: Facade Class Diagram
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Bill Sanders
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