Facade

The facade pattern is used to provide a simple interface to a complex set of interfaces. Often facades are produced to further simplify common APIs or adapt them to a more specific use.

Structure

TODO: add UML diagram

Participants

• Facade: The Facade knows the internal workings of the subsystem and which classes have which responsibilities. It delegates client request to the correct part of the subsystem.
• Subsystem classes: These classes implement the functionality of the subsystem and handle work assigned to them by the facade. However, they are unaware of the existence of the facade.

Collaborations

• Clients communicate with the subsystem through the facade and don't have access to the subsystem objects directly.

Use When

• You want to provide a simple interface to a complex subsystem.
• Clients are closely coupled with the implementation of an abstraction. Use Facade to decouple clients from the implementation.
• You want to layer subsystems. Create a Facade for each subsystem level as an access point and get the subsystems to communicate with each other through their facades.

The design pattern

• provides convenient methods
• makes the inner system more flexible since less methods are called from the outside
• can turn a badly designed API into a very well usable one

Example

The boot process of a computer can be modeled as a Facade. The system's boot() method is called. This method now starts and configures all the services and programs needed to use the computer.