Facade pattern

Purpose

• Supplies a single interface to a set of interfaces witin a system.

 

Use when

• A simple interface is needed to provide access to a complex system.
• There are many dependencies between system implementations and clients.
• Systems and subsystems should be layered.
• Provides a unified interface to a set of interfaces in a subsystem. It defines a higher-level interface that makes a subsystem easier to use

 

 

Motivation

• In typical OO design,
  - Structuring a system into subsystems helps reduce complexity.
  - Subsystems are groups of classes, or groups of classes and other subsystems.
  - May produces many minimal classes

 

Benefits

• Hides the implementation of the dsubsystem from clients
• Promotes weak coupling between the subsystem and its clients.
• Does not prevent sophisticated clients form accessing the underlying classes.
• Facade does not add any functionality, it just simplifies interfaces.

 

Example

• The app only knows and uses the high-level API when using the Db.

 

Comparing the Facade with the Adapter Pattern

• Are there exisiting classes ? Facade : yes, Adapter : yes
• Is there an interface we must design to? Facade : no, Adapter : yes
• Is a simpler interface needed ? Facade : yes, Adapater : no

 

The Principle of Least Knowledge (Law of Demeter)

• Talk only to your imediate friends