Annex B: Design Patterns

B5.Facade

Description

When we wish to facilitate the use of a subsystem, this pattern allows to provide a simplified interface, which avoids the direct coupling between the clients and the elements of the subsystem.

Structure and Components

Structure and Components of the Facade design pattern

• Facade defines an interface unified for the subsystem and gives access to certain public components,
• C1 to C7 establish the functions of the subsystem,
• FacadeClient handles the subsystem by passing through the facade.

Collaborations

• the client handles the subsystem by addressing itself to the facade or to the elements of the subsystem made public by the facade,
• the facade transmits the requests to the subsystem after transformation, if necessary.

Strengths and Weaknesses

decrease the coupling between the client and the subsystem,

mask the private elements of the subsystem,

the unified interface presented by the facade can be too restrictive to use the whole set of the functions of the subsystem.

Use in CBR*Tools

The controller of the reasoning ( Reasoner class) is a facade for the whole system of reasoning, the memory ( Memory class) is a facade simplifying the management of the case bases and the indices, finally the behavioural part ( BehaviouralSituation class) of a behavioural situation is a facade of the behavioural indices to simplify their management.

Brigitte Trousse
Last modified: Fri Sep 28 13:51:49 MEST 2001