Façade Design PatternSource: Design Patterns – Elements of Reusable Object-

Oriented Software; Gamma, et. al.

Problem

• A subsystem contains lots of classes, all of which are necessary to perform its function

• Most of the subsystem's internal complexity is not directly relevant to its clients

• We want to simplify the client's interface to the subsystem to make it easier to use

• We want to minimize client dependencies on the subsystem's internal details

Solution

• Introduce a Façade object that provides a simplified interface to the subsystem

• The Façade's interface provides exactly those operations needed by most clients, and no more

• The Façade translates high-level client requests into lower-level requests on subsystem objects

• Internal subsystem objects have no knowledge of the Façade

Solution

• Most clients interact with the subsystem strictly through the Façade

• Advanced clients may still be allowed to access the full scope of subsystem functionality, but most clients don't need or want to

• Similar to keeping a class' implementation details private, and making public only those operations directly needed by clients (i.e., information hiding)

Consequences

• Makes using the subsystem easier

• Reduces coupling between clients and the subsystem

• Reduces compilation dependencies, thus minimizing recompilation time

• Useful for defining the interfaces between layers in a layered system

Known Uses: Compilers

Known Uses: Web Applications

Application Server

Browser WebServer Facade

DatabaseServer

Known Uses: CS240 Chess Program

Hardware

Linux

XML Reading/Writing GTK GUI Toolkit

Chess Logic

Chess User Interface

Facade

Known Uses: CS240 Chess Program

PieceSelector

Chess

+Game(in selector : PieceSelector)+Game(in fileName, in selector : PieceSelector)+GetTurn() : ChessColor+IsCheck() : bool+IsMate() : bool+GetColor(in pos : BoardPosition) : ChessColor+GetPiece(in pos : BoardPosition) : Piece+GetLegalMoves(in pos : BoardPosition) : <unspecified>+MakeMove(in pos : BoardPosition, in newPos : BoardPosition) : void+UndoMove() : void+Save(in fileName) : void

Game

1 1

has Board

History

11 has

1

1

hasSquare

1

64

contains

Move

1

*

contains

Piece

1

0..1

has

Pawn

BoardPosition

1 1

has

PieceFactory

1

1has

XMLReaderXMLWriter

XMLEvent

«enumeration»XMLEventType

«enumeration»ChessColor

«enumeration»ChessDirection

«enumeration»PieceType

Rook Knight Bishop King Queen