Façade Design Pattern
Source: Design Patterns – Elements of Reusable ObjectOriented 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
Web
Server
Database
Server
Facade
Known Uses: CS240 Chess Program
Chess User Interface
Facade
Chess Logic
XML Reading/Writing
GTK GUI Toolkit
Linux
Hardware
Known Uses: CS240 Chess Program
Game
PieceSelector
has
1
Chess
1
+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
has
1
1
has
1
contains
64
1
1
Board
has
1
1
Square
1
1
XMLWriter
BoardPosition
has
1
XMLReader
History
PieceFactory
contains
1
has
*
XMLEvent
Move
Piece
«enumeration»
XMLEventType
«enumeration»
ChessColor
«enumeration»
ChessDirection
«enumeration»
PieceType
Pawn
0..1
Rook
Knight
Bishop
King
Queen