Design Patterns

1
Design Patterns

• Shyh-Kang Jeng
• Department of Electrical Engineering
• National Taiwan University

2
Reference

• E. Gamma, R. Helm, R. Johnson, and J. Vlissides,
  Design Patterns Elements of Reusable
  Object-Oriented Software, Addison-Wesley, 1994.
• A. Shalloway and J. R. Trott, Design Patterns
  Explained, Addison-Wesley, 2002

3
Concept of Design Patterns

• Each pattern describes a problem which occurs
  over and over again in our environments, and then
  describes the core of the solution to that
  problem, in such a way that you can use this
  solution a million times over, without ever doing
  it the same way twice.
• -- Christopher Alexander,
• A Pattern Language, 1977

4
Basics of Design Patterns

• Design patterns are not about designs such as
  linked lists and hash tables that can be encoded
  in classes and reused as is
• Nor are they complex, domain specific designs for
  an entire application or subsystem
• Design patterns are descriptions of communicating
  objects and classes that are customized to solve
  a general design problem in a particular context
• Encapsulate the concept that varies

5
Elements of a Pattern

• Pattern Name
• Problem
• Solution
• Consequence

6
Design Pattern Space
7
Model-View-Controller
D. Collins, Designing Object-Oriented User
Interfaces,
Benjamin/Cummings, 1995.
8
Presentations of Data
9
Observer Pattern

• Define a one-to-many dependency between objects
  so that when one object changes state, all its
  dependents are notified and updated automatically

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Observer Pattern
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Observer Pattern
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Class Observer

• class Subject
• class Observer
• public
• virtual Observer( )
• virtual void Update(
• Subject theChangedSubject) 0
• protected
• Observer( )

13
Class Subject

• class Subject
• public
• virtual Subject( )
• virtual void Attach(Observer)
• virtual void Detach(Observer)
• virtual void Notify( )
• protected
• Subject( )
• private
• ListltObservergt observers

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Methods in Subject

• void SubjectAttach(Observer o)
• observers-gtAppend(o)
• void SubjectNotify( )
• ListIteratorltObservergt i(observers)
• for(i.First( ) !i.IsDone( ) i.Next( ))
• i.CurrentItem( )-gtUpdate(this)

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Document-Application Application Framework
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Template Method Pattern

• Define the skeleton of an algorithm in an
  operation, deferring some steps to subclasses
• Let subclasses redefine certain steps of an
  algorithm without changing the algorithms
  structure
• Hook operations

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Template Method Pattern
18
Methods in View

• void ViewDisplay( )
• SetFocus( )
• DoDisplay( )
• ResetFocus( )
• void ViewDoDisplay( )
• void MyViewDoDisplay( )

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Compiler System
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Façade Pattern

• Provide a unified interface to a set of
  interfaces in a subsystem
• Define a higher-level interface that makes the
  subsystems easier to use
• Not prevent applications from using subsystem
  classes if we need to
• Can choose between ease of use and generality

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Façade Pattern
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Class Compiler

• class Compiler
• public
• Compiler( )
• virtual void Compile(
• istream, BytecodeStream)

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Method Compile

• void CompilerCompile(
• istream input,
• BytecodeStream output)
• Scanner scanner(input)
• ProgramNodeBuilder builder
• Parser parser
• parser.parse(scanner, builder)
• RISCCodeGenerator generator(output)
• ProgramNode parseTree builder.GetRootNode(
  )
• parseTree-gtTraverse(generator)

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Font Dialog Box
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Mediator Pattern

• Define an object that encapsulates how a set of
  objects interact
• Promote loose coupling by keeping objects from
  referring to each other explicitly
• Let the client vary interactions of objects
  independently
• Centralize control

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Mediator Pattern
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Mediator Pattern
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Mediator Pattern
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Class DialogDirector

• class DialogDirector
• public
• virtual DialogDirector( )
• virtual void ShowDialog( )
• virtual void WidgetChanged(Widget) 0
• protected
• DialogDirector( )
• virtual void CreateWidgets( ) 0

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Class Widget

• class Widget
• public
• Widget(DialogDirector)
• virtual void Changed( )
• virtual void HandleMouse(
• MouseEvent event)
• private
• DialogDirector director

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Class ListBox

• class ListBox public Widget
• public
• ListBox(DialogDirector)
• virtual string GetSelection( )
• virtual void SetList(
• Listltstringgt listItems)
• virtual void HandleMouse(
• MouseEvent event)

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Class FontDialogDirector

• class FontDialogDirector
• public DialogDirector
• public
• FontDialogDirector( )
• virtual FontDialogDirector( )
• virtual void WidgetChanged(Widget)
• protected
• virtual void CreateWidgets( )
• private
• ListBox fontList

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Singleton Pattern

• Ensure a class only has one instance, and provide
  a global point of access to it
• Provide controlled access to sole instance
• Reduce name space

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Singleton Pattern
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Class MazeFactory

• class MazeFactory
• public
• static MazeFactory Instance( )
• protected
• MazeFactory( )
• private
• static MazeFactory instance

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Invoking Singleton Instance

• MazeFactory
• MazeFactoryinstance NULL
• MazeFactory MazeFactoryInstance( )
• if( instance NULL )
• instance new MazeFactory
• return instance

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Subclass of a Singleton

• class BombedMazeFactory
• public MazeFactory

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Invoking Singleton Instance

• MazeFactory MazeFactoryInstance( )
• if( instance NULL )
• const char mazeStyle
• getenv(MAZESTYLE)
• if(strcmp(mazeStyle,bombed)0)
• instance new BomedMazeFactory
• else if
• else
• instance new MazeFactory
• return instance

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Class Singleton

• class Singleton
• public
• static void Register( char name,
• Singleton)
• static Singleton Instance()
• protected
• static Singleton Lookup(
• const char name)
• private
• static Singleton instance
• static ListltNameSingletonPairgt registry

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Method Instance

• Singleton SingletonInstance()
• if( instance NULL )
• const char singletonName
• getenv(SINGLETON)
• instance
• Lookup( singletonName )
• return instance

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Register a Singleton Object

• class MySingleton public Singleton
• Static MySingleton theSingleton
• MySingletonMySingleton()
• SingletonRegister(MySingleton,
• this)

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TCP Connection
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State Pattern

• Allow an object to alter its behavior when its
  internal state changes
• Localize state-specific behavior and partition
  behavior for different states
• Make state transitions explicit
• State objects can be shared

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State Pattern
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Class TCPConnection

• class TCPState
• class TCPConnection
• public
• TCPConnection( )
• void ActiveOpen( )
• private
• friend class TCPState
• void ChangeState(TCPState)
• private
• TCPState state

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Class TCPState

• class TCPState
• public
• virtual void ActiveOpen(TCPConnection)
• virtual void Synchronize(TCPConnection)
• virtual void Acknowledge(TCPConnection)
• virtual void Send(TCPConnection)
• protected
• void ChangeState(
• TCPConnection,TCPState)

47
State Classes

• class TCPListen public TCPState
• public
• static TCPState Instance( )
• virtual void Send(TCPConnection)
• class TCPClosed public TCPState
• public
• static TCPState Instance( )
• virtual void ActiveOpen(TCPConnection)

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Drawing Editor
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Adapter Pattern

• Convert the interface of a class into another
  interface clients expect
• Allow classes work together that could not
  otherwise because of incompatible interfaces
• Clients call operations on an Adapter instance.
  In turn, the adapter calls Adaptee operations
  that carry out the request

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Adapter Pattern
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Class Shape

• class Shape
• public
• Shape( )
• virtual void BoundingBox (
• Point bottomLeft,
• Point topRight ) const
• virtual Manipulator CreateManipulator( )
  const

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Class TextView

• class TextView
• public
• TextView( )
• void GetOrigin(Coord x, Coord y) const
• void GetExtent(Coord width,
• Coord height) const
• virtual bool IsEmpty( ) const

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Class TextShape

• class TextShape public Shape
• public
• TextShape(TextView)
• virtual void BoundingBox(
• Point bottomLeft,
• Point topRight ) const
• virtual bool IsEmpty( ) const
• virtual Manipulator CreateManipulator( )
  const
• private
• TextView text

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Method BoundingBox

• void TextShapeBoundingBox(
• Point bottomLeft,
• Point topRight ) const
• Coord bottom, left, width, height
• text-gtGetOrigin(bottom, left)
• text-gtGetExtent(width, height)
• bottomLeft Point(bottom, left)
• topRight Point(bottomheight, leftwidth)

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Exercise (1)

• Some library classes like DirectSound (actually a
  COM component) in DirectX can be used to play
  sound directly from a memory buffer. However, if
  we want to play a music of MIDI or WAVE format,
  we have to adapt those library classes to accept
  MIDI or WAVE format. Try to develop such adapter
  classes, assuming that you want to use a method
  play() in your own class, say, CMIDI or CWave, to

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Exercise (2)

• play a piece of MIDI or WAVE music. In the
  play() method, you may have to use methods
  Initialize() and CreateSoundBuffer() in class
  DirectSound and methods Play() in class
  DirectSoundBuffer. Note that this is an
  over-simplified description, you are free to add
  anything you think existent in those two library
  classes. Of course, if you are interested, you
  may get some related books for details.

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Graphics
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Proxy Pattern

• Provide a surrogate or placeholder for another
  object to control access to it
• Forward requests to real subject when
  appropriate, depending on the kind of proxy
• Remote proxy can hide the fact that an object
  resides in a different address space
• Virtual proxy can perform optimizations such as
  creating an object on demand

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Proxy Pattern
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Class Graphic

• class Graphic
• public
• virtual Graphic( )
• virtual void Draw( ) 0
• virtual void HandleMouse(
• Event event)0
• virtual
• const Point GetExtent( ) 0
• virtual void Load(istream from) 0
• virtual void Save(ostream to)
• 0
• protected
• Graphic( )

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Class Image

• class Image public Graphic
• public
• Image(string file)
• virtual Image( )
• virtual void Draw(
• const Point at)
• virtual void HandleMouse(
• Event event)
• virtual const Point GetExtent( )
• virtual void Load(istream from)
• virtual void Save(ostream to)

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Class ImageProxy

• class ImageProxy public Graphic
• public
• ImageProxy(string file)
• virtual ImageProxy( )
• protected
• Image GetImage( )
• private
• Image image
• Point extent
• string fileName

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Method GetImage

• ImageProxyImageProxy(
• string fileName)
• fileName fileName
• extent PointZero
• image NULL
• Image ImageProxyGetImage( )
• if( image NULL )
• image new Image(fileName)
• return image

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Methods in ImageProxy

• void ImageProxyDraw(
• const Point at)
• GetImage( )-gtDraw(at)
• void ImageProxyHandleMouse(
• Event event ) GetImage( )
• -gtHandleMouse(event)
• void ImageProxySave(ostream to)
• to ltlt extent ltlt fileName
• void ImageProxyLoad(
• ifstream from)
• from gtgt extent gtgt fileName

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Invoking ImageProxy

• class TextDocument
• public
• TextDocument( )
• Void Insert(Graphic)
• TextDocument text
• new TextDocument
• text-gtInsert( new ImageProxy(anImageFileName))

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Creating Mazes
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Creating Mazes

• Maze MazeGameCreateMaze( )
• Maze aMaze new Maze
• Room r1 new Room(1)
• Room r2 new Room(2)
• Door theDoor new Door(
• r1, r2 )
• aMaze-gtAddRoom( r1 )
• aMaze-gtAddRoom( r2 )
• Set sides for each room
• return aMaze

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Builder Pattern

• Separate the construction of a complex object
  from its representation so that the same
  construction process can create different
  representations
• The algorithm for creating a complex object
  should be independent of the parts that make up
  the object and how they are assembled
• Gives finer control over the construction process

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Builder Pattern
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Builder Pattern
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Class MazeBuilder

• class MazeBuilder
• public
• virtual void BuildMaze( )
• virtual void BuildRoom( int room )
• virtual void BuildDoor(
• int roomFrom, int roomTo )
• virtual Maze GetMaze( )
• return NULL
• protected
• MazeBuilder( )

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Method CreateMaze

• Maze MazeGameCreateMaze(
• MazeBuilder builder )
• builder.BuildMaze( )
• builder.BuildRoom(1)
• builder.BuildRoom(2)
• builder.BuildDoor(1, 2)
• return builder.GetMaze( )

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Class StandardMazeBuilder

• class StandardMazeBuilder
• public MazeBuilder
• public
• StandardMazeBuilder( )
• virtual void BuildMaze( )
• virtual void BuildRoom(
• int room )
• virtual void BuildDoor(
• int roomFrom, int roomTo )
• virtual Maze GetMaze( )
• private
• Maze currentMaze

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Class CountingMazeBuilder

• class CountingMazeBuilder
• public MazeBuilder
• public
• CountingMazeBuilder( )
• virtual void BuildMaze( )
• virtual void BuildRoom(
• int room )
• virtual void BuildDoor(
• int roomFrom, int roomTo )
• virtual Maze GetCounts( )
• private
• int doors, rooms

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Method BuildRoom

• CountingMazeBuilder
• CountingMazeBuilder()
• rooms 0
• doors 0
• void CountingMazeBuilderBuildRoom(
• int )
• rooms

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Invoking MazeBuilder

• Maze maze
• MazeGame game
• StandardMazeBuilder builder1
• game.CreateMaze( builder1 )
• maze builder1.GetMaze()
• int rooms, doors
• CountingMazeBuilder builder2
• game.CreateMaze( builder2 )
• builder2.GetCounts( rooms, doors )

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Exercise

• Suppose that you want to create x-y plots, which
  can be a linear plot, a semi-log plot, or a
  full-log plot. Write classes LinearPlotBuilder,
  SemiLogPlotBuilder, and FullLogPlotBuilder to
  build those three kinds of plots. Note that an
  x-y plot can be obtained via building the
  horizontal axis, the vertical axis, and the
  curves. Prepare a test program to test these
  classes. No much details required.