OO Methodology Elaboration Iteration 2 - Design Patterns -

2 Table of Contents Iteration 1 –Use-Case Model –Process Sale Use Case –Domain Model –Design Model Iteration 2 Requirements GRASP: More Patterns for Assigning Responsibilities –Polymorphism –Pure Fabrication –Indirection –Protected Variations Designing Use-Case Realizations with GoF Design Patterns –Adapter –Factory –Strategy –Composite –Facade –Observer

3 Iteration 2 Requirements Support for variations in third-party external services. Complex pricing rules Pluggable business rules A design to refresh a GUI window when the sale total changes

4 Refinements of Analysis-oriented Artifacts Use-case Model : Use Cases Use-case Model : SSDs –new system operations –remotely communicating with external systems Domain Model Use-case Model : Operation Contracts

5 More GRASP Patterns Polymorphism Indirection Pure Fabrication Protected Variations

6 Design Patterns What is Design Pattern –reuses design –describes a particular recurring design problem that arises in specific design contexts, and presents a well-proven generic scheme for its solution Four essential elements –pattern name –problem –solution –consequences Gang of Four[GOF95] –creational patterns – 5 patterns concern the process of object creation. –structural patterns – 7 patterns deal with the composition of classes or objects. –behavioral patterns – 11 patterns characterize the ways in which classes or objects interact and distribute responsibility.

7 Design Pattern Space(GoF) Scope –Class patterns deal with relationships between classes and their subclasses established through inheritance they are static—fixed at compile-time –Object patterns deal with object relationships can be changed at run-time and are more dynamic. almost all patterns use inheritance to some extent. Purpose CreationalStructuralBehavioral Scope ClassFactory MethodAdapterInterpreter Template Method ObjectAbstract Factory Builder Prototype Singleton Adapter Bridge Composite Decorator Facade Flyweight Proxy Chain of Responsibility Command Iterator Mediator Memento Observer State Strategy Visitor

8 Describing Design Patterns(GoF) Pattern Name and Classification Intent Also Known As Motivation Applicability Structure Participants Collaborations Consequences Implementation Sample Code Known Uses Related Patterns

9 How Design Patterns Solve Design Problems –Finding Appropriate Objects most objects come from the analysis model abstraction pure fabrication –Determining Object Granularity describe specific ways of decomposing an object into smaller objects –Specifying Object Interfaces subtype dynamic binding polymorphism –Specifying Object Implementations class inheritance cf. interface inheritance –Putting Reuse Mechanisms to Work Inheritance Composition Delegation –Relating Run-Time and Compile-Time Structures –Designing for Change indirect creation of objects avoiding hard-coded requests limiting platform dependencies hiding object implementations avoiding algorithmic dependencies loose coupling using composition and delegation over inheritance etc.

10 How to Use a Design Pattern Issues –Is there a pattern that addresses a similar problem –Does the pattern trigger an alternative solution that may be more acceptable –Is there a simple solution? –Is the context of the pattern consistent with that of the problem –Are the consequences of using the pattern acceptable –Are constraints imposed by the software environment that would conflict with the use of the pattern GoF suggest a procedure 1.Read the pattern once through for an overview. 2.Go back and study the Structure, Participants, and Collaborations sections. 3.Look at the Sample Code section to see a concrete example of the pattern in code. 4.Choose names for pattern participants that are meaningful in the application context. 5.Define the classes. 6.Define application-specific names for operations in the pattern 7.Implement the operations to carry out the responsibilities and collaborations in the pattern.

11 Adapter Pattern The NextGen POS – needs to support several kinds of external third-party services, including tax calculators, credit authorization services Adapter pattern –Context/Problem How to resolve incompatible interfaces, or provide a stable interface to similar components with different interfaces? –Solution Convert the original interface of a component into another interface, through an intermediate adapter object, that adapts the varying external interfaces

12 Adapter Pattern NextGen POS system needs to support several kinds of external third-party services, including tax calculators, credit authorization services, inventory systems etc.

13 Adapter Pattern Using an Adapter Adapter pattern offers Protected Variations from changing external interfaces or third-party packages through the use of an Indirection object that applies interfaces and Polymorphism

14 Adapter Pattern Class Adapter –uses multiple inheritance to adapt one interface to another Object Adapter –uses object composition

15 Adapter Pattern Example (Drawing Editor) –TextView interface does not match the domain-specific interface an application requires

16 Adapter Pattern Sample Code(Drawing Editor) class Shape { public: Shape(); virtual void BoundingBox( Point& bottomLeft, Point& topRight ) const; virtual Manipulator* CreateManipulator() const; }; class TextView { public: TextView(); void GetOrigin(Coord& x, Coord& y) const; void GetExtent(Coord& width, Coord& height) const; virtual bool IsEmpty() const; }; class TextShape : public Shape, private TextView { // class adater public: TextShape(); virtual void BoundingBox( Point& bottomLeft, Point& topRight ) const; virtual bool IsEmpty() const; virtual Manipulator* CreateManipulator() const; }; void TextShape::BoundingBox ( Point& bottomLeft, Point& topRight ) const { Coord bottom, left, width, height; GetOrigin(bottom, left); GetExtent(width, height); bottomLeft = Point(bottom, left); topRight = Point(bottom + height, left + width); }

17 Adapter Pattern Sample Code(Drawing Editor) class TextShape : public Shape { // object adater public: TextShape(TextView*); virtual void BoundingBox( Point& bottomLeft, Point& topRight ) const; virtual bool IsEmpty() const; virtual Manipulator* CreateManipulator() const; private: TextView* _text; }; TextShape::TextShape (TextView* t) { _text = t; } void TextShape::BoundingBox ( Point& bottomLeft, Point& topRight ) const { Coord bottom, left, width, height; _text->GetOrigin(bottom, left); _text->GetExtent(width, height); bottomLeft = Point(bottom, left); topRight = Point(bottom + height, left + width); }

18 Factory Pattern NextGen POS –who creates the adapters? –how to determine which class of adapter to create? –Choosing a domain object to create the adapters does not support the goal of a separation of concerns and lowers its cohesion Factory (Concrete Factory) –Context Who should be responsible for creating objects when there are special considerations, such as complex creation logic, a desire to separate to creation responsibilities for better cohesion, etc.? –Solution Create a Pure Fabrication object called a Factory that handles the creation –Advantages Separate the responsibility of complex creation into cohesive helper objects Hide potentially complex creation logic Allow introduction of performance-enhancing memory management strategies such as object caching or recycling GoF –Abstract Factory –Factory Method

19 Factory Pattern Factory Using data-driven design (Protected Variation)

20 Singleton Pattern NextGen POS –who creates the factory itself, and how is it accessed? –only one instance of the factory is needed –this factory may need to be called from various places in the code Singleton –Context Exactly one instance of a class is allowed. Objects need a global and single point of access –Solution Define a static method of the class that returns the singleton

21 Singleton Pattern Factory for external services

22 Singleton Pattern Visibility to the singleton (global) public class Register { public void initialize() {... accountingAdapter = ServiceFactory.getInstance(). getAccountingAdapter();... }... } UML Shorthand

23 Adapter, Factory, Singleton

24 Abstract Factory Context –when a system should be configured with one of multiple families of products –a family of related product objects is designed to be used together –how can these families of products be created Solution –provide an interface for creating families of related or dependent objects without specifying their concrete classes Structure

25 Abstract Factory Example : User Interface Toolkit Consequences 1.It isolates concrete classes 2.It makes exchanging product families easy. 3.It promotes consistency among products 4.Supporting new kinds of products is difficult

26 Abstract Factory Sample : Maze Game class MapSite { public: virtual void Enter() = 0; }; class Room : public MapSite { public: Room(int roomNo); MapSite* GetSide(Direction) const; void SetSide(Direction, MapSite*); virtual void Enter(); private: MapSite* _sides[4]; int _roomNumber; };

27 Abstract Factory Sample : Maze Game class Wall : public MapSite { public: Wall(); virtual void Enter(); }; class Door : public MapSite { public: Door(Room* = 0, Room* = 0); virtual void Enter(); Room* OtherSideFrom(Room*); private: Room* _room1; Room* _room2; bool _isOpen; }; class Maze { public: Maze(); void AddRoom(Room*); Room* RoomNo(int) const; private: //... };

28 Abstract Factory Sample : Maze Game –In another class MazeGame, a Maze is created Maze* MazeGame::CreateMaze () { Maze* aMaze = new Maze; Room* r1 = new Room(1); Room* r2 = new Room(2); Door* theDoor = new Door(r1, r2); aMaze->AddRoom(r1); aMaze->AddRoom(r2); r1->SetSide(North, new Wall); r1->SetSide(East, theDoor); r1->SetSide(South, new Wall); r1->SetSide(West, new Wall); r2->SetSide(North, new Wall); r2->SetSide(East, new Wall); r2->SetSide(South, new Wall); r2->SetSide(West, theDoor); return aMaze; } –hard-coded –how can you change CreateMaze easily so that it creates another family of components of maze for example, enchanted maze

29 Abstract Factory Alternative: Abstract Factory MazeFactory Enchanted MazeFactory Bombed MazeFactory Maze Enchanted Maze Bombed Maze Room Enchanted Room Bombed Room Wall Enchanted Wall Bombed Wall MazeGame

30 Abstract Factory Alternative: Abstract Factory –MazeFactory class class MazeFactory { public: MazeFactory(); virtual Maze* MakeMaze() const { return new Maze; } virtual Wall* MakeWall() const { return new Wall; } virtual Room* MakeRoom(int n) const { return new Room(n); } virtual Door* MakeDoor(Room* r1, Room* r2) const { return new Door(r1, r2); } }; –creating a maze using an abstract factory Maze* MazeGame::CreateMaze (MazeFactory& factory) { Maze* aMaze = factory.MakeMaze(); Room* r1 = factory.MakeRoom(1); Room* r2 = factory.MakeRoom(2); Door* aDoor = factory.MakeDoor(r1, r2); aMaze->AddRoom(r1); aMaze->AddRoom(r2); r1->SetSide(North, factory.MakeWall()); r1->SetSide(East, aDoor); r1->SetSide(South, factory.MakeWall());.... r2->SetSide(West, aDoor); return aMaze; }

31 Abstract Factory Alternative: Abstract Factory –enchanted maze factory class EnchantedMazeFactory : public MazeFactory { public: EnchantedMazeFactory(); virtual Room* MakeRoom(int n) const { return new EnchantedRoom(n, CastSpell()); } virtual Door* MakeDoor(Room* r1, Room* r2) const { return new DoorNeedingSpell(r1, r2); } protected: Spell* CastSpell() const; }; –create a bombed maze MazeGame game; BombedMazeFactory factory; game.CreateMaze(factory);

32 Factory Method Intent –Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses. Motivation –Consider a framework for applications that can present multiple documents to the user.

33 Factory Method Applicability –a class can't anticipate the class of objects it must create. –a class wants its subclasses to specify the objects it creates. Structure Consequences –eliminates the need to bind application-specific classes –provides a hooks for subclasses

34 Factory Method Sample Code class MazeGame { public: Maze* CreateMaze(); // factory methods: virtual Maze* MakeMaze() const { return new Maze; } virtual Room* MakeRoom(int n) const { return new Room(n); } virtual Wall* MakeWall() const { return new Wall; } virtual Door* MakeDoor(Room* r1, Room* r2) const { return new Door(r1, r2); } }; Maze* MazeGame::CreateMaze () { Maze* aMaze = MakeMaze(); Room* r1 = MakeRoom(1); // using factory methods Room* r2 = MakeRoom(2); Door* theDoor = MakeDoor(r1, r2); aMaze->AddRoom(r1); aMaze->AddRoom(r2); r1->SetSide(North, MakeWall()); r1->SetSide(East, theDoor); r1->SetSide(South, MakeWall());.... return aMaze; }

35 Factory Method Sample Code –EnchantedMazeGame subclass refines factory methods class EnchantedMazeGame : public MazeGame { public: EnchantedMazeGame(); virtual Room* MakeRoom(int n) const { return new EnchantedRoom(n, CastSpell()); } virtual Door* MakeDoor(Room* r1, Room* r2) const { return new DoorNeedingSpell(r1, r2); } protected: Spell* CastSpell() const; }; –creating a maze EnchantedMazeGame game; game.CreateMaze();