GoF Design Pattern Categories

Transcription

1 GoF Design Pattern Categories Purpose Creational Structural Behavioral Scope Class Factory Method Adapter Interpreter Template Method Object Abstract Factory Builder Prototype Singleton Adapter Bridge Composite Decorator Facade Proxy Flyweight Chain of Responsibility Command Iterator Mediator Memento Observer State Strategy Visitor 1

2 Intent: BUILDER (Object Creational) Separate the construction of a complex object from its representation so that the same construction process can create different representations Builders specify abstract interfaces for creating parts of a Product object 2

3 BUILDER (Object Creational) Usage: Rarely used, only useful if complex objects consisting of multiple parts need to be constructed (composite objects for example) 3

4 BUILDER (Object Creational) Motivation: ortf reader should be able to convert RTF to many text format oadding new conversions without modifying the reader should be easy 4

5 Builder Motivation Each kind of converter class takes the mechanism for creating and assembling a complex object and puts it behind an abstract interface. The converter is separate from the reader, which is responsible for parsing an RTF document. The Builder pattern captures all these relationships. Each converter class is called a builder in the pattern, and the reader is called the director. 5

6 Builder motivation 6

7 Builder motivation (example) 7

8 Builder Motivation Applied to this example, the Builder pattern separates the algorithm for interpreting a textual format (that is, the parser for RTF documents) from how a converted format gets created and represented. This lets us reuse the RTFReader's parsing algorithm to create different text representations from RTF documents just configure the RTFReader with different subclasses of Textconverter. 8

9 Applicability Use the Builder pattern when othe algorithm for creating a complex object should be independent of the parts that make up the object and how they are assembled othe construction process must allow different representations for the object that is constructed 9

10 BUILDER Structure Director Construct () builders Builder BuildPart () for all objects in structure { builder->buildpart () } ConcreteBuilder BuildPart () GetResult () Product 10

11 Builder - participants Builder (TextConverter): - specifies an abstract interface for creating parts of a Product object. ConcreteBuilder (ASCIIConverter, TeXConverter, TextWidgetConverter) - constructs and assembles parts of the product by implementing the Builder interface. - defines and keeps track of the representation it creates. - provides an interface for retrieving the product (e.g., GetASCIIText, Get-TextWidget). 11

12 Builder - participants Director (RTFReader) - constructs an object using the Builder interface. Product (ASCIIText, TeXText, TextWidget) - represents the complex object under construction. ConcreteBuilder builds the product's internal representation and defines the process by which it's assembled. - includes classes that define the constituent parts, including interfaces for assembling the parts into the final result. 12

13 Builder - Collaborations Client creates Director object and configures it with the desired Builder object Director notifies Builder whenever a part of the product should be built Builder handles requests from the Director and adds parts to the product Client retrieves the product from the Builder 13

14 Builder - Collaborations 14

15 Consequences It lets you vary a product's internal representation. It isolates code for construction and representation. It gives you finer control over the construction process. 15

16 Implementation issues Assembly and construction interface. Why no abstract class for products? Empty methods as default in Builder. 16

17 Related Patterns Abstract Factory is similar to Builder in that it too may construct complex objects. The primary difference is that the Builder pattern focuses on constructing a complex object step by step. Abstract Factory's emphasis is on families of product objects (either simple or complex). 17

18 Related Patterns Builder returns the product as a final step, but as far as the Abstract Factory pattern is concerned, the product gets returned immediately. A Composite is what the builder often builds. 18

19 Known Uses RTF converter application is from ET++. Builder is a common pattern in Smalltalk-80. The Service Configurator framework from the Adaptive Communications Environment uses a builder to construct network service components that are linked into a server at run-time. 19

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21 A scenario You ve just been asked to build a vacation planner for Patternsland, a new theme park just outside of Objectville. Park guests can choose a hotel and various types of admission tickets, make restaurant reservations, and even book special events. To create a vacation planner, you need to be able to create structures like this: 21

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25 enum VehicleType { Car, MotorCycle} enum Wheels {Wheels_2, Wheels_3, Wheels_4 } enum Door { Door_0, Door_2, Door_4} enum Frame { Car_Frame, MotorCycle_Frame } enum Engine { Car_Engine_2500CC, MotorCycle_Engine_50CC} 26

26 class Vehicle { //product public Vehicle(VehicleType VType) } { this.vtype = VType; } public VehicleType VType; public Engine engin; public Wheels wheels; public Door door; public Frame frame; 27

27 abstract class VehicleBuilder { protected Vehicle NewVehicle; } public Vehicle GetVehicle() { return NewVehicle; } public abstract void BuildFrame(); public abstract void BuildWheels(); public abstract void BuildDoor(); public abstract void BuildEngine(); 28

28 class CarBuilder:VehicleBuilder //concrete builder { public CarBuilder() } { NewVehicle = new Vehicle(VehicleType.Car); } public override void BuildDoor() { NewVehicle.door = Door.Door_4; } public override void BuildFrame() { NewVehicle.frame = Frame.Car_Frame; } public override void BuildWheels() { NewVehicle.wheels = Wheels.Wheels_4; } public override void BuildEngine() { NewVehicle.engin = Engine.Car_Engine_2500CC; } 29

29 class MotorCycle:VehicleBuilder { public MotorCycle() } { NewVehicle = new Vehicle(VehicleType.MotorCycle); } public override void BuildDoor() { NewVehicle.door = Door.Door_0; } public override void BuildEngine() { NewVehicle.engin = Engine.MotorCycle_Engine_50CC; } public override void BuildWheels() { NewVehicle.wheels = Wheels.Wheels_2; } public override void BuildFrame() { NewVehicle.frame = Frame.MotorCycle_Frame; } 30

30 class Shop //director { public void ConStruct(VehicleBuilder VB) } { } VB.BuildFrame(); VB.BuildEngine(); VB.BuildWheels(); VB.BuildDoor(); 31

31 private void button1_click(object sender, EventArgs e) { VehicleBuilder builder = new CarBuilder(); } Shop shop = new Shop(); shop.construct(builder); Vehicle c = builder.getvehicle(); MessageBox.Show(c.VType.ToString()); //output=car 32

32 FACTORY METHOD (Class Creational) Intent: o Define an interface for creating an object, but let subclasses decide which class to instantiate. o Factory Method lets a class defer instantiation to subclasses. Also Known As Virtual Constructor 33

33 FACTORY METHOD (Class Creational) Usage: Frequently used, fairly easy to implement and useful for centralizing object lifetime management and avoiding object creation code duplication 34

34 FACTORY METHOD (Class Creational) Motivation: o Framework use abstract classes to define and maintain relationships between objects o Framework has to create objects as well - must instantiate classes but only knows about abstract classes - which it cannot instantiate o Factory method encapsulates knowledge of which subclass to create - moves this knowledge out of the framework 35

35 FACTORY METHOD Motivation Document docs Application Open() Close() Save() Revert() CreateDocument() NewDocument() OpenDocument() Document* doc=createdocument(); docs.add(doc); doc->open(); MyDocument MyApplication CreateDocument() return new MyDocument 36

36 Applicability Use the Factory Method pattern when oa class can t anticipate the class of objects it must create. oa class wants its subclasses to specify the objects it creates. oclasses delegate responsibility to one of several helper subclasses, and you want to localize the knowledge of which helper subclass is the delegate. 37

37 FACTORY METHOD structure (Class Creational) 38

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39 Product Participants o Defines the interface of objects the factory method creates ConcreteProduct o Implements the product interface Creator o Declares the factory method which returns object of type product o May contain a default implementation of the factory method o Creator relies on its subclasses to define the factory method so that it returns an instance of the appropriate Concrete Product. ConcreteCreator o Overrides factory method to return instance of ConcreteProduct 40

40 Collaborations Creator relies on its subclasses to define the factory method so that it returns an instance of the appropriate Concrete Product. 41

41 Provides hooks for subclasses. Consequences Connects parallel class hierarchies. 42

42 Implementation Two major varieties. Parameterized factory methods. Language-specific variants and issues. Using templates to avoid subclassing. Naming conventions. 43

43 Known Uses Factory methods pervade toolkits and frameworks. Class View in the Smalltalk-8 0Model/View/Controller framework has a method default Controller that creates a controller, and this might appear to be a factory method. The Orbix ORB system from IONA Technologies uses Factory Method to generate an appropriate type of proxy when an object requests a reference to a remote object. 44

44 Related Patterns Abstract Factory is often implemented with factory methods. The Motivation example in the Abstract Factory pattern illustrates Factory Method as well. Factory methods are usually called within Template Methods. In the document example above, NewDocument is a template method. 45

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62 PROTOTYPE (Object Creational) Intent: ospecify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype. Motivation: oframework implements Graphic class for graphical components and GraphicTool class for tools manipulating/creating those components 65

63 Motivation Actual graphical components are application-specific How to parameterize instances of Graphic Tool class with type of objects to create? Solution: create new objects in Graphic Tool by cloning a prototype object instance 66

64 PROTOTYPE Motivation Tool Manipulate() prototype Graphic Draw(Position) Clone() Staff MusicalNote Rotate Tool Manipulate() Graphic Tool Manipulate() Draw(Position) Clone() p = prototype ->Clone() while(user drags mouse){ p ->Draw(new position) } Insert p into drawing WholeNote Draw(Position) Clone() Return copy of self HalfNote Draw(Position) Clone() Return copy of self 67

65 PROTOTYPE Motivation Usage: Easy pattern, usage depends on the preferred software design, provides an alternative to the other creational patterns that are mainly based on external classes for creation 68

66 Applicability Use the Prototype pattern when a system should be independent of how its products are created, composed, and represented; owhen the classes to instantiate are specified at runtime, for example, by dynamic loading; or oto avoid building a class hierarchy of factories that parallels the class hierarchy of products; or 69

67 Applicability when instances of a class can have one of only a few different combinations of state. It may be more convenient to install a corresponding number of prototypes and clone them rather than instantiating the class manually, each time with the appropriate state. 70

68 PROTOTYPE Structure client prototype Prototype Operation() Clone() p = prototype ->Clone() ConcretePrototype1 Clone() ConcretePrototype2 Clone() return copy of self return copy of self 71

69 Participants: Prototype (Graphic) odeclares an interface for cloning itself ConcretePrototype (Staff, WholeNote, HalfNote) oimplements an interface for cloning itself Client (GraphicTool) ocreates a new object by asking a prototype to clone itself L5 Collaborations: A client asks a prototype to clone Itself. UNIT-III 72

70 Consequences Adding and removing products at run-time. Specifying new objects by varying values. Specifying new objects by varying structure. Reduced subclassing. Configuring an application with classes dynamically. 73

71 Implementation Using a prototype manager. Implementing the Clone operation. Initializing clones. 74

72 Known Uses first example of the Prototype pattern was in Ivan Sutherland's Sketchpad system first widely known application of the pattern in an object oriented language was in ThingLab, where users could form a composite object and then promote it to a prototype by installing it in a library of reusable objects 75

73 Related Patterns Prototype and Abstract Factory are competing patterns in some ways, as we discuss at the end of this chapter. They can also be used together, however. An Abstract Factory might store a set of prototypes from which to clone and return product objects. Designs that make heavy use of the Composite and Decorator patterns often can benefit from Prototype as well. 76

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77 abstract class ColorPrototype { public abstract ColorPrototype Clone(); } class Color:ColorPrototype { public int Red; public int Green; public int Blue; public override ColorPrototype Clone() { return this.memberwiseclone() as ColorPrototype; } } 81

78 private void button1_click(object sender, EventArgs e) { Color c1 = new Color(); c1.green = 255; Color c2 = c1.clone() as Color; } 82

79 Intent: SINGELTON oensure a class only has one instance, and provide a global point of access to it. Motivation: osome classes should have exactly one instance (one print spooler, one file system, one window manager) oa global variable makes an object accessible but doesn t prohibit instantiation of multiple objects oclass should be responsible for keeping track of its sole interface 83

80 SINGELTON Usage: Often used for objects that need to provide global access with the constraint of only one single instance in the application 84

81 Applicability Use the Singleton pattern when othere must be exactly one instance of a class, and it must be accessible to clients from a well-known access point. owhen the sole instance should be extensible by subclassing, and clients should be able to use an extended instance without modifying their code. 85

82 SINGLETON Structure Singleton static Instance() return uniquelnstance SingletonOperation() GetSingletonData() Static uniquelnstance singletondata 86

83 Singleton: Participants and Collaborations Defines an instance operation that lets clients access its unique interface Instance is a class operation (static in Java) May be responsible for creating its own unique instance Collaborations: Clients access a Singleton instance solely through Singleton s Instance operation. 87

84 Consequences Controlled access to sole instance. Reduced name space. Permits refinement of operations and representation. Permits a variable number of instances. More flexible than class 88

85 Implementation Points Generally, a single instance is held by the object, and controlled by a single interface. Sub classing the Singleton may provide both default and overridden functionality. 89

86 Known Uses A more subtle example is the relationship between classes and their metaclasses. A metaclass is the class of a class, and each metaclass has one instance. Metaclasses do not have names (except indirectly through their sole instance), but they keep track of their sole instance and will not normally create another. 90

87 Related pattern Many patterns can be implemented using the Singleton pattern. See Abstract Factory, Builder, and Prototype. 91

88 Discussion of Creational Patterns There are two common ways to parameterize a system by the classes of objects it creates: One way is to subclass the class that creates the objects The other way to parameterize a system relies more on object composition 94

89 Discussion of Creational Patterns Which pattern is best depends on many factors. Abstract Factory doesn't offer much of an improvement, because it requires an equally large Graphics Factory class hierarchy. the Prototype pattern is probably the best for the drawing editor framework, because it only requires implementing a Clone operation on each Graphics class. Factory Method makes a design more customizable and only a little more complicated. 97

90 Discussion of Creational Patterns Designs that use Abstract Factory, Prototype, or Builder are even more flexible than those that use Factory Method, but they're also more complex. Often, designs start out using Factory Method and evolve toward the other creational patterns as the designer discovers where more flexibility is needed. 98