设计模式之工厂方法

Factory Method Design Pattern

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definition Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses. Frequency of use:   high

UML class diagram

participants

    The classes and/or objects participating in this pattern are:

• Product  (Page)
  • defines the interface of objects the factory method creates
• ConcreteProduct  (SkillsPage, EducationPage, ExperiencePage)
  • implements the Product interface
• Creator  (Document)
  • declares the factory method, which returns an object of type Product. Creator may also define a default implementation of the factory method that returns a default ConcreteProduct object.
  • may call the factory method to create a Product object.
• ConcreteCreator  (Report, Resume)
  • overrides the factory method to return an instance of a ConcreteProduct.

sample code in C#

This structural code demonstrates the Factory method offering great flexibility in creating different objects. The Abstract class may provide a default object, but each subclass can instantiate an extended version of the object.

// Factory Method pattern -- Structural example

using System; using System.Collections; namespace DoFactory.GangOfFour.Factory.Structural {    // MainApp test application    class MainApp   {      static void Main()     {        // An array of creators       Creator[] creators = new Creator[2];       creators[0] = new ConcreteCreatorA();       creators[1] = new ConcreteCreatorB();        // Iterate over creators and create products        foreach(Creator creator in creators)       {         Product product = creator.FactoryMethod();         Console.WriteLine("Created {0}",           product.GetType().Name);       }        // Wait for user       Console.Read();     }   }    // "Product"    abstract class Product   {   }    // "ConcreteProductA"    class ConcreteProductA : Product   {   }    // "ConcreteProductB"    class ConcreteProductB : Product   {   }    // "Creator"    abstract class Creator   {      public abstract Product FactoryMethod();   }    // "ConcreteCreator"    class ConcreteCreatorA : Creator   {      public override Product FactoryMethod()     {        return new ConcreteProductA();     }   }    // "ConcreteCreator"    class ConcreteCreatorB : Creator   {      public override Product FactoryMethod()     {        return new ConcreteProductB();     }   } }

Output Created ConcreteProductA Created ConcreteProductB

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This real-world code demonstrates the Factory method offering flexibility in creating different documents. The derived Document classes Report and Resume instantiate extended versions of the Document class. Here, the Factory Method is called in the constructor of the Document base class.

 

// Factory Method pattern -- Real World example

using System; using System.Collections; namespace DoFactory.GangOfFour.Factory.RealWorld {    // MainApp test application    class MainApp   {      static void Main()     {        // Note: constructors call Factory Method       Document[] documents = new Document[2];       documents[0] = new Resume();       documents[1] = new Report();        // Display document pages        foreach (Document document in documents)       {         Console.WriteLine("/n" + document.GetType().Name+ "--");          foreach (Page page in document.Pages)         {           Console.WriteLine(" " + page.GetType().Name);         }       }        // Wait for user       Console.Read();         }   }    // "Product"    abstract class Page   {   }    // "ConcreteProduct"    class SkillsPage : Page   {   }    // "ConcreteProduct"    class EducationPage : Page   {   }    // "ConcreteProduct"    class ExperiencePage : Page   {   }    // "ConcreteProduct"    class IntroductionPage : Page   {   }    // "ConcreteProduct"    class ResultsPage : Page   {   }    // "ConcreteProduct"    class ConclusionPage : Page   {   }    // "ConcreteProduct"    class SummaryPage : Page   {   }    // "ConcreteProduct"    class BibliographyPage : Page   {   }    // "Creator"    abstract class Document   {      private ArrayList pages = new ArrayList();      // Constructor calls abstract Factory method      public Document()     {        this.CreatePages();     }      public ArrayList Pages     {        get{ return pages; }     }      // Factory Method      public abstract void CreatePages();   }    // "ConcreteCreator"    class Resume : Document   {      // Factory Method implementation      public override void CreatePages()     {       Pages.Add( new SkillsPage());       Pages.Add( new EducationPage());       Pages.Add( new ExperiencePage());     }   }    // "ConcreteCreator"    class Report : Document   {      // Factory Method implementation      public override void CreatePages()     {       Pages.Add( new IntroductionPage());       Pages.Add( new ResultsPage());       Pages.Add( new ConclusionPage());       Pages.Add( new SummaryPage());       Pages.Add( new BibliographyPage());     }   } }

Output Resume -------  SkillsPage  EducationPage  ExperiencePage Report -------  IntroductionPage  ResultsPage  ConclusionPage  SummaryPage  BibliographyPage