C#说明设计模式（四） 工厂方法

Factory Method
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.

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.

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.
UML Code

using System;
using System.Collections;

// "Product"

abstract class Product
{
}

// "ConcreteProductA"

class ConcreteProductA : Product
{
}

// "ConcreteProductB"

class ConcreteProductB : Product
{
}

// "Creator"

abstract class Creator
{
  // Methods
  abstract public Product FactoryMethod();
}

// "ConcreteCreatorA"

class ConcreteCreatorA : Creator
{
  // Methods
  override public Product FactoryMethod()
  {
    return new ConcreteProductA();
  }
}

// "ConcreteCreatorB"

class ConcreteCreatorB : Creator
{
  // Methods
  override public Product FactoryMethod()
  {
    return new ConcreteProductB();
  }
}

/// <summary>
/// Client test
/// </summary>
class Client
{
  public static void Main( string[] args )
  {

    // FactoryMethod returns ProductA
    Creator c = new ConcreteCreatorA();
    Product p = c.FactoryMethod();
    Console.WriteLine( "Created {0}", p );

    // FactoryMethod returns ProductB
    c = new ConcreteCreatorB();
    p = c.FactoryMethod();
    Console.WriteLine( "Created {0}", p );  

  }
}

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.


using System;
using System.Collections;

// "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
{
  // Fields
  protected ArrayList pages = new ArrayList();

  // Constructor
  public Document()
  {
    this.CreatePages();
  }

  // Properties
  public ArrayList Pages
  {
    get{ return pages; }
  }

  // Factory Method
  abstract public void CreatePages();
}

// "ConcreteCreator"

class Resume : Document
{
  // Factory Method implementation
  override public void CreatePages()
  {
    pages.Add( new SkillsPage() );
    pages.Add( new EducationPage() );
    pages.Add( new ExperiencePage() );
  }
}

// "ConcreteCreator"

class Report : Document
{
  // Factory Method implementation
  override public void CreatePages()
  {
    pages.Add( new IntroductionPage() );
    pages.Add( new ResultsPage() );
    pages.Add( new ConclusionPage() );
    pages.Add( new SummaryPage() );
    pages.Add( new BibliographyPage() );
  }
}

/// <summary>
///  FactoryMethodApp test
/// </summary>
class FactoryMethodApp
{
  public static void Main( string[] args )
  {
    Document[] docs = new Document[ 2 ];

    // Note: constructors call Factory Method
    docs[0] = new Resume();
    docs[1] = new Report();

    // Display document pages
    foreach( Document document in docs )
    {
      Console.WriteLine( "\n" + document + " ------- " );
      foreach( Page page in document.Pages )
        Console.WriteLine( " " + page );
    }
  }
}