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An interface with Create operations for each of the abstract products
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Factory1, Factory2
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Implementations of all the AbstractFactory creation operations
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An interface for a kind of product with its own operations
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ProductA1, ProductA2, ProductB1, ProductB2 Classes that implement the AbstractProduct interface and define product objects
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to be created by the corresponding factories
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A class that accesses only the AbstractFactory and AbstractProduct interfaces An interesting aspect of the Abstract Factory pattern is that the whole product family can be changed while the application is running. For example, if the client is using the family of objects created by FactoryA, it can instantiate FactoryB and switch to that. Because the products implement the same abstract interface, any operations it may contain will be the same, although their implementations will differ.
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Abstract Factory Pattern |
Client a : IProductA b : IProductB +Client(parameter : IFactory)
IFactory
IProductA
ProductA1 Factory1 Factory2
ProductA2
IProductB
ProductB1
ProductB2
QUIZ
Match the Abstract Factory Pattern Players with the Replica Handbag Illustration
To test whether you understand the Abstract Factory pattern, cover the lefthand column of the table below and see if you can match the players in the righthand column to the items in the illustrative example (Figure 6-1). Then check your answers against the lefthand column.
Client AbstractFactory FactoryA FactoryB AbstractProduct ProductA1 ProductB1
Customer buying a handbag Returns a handbag Gucci Poochy Handbags Genuine bags Replica bags
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6: Creational Patterns: Abstract Factory and Builder
Implementation and Example: Gucci and Poochy
The code that we shall develop here is based on the preceding illustration. The implementation we have chosen for the Abstract Factory pattern is not the usual inheritance-based one it also makes use of generics to simplify the creation of the factories. Instead of several factory subclasses, we have only one, and it is generic. A factory and its interface look like this:
interface IFactory<Brand> where Brand : IBrand { IBag CreateBag( ); IShoes CreateShoes( ); } // Factories (both in the same one) class Factory<Brand> : IFactory<Brand> where Brand : IBrand, new( ) { public IBag CreateBag( ) { return new Bag<Brand>( ); } public IShoes CreateShoes( ) { return new Shoes<Brand>( ); } }
Our factory is going to make bags and shoes. The structure of the factories is always the same, so we create a generic one based on brand. The generic specification for Factory includes some constraints.
C# Feature Generic Constraints
Generic type and method declarations can optionally specify constraints on the types that can be used to instantiate them. Constraints can indicate which interfaces the type should implement. For example:
class SortedList<T> where T: IComparable<T> {...}
Here, a type T used to instantiate SortedList must implement IComparable.
new( ) as a constraint indicates that the type must have a constructor.
cf. C# Language Specification Version 3.0, September 2007, Section 10.1.5
In our example, the two Products are bags and shoes. Bags indicate what they are made of, and shoes indicate their price. Once again, we declare the products as generic:
Abstract Factory Pattern |
// Product 1 interface IBags { string Material { get; } } // Concrete Product 1 class Bag<Brand> : IBag where Brand : IBrand, new( ) { private Brand myBrand; public Bag( ) { myBrand = new Brand( ); } public string Material { get { return myBrand.Material; } } }
When the Factory to be used is instantiated, as in:
IFactory<Brand> factory = new Factory<Brand>( );
one of the small brand specifications, such as:
class Gucci : IBrand { public int Price { get { return 1000; } } public string Material { get { return "Crocodile skin"; } } }
is passed through to the Factory. Then, when a product is to be made, as in:
IBag bag = factory.CreateBag( ); IShoes shoes = factory.CreateShoes( );
the Factory uses the same brand to instantiate the generic Bag and Shoe classes. The full example code is in Example 6-1.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 using System; namespace AbstractFactoryPattern { // Abstract Factory D-J Miller and Judith Bishop Sept 2007 // Uses generics to simplify the creation of factories interface IFactory<Brand> where Brand : IBrand { IBag CreateBag( ); IShoes CreateShoes( ); } // Concrete Factories (both in the same one) class Factory<Brand> : IFactory<Brand> where Brand : IBrand, new( ) { public IBag CreateBag( ) { return new Bag<Brand>( ); } public IShoes CreateShoes( ) { return new Shoes<Brand>( );
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