Factory Pattern

Factories

• A Very Simple Factory
• Factory Method
• Abstract Factory

When to use factories?

• Unsure which concrete implementation of an interface I want to return
• Creation should be seperated from representation of an object
• Lots of if/else blocks when deciding which concrete class to create
• Switch statements when deciding which concrete class to create

Simple Factory

Intent

• Creates objects without exposing the instantiation logic to the client
• Refers to the newly created object through a common interface

Implementation

Example Model

Demo Source Code (Git)

Factory Method

Intent

• Defines an interface for creating objects, but let subclasses to decide which class to instantiate
• Refers to the newly created object through a common interface

Implementation

Example Model

Demo Source Code (Git)

Advantage

• Eliminate references to concrete classes
  • Factories
  • Objects created by factories
• Factories can be inherited to provide even more specialized object creation
• Rules for object initialization is centralized

Disadvantage

• May ned to create a factory just to get a concrete class delivered
• The inheritance hierarchy gets deeper with coupling between concrete factories and created classes

Abstract Factory

Intent

• Offers the interface for creating a family of related objects, without explicitly specifying their classes
• Factories create different types of concrete objects(Products)
• A Factory now represents a "family" of objects that it can create
• Factories may have more than one factory method

Implementation

• AbstractFactory - declares a interface for operations that create abstract products
• ConcreteFactory - implements operations to create concrete products
• AbstractProduct - declares an interface for a type of product objects
• Product - defines a product to be created by the corresponding ConcreteFactory; it implements the AbstractProduct interface
• Client - uses the interfaces declared by the AbstractFactory and AbstractProduct classes

Example Model

Demo Source Code (Git)

Consequences

• Add new factories and classes without breaking OCP
• Defer choosing classes to classes that specialized in making that decision
• Using private or internal constructors hides direct construction with the "new" keyword

Resources

• Pluralsight Design Patterns Library Course
• oodesign.com
• C# 3.0 Design Patterns
  • By Judith BISHOP
  • Publisher: O'Reilly Media

• Head First Design Patterns 

  • By Eric Freeman, Elisabeth Robson, Bert Bates, Kathy Sierra 

  • Publisher: O'Reilly Media

thanks....

@brs_snmz