Abstract Factory pattern Intent Provide an interface for creating families of related or dependent objects without specifying their concrete classes

Abstract Factory pattern Motivation MotifWidgetFactory CreateScrollBar() CreateWindow() PMWidgetFactory CreateScrollBar() CreateWindow() PMWindowMotifWindow PMScrollBarMotifScrollBar WidgetFactory (abstract) CreateScrollBar() CreateWindow() Window ScrollBar Client creates

Abstract Factory Motivation

Abstract Factory Structure Provide an interface for creating families of related or dependent objects without specifying their concrete class

Abstract Factory pattern Structure ConcreteFactory1 CreateProductA() CreateProductB() ConcreteFactory2 CreateProductA() CreateProductB() ProductA2 ProductA1 ProductB2 ProductB1 AbstractFactory CreateProductA() CreateProductB() AbstractProductA AbstractProductB Client creates

Abstract Factory pattern Applicability Use the Abstract Factory pattern when – a system should be independent of how its products are created, composed and represented – a system should be configured with one of multiple families of products – a family of related product objects is designated to be used together, and you need to enforce this constraint – you want to provide a class library of products, and you want to reveal just their interfaces, not their implementations

Abstract Factory pattern Participants AbstractFactory (WidgetFactory) – declares an interface for operations that create abstract product objects ConcreteFactory (MotifWidgetFactory,…) – implements the operations to create concrete product objects AbstractProduct (Window, ScrollBar) – declared an interface for a type of product object ConcreteProduct (MotifWindow, MotifScrollBar) – defines a product object to be created by the corresponding concrete factory – implements the AbstractProduct interface Client – uses only interfaces declared by AbstractFactory and AbstractProduct classes

Abstract Factory - consequences Isolates concrete classes Makes exchanging product families easy – the class of concrete factory appears only once in the application - that is, where it’s instantiated Promotes consistency among products – enforces, that products from one family are used together Supporting new kinds of products is difficult – AbstractFactory interface fixes the set of products that can be created – involves changing AbstractFactory and all its subclasses interfaces

Abstract Factory - implementation Factories as singletons Creating the products – use Factory Method pattern declare FactoryMethod for each kind of product in AbstractFactory override FactoryMethod in ConcreteFactory – use Prototype pattern for ConcreteFactory if many product families are possible initialize the concrete factory with a prototypical instance of each product in the family concrete factory will create a new product by cloning the prototype no need for a new concrete factory class for each new family Defining extensible factories – Via parameterization -- I.e., object = factory.make ( typeA ) ; – Via mixing concrete and abstract factory hierarchies.