Factory Design Pattern

Video Lecture

Section	Video Links
Factory Pattern
Factory Use Case

When developing code, you may instantiate objects directly in methods or in classes. While this is quite normal, you may want to add an extra abstraction between the creation of the object and where it is used in your project.

You can use the Factory pattern to add that extra abstraction. The Factory pattern is one of the easiest patterns to understand and implement.

Adding an extra abstraction will also allow you to dynamically choose classes to instantiate based on some kind of logic.

Before the abstraction, your client, class or method would directly instantiate an object of a class. After adding the factory abstraction, the concrete product (object) is now created outside the current class/method, and now in a subclass instead.

Imagine an application for designing houses and the house has a chair already added on the floor by default. By adding the factory pattern, you could give the option to the user to choose different chairs, and how many at runtime. Instead of the chair being hard coded into the project when it started, the user now has the option to choose.

Adding this extra abstraction also means that the complications of instantiating extra objects can now be hidden from the class or method that is using it.

This separation also makes your code easier to read and document.

The Factory pattern is really about adding that extra abstraction between the object creation and where it is used. This gives you extra options that you can more easily extend in the future.

Terminology

Concrete Creator: The client application, class or method that calls the Creator (Factory method).
Product Interface: The interface describing the attributes and methods that the Factory will require in order to create the final product/object.
Creator: The Factory class. Declares the Factory method that will return the object requested from it.
Concrete Product: The object returned from the Factory. The object implements the Product interface.

Factory UML Diagram

Factory Pattern Overview

Source Code

In this concept example, the client wants an object named b

Rather than creating b directly in the client, it asks the creator (factory) for the object instead.

The factory finds the relevant class using some kind of logic from the attributes of the request. It then asks the subclass to instantiate the new object that it then returns as a reference back to the client asking for it.

./src/factory/factory-concept.ts

// The Factory Concept

interface IProduct {
    name: string
}

class ConcreteProduct implements IProduct {
    name = ''
}

class ConcreteProductA extends ConcreteProduct {
    constructor() {
        super()
        this.name = 'ConcreteProductA'
    }
}

class ConcreteProductB extends ConcreteProduct {
    constructor() {
        super()
        this.name = 'ConcreteProductB'
    }
}

class ConcreteProductC extends ConcreteProduct {
    constructor() {
        super()
        this.name = 'ConcreteProductC'
    }
}

class Creator {
    static createObject(someProperty: string): IProduct {
        if (someProperty === 'a') {
            return new ConcreteProductA()
        } else if (someProperty === 'b') {
            return new ConcreteProductB()
        } else {
            return new ConcreteProductC()
        }
    }
}

// The Client
const PRODUCT = Creator.createObject('b')
console.log(PRODUCT.name)

Output

node ./dist/factory/factory-concept.js
ConcreteProductB

Factory Use Case

An example use case may be a user interface where the user can select from a menu of items, such as chairs.

The user has been given a choice using some kind of navigation interface, and it is unknown what choice, or how many chairs the user will add until the application is actually running, and the user starts using it.

So, when the user selected the chair, the factory then takes some property involved with that selection, such as an ID, Type or other attribute and then decides which relevant subclass to instantiate in order to return the appropriate object.

While there are is a large amount of code in this example, and it is spread across several files, the actual factory is the ChairFactory class in the file chair-factory.ts. So, the factory is the part of your program that is creating a separation or abstraction between the instantiating of your object and where it is used.

Factory Example UML Diagram

Chair Factory

Source Code

./src/factory/client.ts

// Factory Use Case Example Code
import ChairFactory from './chair-factory'

const CHAIR = ChairFactory.getChair('SmallChair')
console.log(CHAIR.getDimensions())

./src/factory/dimension.ts

export type dimension = {
    height: number
    width: number
    depth: number
}

./src/factory/chair.ts

import { dimension } from './dimension'

// A Chair Interface
interface IChair {
    height: number
    width: number
    depth: number
    getDimensions(): dimension
}

// The Chair Base Class
export default class Chair implements IChair {
    height = 0
    width = 0
    depth = 0

    getDimensions(): dimension {
        return {
            width: this.width,
            depth: this.depth,
            height: this.height,
        }
    }
}

./src/factory/chair-factory.ts

import SmallChair from './small-chair'
import MediumChair from './medium-chair'
import BigChair from './big-chair'
import IChair from './chair'

export default class ChairFactory {
    static getChair(chair: string): IChair {
        if (chair == 'BigChair') {
            return new BigChair()
        } else if (chair == 'MediumChair') {
            return new MediumChair()
        } else {
            return new SmallChair()
        }
    }
}

./src/factory/small-chair.ts

import Chair from './chair'

export default class SmallChair extends Chair {
    constructor() {
        super()
        this.height = 40
        this.width = 40
        this.depth = 40
    }
}

./src/factory/medium-chair.ts

import Chair from './chair'

export default class MediumChair extends Chair {
    constructor() {
        super()
        this.height = 60
        this.width = 60
        this.depth = 60
    }
}

./src/factory/big-chair.ts

import Chair from './chair'

export default class BigChair extends Chair {
    constructor() {
        super()
        this.height = 80
        this.width = 80
        this.depth = 80
    }
}

Output

node ./dist/factory/client.js
{'width': 40, 'depth': 40, 'height': 40}

Summary

The Factory Pattern defers the creation of the final object to a subclass.
The Factory pattern is about inserting another layer/abstraction between instantiating an object and where in your code it is actually used.
It is unknown what or how many objects you will need to be created until runtime.
You want to localize knowledge of the specifics of instantiating a particular object to the subclass so that the client doesn't need to be concerned about the details.
You want to create an external framework, that an application can import/reference, and hide the details of the specifics involved in creating the final object/product.
The unique factor that defines the Factory pattern, is that your project now defers the creation of objects to the subclass that the factory had delegated it to.

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