Singleton Design Pattern
Video Lecture
Overview
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Singleton UML Diagram
Source Code
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./src/singleton/singleton-concept.ts
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25 | // Singleton Concept Sample Code
export class Singleton {
// The Singleton Class
static instance: Singleton
id: number
constructor(id: number) {
this.id = id
if (Singleton.instance) {
return Singleton.instance
}
Singleton.instance = this
}
}
// The Client
// All uses of the singleton point to the same original object
const OBJECT1 = new Singleton(1) // setting its id property to 1
const OBJECT2 = new Singleton(2) // setting its id property to 2
console.log(OBJECT1 === OBJECT2) // = true
console.log(OBJECT1.id) // returns 1
console.log(OBJECT2.id) // returns 1
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Output
node ./dist/singleton/singleton-concept.js
true
Singleton Use Case
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Example UML Diagram
Source Code
./src/singleton/client.ts
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21 | // Singleton Use Case Example Code
import { Game1 } from './game1'
import { Game2 } from './game2'
import { Game3 } from './game3'
// The Client
// Despite all games instantiating a leaderboard, they all point
// to the same memory object since the leaderboard is a singleton.
const GAME1 = new Game1()
GAME1.addWinner(2, 'Cosmo')
const GAME2 = new Game2()
GAME2.addWinner(3, 'Sean')
const GAME3 = new Game3()
GAME3.addWinner(1, 'Emmy')
GAME1.leaderboard.print()
GAME2.leaderboard.print()
GAME3.leaderboard.print()
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./src/singleton/game1.ts
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16 | // A Game Class that uses the Leaderboard Singleton
import Leaderboard from './leaderboard'
import Game from './igame'
export class Game1 implements Game {
leaderboard: Leaderboard
constructor() {
this.leaderboard = new Leaderboard()
}
addWinner(position: number, name: string): void {
this.leaderboard.addWinner(position, name)
}
}
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./src/singleton/game2.ts
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16 | // A Game Class that uses the Leaderboard Singleton
import Leaderboard from './leaderboard'
import Game from './igame'
export class Game2 implements Game {
leaderboard: Leaderboard
constructor() {
this.leaderboard = new Leaderboard()
}
addWinner(position: number, name: string): void {
this.leaderboard.addWinner(position, name)
}
}
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./src/singleton/game3.ts
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16 | // A Game Class that uses the Leaderboard Singleton
import Leaderboard from './leaderboard'
import Game from './igame'
export class Game3 implements Game {
leaderboard: Leaderboard
constructor() {
this.leaderboard = new Leaderboard()
}
addWinner(position: number, name: string): void {
this.leaderboard.addWinner(position, name)
}
}
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./src/singleton/leaderboard.ts
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25 | // A Leaderboard Singleton Class
export default class Leaderboard {
static instance: Leaderboard
#table: { [id: number]: string } = {}
constructor() {
if (Leaderboard.instance) {
return Leaderboard.instance
}
Leaderboard.instance = this
}
public addWinner(position: number, name: string): void {
this.#table[position] = name
}
public print(): void {
console.log('-----------Leaderboard-----------')
for (const key in this.#table) {
console.log(`|\t${key}\t|\t${this.#table[key]}\t|`)
}
console.log()
}
}
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./src/singleton/igame.ts
| // A Game Interface
export default interface IGame {
addWinner(position: number, name: string): void
}
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Output
node ./dist/singleton/client
-----------Leaderboard-----------
| 1 | Emmy |
| 2 | Cosmo |
| 3 | Sean |
-----------Leaderboard-----------
| 1 | Emmy |
| 2 | Cosmo |
| 3 | Sean |
-----------Leaderboard-----------
| 1 | Emmy |
| 2 | Cosmo |
| 3 | Sean |
Summary
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