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Commit 4749af5

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author
Badacadabra
committed
Add Strategy (ES5 + ES6 + CoffeeScript + TypeScript)
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‎doc/GoF/Behavioral/Strategy/README.md

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# Synopsis
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I am playing a fighting game and I do not know my opponent. Should I prefer an offensive or defensive style?
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# Problem
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Since the opponent is unknown, the player has to choose a fighting strategy on the fly.
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In programming terms, we can say that an algorithm has to be selected at runtime, among several available algorithms.
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# Solution
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The Strategy design pattern is exactly what we need in this kind of situation. To implement this pattern, we can have:
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* A concrete context (FightingGame) which contains a reference to a strategy
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* An abstract representation of a strategy (abstract class or interface)
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* Concrete strategies (Offense & Defense)
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# ==============================
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# CONCRETE CONTEXT
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# ==============================
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class FightingGame
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constructor: ->
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@_strategy = null
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play: ->
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@_strategy.fight()
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setStrategy: (fightStyle) ->
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@_strategy = fightStyle
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# ==============================
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# ABSTRACT STRATEGY
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# ==============================
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class Strategy
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constructor: ->
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throw new Error "You cannot instantiate an abstract class!" if @constructor is Strategy
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fight: ->
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throw new Error "You cannot call an abstract method!"
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# ==============================
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# CONCRETE STRATEGIES
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# ==============================
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class Offense extends Strategy
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fight: ->
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"Fight with an offensive style"
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class Defense extends Strategy
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fight: ->
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"Fight with a defensive style"
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# ==============================
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# CLIENT CODE
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# ==============================
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game = new FightingGame
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offense = new Offense
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defense = new Defense
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game.setStrategy defense
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console.log "ROUND 1 - #{game.play()}"
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game.setStrategy offense
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console.log "ROUND 2 - #{game.play()}"
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// ==============================
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// CONCRETE CONTEXT
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// ==============================
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class FightingGame {
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private strategy: Strategy;
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play(): string {
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return this.strategy.fight();
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}
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setStrategy(fightStyle: Strategy): void {
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this.strategy = fightStyle;
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}
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}
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// ==============================
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// ABSTRACT STRATEGY
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// ==============================
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interface Strategy {
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fight(): string;
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}
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// ==============================
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// CONCRETE STRATEGIES
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// ==============================
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class Offense implements Strategy {
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fight(): string {
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return "Fight with an offensive style";
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}
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}
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class Defense implements Strategy {
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fight(): string {
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return "Fight with a defensive style";
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}
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}
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// ==============================
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// CLIENT CODE
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// ==============================
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let game: FightingGame = new FightingGame(),
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offense: Strategy = new Offense(),
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defense: Strategy = new Defense();
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game.setStrategy(defense);
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console.log(`ROUND 1 - ${game.play()}`);
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game.setStrategy(offense);
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console.log(`ROUND 2 - ${game.play()}`);
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'use strict';
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// ==============================
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// CONCRETE CONTEXT
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// ==============================
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var FightingGame = (function () {
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var strategy = null;
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function FightingGame() {}
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FightingGame.prototype.play = function () {
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return strategy.fight();
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};
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FightingGame.prototype.setStrategy = function (fightStyle) {
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strategy = fightStyle;
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};
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return FightingGame;
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})();
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// ==============================
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// ABSTRACT STRATEGY
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// ==============================
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var Strategy = (function() {
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function Strategy() {
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if (this.constructor === Strategy) {
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throw new Error("You cannot instantiate an abstract class!");
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}
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}
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Strategy.prototype.fight = function () {
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throw new Error("You cannot call an abstract method!");
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};
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return Strategy;
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})();
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// ==============================
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// CONCRETE STRATEGIES
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// ==============================
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var Offense = (function () {
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function Offense() {}
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Offense.prototype = Object.create(Strategy.prototype);
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Offense.prototype.constructor = Offense;
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Offense.prototype.fight = function () {
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return "Fight with an offensive style";
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};
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return Offense;
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})();
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var Defense = (function () {
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function Defense() {}
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Defense.prototype = Object.create(Strategy.prototype);
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Defense.prototype.constructor = Defense;
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Defense.prototype.fight = function () {
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return "Fight with a defensive style";
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};
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return Defense;
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})();
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// ==============================
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// CLIENT CODE
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// ==============================
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var game = new FightingGame(),
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offense = new Offense(),
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defense = new Defense();
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game.setStrategy(defense);
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console.log("ROUND 1 - " + game.play());
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game.setStrategy(offense);
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console.log("ROUND 2 - " + game.play());
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// ==============================
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// CONCRETE CONTEXT
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// ==============================
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class FightingGame {
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constructor() {
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this._strategy = null;
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}
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play() {
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return this._strategy.fight();
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}
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setStrategy(fightStyle) {
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this._strategy = fightStyle;
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}
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}
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// ==============================
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// ABSTRACT STRATEGY
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// ==============================
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class Strategy {
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constructor() {
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if (new.target !== undefined) {
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throw new Error("You cannot instantiate an abstract class!");
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}
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}
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fight() {
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throw new Error("You cannot call an abstract method!");
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}
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}
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// ==============================
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// CONCRETE STRATEGIES
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// ==============================
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class Offense extends Strategy {
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fight() {
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return "Fight with an offensive style";
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}
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}
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class Defense extends Strategy {
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fight() {
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return "Fight with a defensive style";
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}
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}
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// ==============================
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// CLIENT CODE
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// ==============================
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let game = new FightingGame(),
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offense = new Offense(),
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defense = new Defense();
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game.setStrategy(defense);
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console.log(`ROUND 1 - ${game.play()}`);
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game.setStrategy(offense);
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console.log(`ROUND 2 - ${game.play()}`);

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