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Commit 3ddfad1

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

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# Synopsis
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In the savanna, there are predators like lions or crocodiles and there are preys like gazelles. So lions and crocodiles often attack gazelles.
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# Problem
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Lions and crocodiles do not attack at any moment. A lion will generally attack if a gazelle is a bit too far from the herd. A crocodile will generally attack if a gazelle is a bit too close to water, especially for drinking.
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In programming terms, predators execute some action when a specific event occurs on the preys side. This implies the implementation of an event-handling mechanism in the code, which may be unclear if it is not properly done.
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# Solution
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The Observer design pattern helps a lot when we have to implement an event-driven system like this. Here this pattern is composed of:
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* An abstract representation of preys which are observable objects (abstract class is preferred because each prey may have multiple predators)
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* An abstract representation of predators which are observers (abstract class or interface)
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* A concrete prey (Gazelle)
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* Concrete predators (Lion & Crocodile)
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# ==============================
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# ABSTRACT OBSERVER
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# ==============================
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class Predator
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constructor: ->
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throw new Error "You cannot instantiate an abstract class!" if @constructor is Predator
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attack: ->
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throw new Error "You cannot call an abstract method!"
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# ==============================
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# ABSTRACT OBSERVABLE
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# ==============================
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class Prey
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constructor: ->
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throw new Error "You cannot instantiate an abstract class!" if @constructor is Prey
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@_predators = []
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addPredator: (predator) ->
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@_predators.push predator
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notifyPredators: ->
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situation = ""
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situation += predator.attack() for predator in @_predators
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situation
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# ==============================
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# CONCRETE OBSERVERS
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# ==============================
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class Lion extends Predator
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attack: ->
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"Lion attack!\n"
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class Crocodile extends Predator
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attack: ->
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"Crocodile attack!\n"
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# ==============================
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# CONCRETE OBSERVABLES
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# ==============================
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class Gazelle extends Prey
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constructor: ->
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super
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# ==============================
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# CLIENT CODE
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# ==============================
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lion = new Lion
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crocodile = new Crocodile
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gazelle = new Gazelle
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gazelle.addPredator lion
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gazelle.addPredator crocodile
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console.log gazelle.notifyPredators()
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// ==============================
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// ABSTRACT OBSERVER
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// ==============================
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interface Predator {
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attack(): string;
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}
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// ==============================
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// ABSTRACT OBSERVABLE
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// ==============================
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abstract class Prey {
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protected predators: Predator[] = [];
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public addPredator(predator: Predator): void {
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this.predators.push(predator);
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}
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public notifyPredators(): string {
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let situation: string = "";
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for (let predator of this.predators) {
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situation += predator.attack();
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}
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return situation;
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}
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}
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// ==============================
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// CONCRETE OBSERVERS
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// ==============================
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class Lion implements Predator {
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public attack(): string {
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return "Lion attack!\n";
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}
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}
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class Crocodile implements Predator {
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public attack(): string {
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return "Crocodile attack!\n";
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}
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}
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// ==============================
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// CONCRETE OBSERVABLES
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// ==============================
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class Gazelle extends Prey {
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constructor() {
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super();
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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 lion: Predator = new Lion(),
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crocodile: Predator = new Crocodile(),
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gazelle: Prey = new Gazelle();
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gazelle.addPredator(lion);
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gazelle.addPredator(crocodile);
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console.log(gazelle.notifyPredators());
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'use strict';
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// ==============================
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// ABSTRACT OBSERVER
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// ==============================
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var Predator = (function() {
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function Predator() {
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if (this.constructor === Predator) {
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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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Predator.prototype.attack = function () {
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throw new Error("You cannot call an abstract method!");
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};
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return Predator;
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})();
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// ==============================
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// ABSTRACT OBSERVABLE
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// ==============================
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var Prey = (function () {
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function Prey() {
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if (this.constructor === Prey) {
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throw new Error("You cannot instantiate an abstract class!");
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}
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this._predators = [];
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}
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Prey.prototype.addPredator = function (predator) {
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this._predators.push(predator);
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};
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Prey.prototype.notifyPredators = function () {
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var situation = "";
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for (var i = 0, len = this._predators.length; i < len; i++) {
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situation += this._predators[i].attack();
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}
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return situation;
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};
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return Prey;
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})();
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// ==============================
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// CONCRETE OBSERVERS
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// ==============================
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var Lion = (function () {
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function Lion() {}
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Lion.prototype = Object.create(Predator.prototype);
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Lion.prototype.constructor = Lion;
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Lion.prototype.attack = function () {
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return "Lion attack!\n";
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};
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return Lion;
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})();
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var Crocodile = (function () {
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function Crocodile() {}
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Crocodile.prototype = Object.create(Predator.prototype);
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Crocodile.prototype.constructor = Crocodile;
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Crocodile.prototype.attack = function () {
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return "Crocodile attack!\n";
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};
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return Crocodile;
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})();
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// ==============================
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// CONCRETE OBSERVABLES
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// ==============================
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var Gazelle = (function () {
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function Gazelle() {
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Prey.call(this);
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}
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Gazelle.prototype = Object.create(Prey.prototype);
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Gazelle.prototype.constructor = Gazelle;
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return Gazelle;
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})();
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// ==============================
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// CLIENT CODE
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// ==============================
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var lion = new Lion(),
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crocodile = new Crocodile(),
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gazelle = new Gazelle();
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gazelle.addPredator(lion);
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gazelle.addPredator(crocodile);
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console.log(gazelle.notifyPredators());
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// ==============================
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// ABSTRACT OBSERVER
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// ==============================
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class Predator {
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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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attack() {
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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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// ABSTRACT OBSERVABLE
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// ==============================
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class Prey {
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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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this._predators = [];
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}
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addPredator(predator) {
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this._predators.push(predator);
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}
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notifyPredators() {
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let situation = "";
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for (let predator of this._predators) {
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situation += predator.attack();
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}
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return situation;
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}
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}
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// ==============================
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// CONCRETE OBSERVERS
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// ==============================
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class Lion extends Predator {
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attack() {
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return "Lion attack!\n";
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}
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}
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class Crocodile extends Predator {
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attack() {
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return "Crocodile attack!\n";
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}
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}
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// ==============================
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// CONCRETE OBSERVABLES
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// ==============================
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class Gazelle extends Prey {
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constructor() {
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super();
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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 lion = new Lion(),
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crocodile = new Crocodile(),
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gazelle = new Gazelle();
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gazelle.addPredator(lion);
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gazelle.addPredator(crocodile);
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console.log(gazelle.notifyPredators());

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