add observer design pattern

Signed-off-by: Nimit <nimitagg95@gmail.com>

Author: nimit95

Behavioral/observer.js

@@ -0,0 +1,62 @@
+const fs = require('fs');
+// Manages a Subject interactions with observer
+class SubjectManager {
+ constructor() {
+ this._observers = {};
+ }
+ addObserver(eventType, observer) {
+ if(!this._observers[eventType]) {
+ this._observers[eventType] = [];
+ }
+ this._observers[eventType].push(observer);
+ }
+ removeObserver(eventType, observer) {
+ const idx = this._observers[eventType].indexOf(observer);
+ if(idx > -1){
+ this._observers[eventType].splice(idx);
+ }
+ }
+ notify(eventType, data) {
+ this._observers[eventType].forEach(observer => observer.update(data));
+ }
+}
+
+class FileManager {
+ constructor() {
+ this.subjectManager = new SubjectManager();
+ }
+ monitorFile(path) {
+ // eslint-disable-next-line node/no-unsupported-features/es-syntax
+ fs.watch(path, (data) => this.subjectManager.notify("change", {path, data}));
+ }
+ addObserver(eventType, observer) {
+ this.subjectManager.addObserver(eventType, observer);
+ }
+ removeObserver(eventType, observer) {
+ this.subjectManager.removeObserver(eventType, observer);
+ }
+}
+class LoggingObserver {
+ update({ data }) {
+ console.log(data);
+ }
+}
+class SizeChangeObserver {
+ constructor() {
+ this.size = 0;
+ }
+ update({ path }) {
+ const newSize = fs.statSync(path).size;
+ if(newSize > this.size) {
+ console.log("size increase to " + newSize);
+ this.size = newSize;
+ }
+ }
+}
+
+// Subject class
+const fileManager = new FileManager();
+// Adding observers
+fileManager.addObserver("change", new LoggingObserver());
+fileManager.addObserver("change", new SizeChangeObserver());
+fileManager.monitorFile(process.argv[1]);

buildDocs.js

@@ -35,6 +35,7 @@ const sections = [
  { name: 'iterator', path: './Behavioral' },
  { name: 'mediator', path: './Behavioral' },
  { name: 'memento', path: './Behavioral' },
+ { name: 'observer', path: './Behavioral' },
  ]
  }
 ];

docs.md

@@ -22,6 +22,7 @@
 * [Iterator](#iterator)
 * [Mediator](#mediator)
 * [Memento](#memento)
+* [Observer](#observer)
 
 
 
@@ -881,5 +882,72 @@ board.printFormattedBoard();
 
 ```
 
+### Observer
+##### observer.js
+```Javascript
+const fs = require('fs');
+// Manages a Subject interactions with observer
+class SubjectManager {
+ constructor() {
+ this._observers = {};
+ }
+ addObserver(eventType, observer) {
+ if(!this._observers[eventType]) {
+ this._observers[eventType] = [];
+ }
+ this._observers[eventType].push(observer);
+ }
+ removeObserver(eventType, observer) {
+ const idx = this._observers[eventType].indexOf(observer);
+ if(idx > -1){
+ this._observers[eventType].splice(idx);
+ }
+ }
+ notify(eventType, data) {
+ this._observers[eventType].forEach(observer => observer.update(data));
+ }
+}
+
+class FileManager {
+ constructor() {
+ this.subjectManager = new SubjectManager();
+ }
+ monitorFile(path) {
+ // eslint-disable-next-line node/no-unsupported-features/es-syntax
+ fs.watch(path, (data) => this.subjectManager.notify("change", {path, data}));
+ }
+ addObserver(eventType, observer) {
+ this.subjectManager.addObserver(eventType, observer);
+ }
+ removeObserver(eventType, observer) {
+ this.subjectManager.removeObserver(eventType, observer);
+ }
+}
+class LoggingObserver {
+ update({ data }) {
+ console.log(data);
+ }
+}
+class SizeChangeObserver {
+ constructor() {
+ this.size = 0;
+ }
+ update({ path }) {
+ const newSize = fs.statSync(path).size;
+ if(newSize > this.size) {
+ console.log("size increase to " + newSize);
+ this.size = newSize;
+ }
+ }
+}
+
+// Subject class
+const fileManager = new FileManager();
+// Adding observers
+fileManager.addObserver("change", new LoggingObserver());
+fileManager.addObserver("change", new SizeChangeObserver());
+fileManager.monitorFile(process.argv[1]);
+```
+