Events and the Event Emitter

• Events

  • Something that has happened in our app that we can respond to.

  • Two kinds:

    • System Events:

      • C++ side libuv

        • file reading / data from internet

    • Custom Events:

      • Inside js core

        • Self created

      • Event emitter

      • Not real, faked

Building Our Own Event Emitter

emitter.js

function Emitter() {
  this.events = [];
}

Emitter.prototype.on = function(type, listener) {
    // check if existing
    this.events[type] = this.events[type] || [];
    // add to container
    this.events[type].push(listener);
};

Emitter.prototype.emit = function(type) {
    if (this.events[type]) {
      this.events[type].forEach(function(listener) {
        listener();
      })
    }
}

module.exports = Emitter;

app.js

var Emitter = require("./emitter.js");

var emtr = new Emitter();



emtr.on('greet', function() {
    console.log('listener envoked1');
});

emtr.on('greet', function() {
    console.log('listener envoked2');
})

console.log("hello")
emtr.emit('greet');

// hello
// listener envoked1
// listener envoked2

• The point of an emitter.

  • Do something when something happens. In our case, we waited for console.log(), and then manually invoked the emitter with keyword greet.

  • Better design to encapsulate an Emitter object, rather than having if and else statements.

The Node Event Emitter

• The node api has this built in library.

• It has the same basic concept of above, and it will have the same output:

var Emitter = require("events");

var emtr = new Emitter();

emtr.on('greet', function() {
    console.log('listener envoked1');
});

emtr.on('greet', function() {
    console.log('listener envoked2');
})

console.log("hello")
emtr.emit('greet');

Using the Event Emitter

• Dont use magic numbers or strings

  • e.g. constants with special meanings

• So instead, it would become better practice to encapulate our type of events within our own modules.

module.exports = {
    events: {
        GREET: 'greet',
        FILEOPENED: 'file opened',
        FILECLOSED: 'file closed'
    }
}

and envoke them like this:

var Emitter = require("events");
var configEvents = require("./config.js").events;
var emtr = new Emitter();

emtr.on(configEvents.GREET, function() {
    console.log('listener envoked1');
});

emtr.on(configEvents.GREET, function() {
    console.log('listener envoked2');
})

console.log("hello")
emtr.emit('greet');

Inheriting From the Event Emitter