Events and Emitters

in Node JS

var EventEmitter = require('events');

Objects that are subclasses of EventEmitter

may have functions attached to them. 
These functions are called listeners, or event handlers,

and are invoked when an event is emitted by that object. 

 

Events are named using a string, such as "complete" or "error". 

 

When the event handler is invoked by an event being emitted,

the context of the event handler function

is the object that it is listening on.

Adding Event Listeners

emitter.addListener(event:String, listener:Event->Void)
alias emitter.on(args)

^---- callback function

v---- event name

Emit an Event

emitter.emit(event:String);

Removing an Event Listener

emitter.removeListener(event:String, listener:Event->Void)

^---- event handle function

v---- event name

Creating an

Event Emitter

Define a class

function Timer () {

  setInterval(function () {
    // do something every 1000ms
  }, 1000);
}

Extend

EventEmitter

var EventEmitter = require('events');

function Timer () {
  EventEmitter.call(this);

  setInterval(function () {
    // do something every 1000ms
  }, 1000);
}

Timer.prototype = new Object(EventEmitter.prototype, {
  constructor: {
    value: EventEmitter,
    configurable: true,
    enumerable: true,
    writable: true
  }
});

Use Node util

var EventEmitter = require('events');
var util = require('util');

function Timer () {
  EventEmitter.call(this);
  
  setInterval(function () {
    // do something every 1000ms
  }, 1000);
}

util.inherits(Timer, EventEmitter);

for easier inheritence

Create a new instance

var myTimer = new Timer();
myTimer.addListener('tick', function(){
  process.stdout.write('tick \n');
});

and add an event listener

Emit the event

function Timer () {
  EventEmitter.call(this);

  var self = this;
  setInterval(function () {
    self.emit('tick');
  }, 1000);
}

named 'tick'

Send data back

function Timer () {
  EventEmitter.call(this);
  var self = this;
  var i = 0;
  setInterval(function () {
    self.emit('tick', { interval : i++ });
  }, 1000);
}

to the event handler

like callback arguments

use event argument

myTimer.addListener('tick', function(event){
  process.stdout.write('tick ' + event.interval + '\n');
});

name the event handler

function tickHandler(event){
  process.stdout.write('tick ' + event.interval + '\n');
}
myTimer.addListener('tick', tickHandler);

remove the

event handler

function tickHandler(event){
  process.stdout.write('tick ' + event.interval + '\n');
  if(event.interval == 5){
    myTimer.removeListener('tick', tickHandler);
  }
}
myTimer.addListener('tick', tickHandler);

use variables

from the context

function tickHandler(event){
  process.stdout.write('tick ' + this.i + '\n');
  if(this.i == 5){
    this.removeListener('tick', tickHandler);
  }
}
myTimer.addListener('tick', tickHandler);
function Timer () {
  EventEmitter.call(this);
  var self = this;
  this.i = 0;
  setInterval(function () {
    self.emit('tick', { interval : self.i++ });
  }, 1000);
}

Remember this!

look for objects or classes that extend EventEmitter
it means you can listen for events that it will emit

check the class api documentation or source code 
for what events it will emit, and the contents of the event object it passes back to the handler

emitter.on('data', function(event){...});

What are you ? -----v

^----- What other events do you emit?

Resources

Events and Emitters in NodeJS

By Joe Karlsson

Events and Emitters in NodeJS

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