Joe Karlsson
progress. tech. cats. coffee. art. food. photog. mpls.
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
