Event Loop and EventEmitter

Event Loop

• Node.js uses Events extensively, which is why it is much faster than other technologies

• When a Node.js-based server starts,

  • it initializes variables,

  • declares functions,

  • and waits for Events to occur

• In Event-Driven applications, there is a main loop that waits for events

  • And when an event is detected, it calls a Callback function

ff

Event vs Callback Function

• A Callback function is called when an asynchronous function returns a result

• Event Handling works based on the observer pattern

Observer pattern

• A design pattern that registers a list of observers - those who observe state changes of an object - on the object, and notifies each observer in the list through methods whenever a state change occurs

• It is mainly used to implement Distributed event handling systems

EventListeners functions that wait for events serve as the observers!

-> Observers wait for events, and when an event fires, the function that handles the event is executed!

events module & EventEmitter Class

: You can bind events and event handlers using Node.js's event module and EventEmitter Class

How to Use

// Use the events module
var events = require('events')

// Create an EventEmitter object
var eventEmitter = new events.EventEmitter();

// Bind event & EventHandler
eventEmitter.on('eventName', eventHandler);

// Trigger an event within the program
eventEmtter.emit('eventName');

ex)

event_handle.js

// Use the events module
var events = require('events');

// Create an EventEmitter object
var eventEmitter = new events.EventEmitter();

// Create an EventHandler function
var connectHandler = function connected(){
    console.log("You've connected");

    // Trigger the data_received event
    eventEmitter.emit('data_received');
}

// Bind the connection event with connectHandler
eventEmitter.on('connection', connectHandler);

// Bind the data_received event with an anonymous function
// -> Instead of storing the function in a variable, pass the function directly as an argument to the .on() method
eventEmitter.on('data_received', function() {
    console.log('Data received');
})

// Trigger the connection event
eventEmitter.emit('connection');

console.log('Program has ended');

Result

$ node event_handle.js
You've connected
Data received
Program has ended

EventEmitter Class

• Many objects in a Node emit events

  • Server emits an event each time a peer connects to it

  • An fs.readStream emits an event when the file is opened

• All objects which emit events are instance of events.EventEmitter

Methods

Sr.No.	Method & Description
1	addListener(event, listener) Adds a listener at the end of the listeners array for the specified event. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of event and listener will result in the listener being added multiple times. Returns emitter, so calls can be chained.
2	on(event, listener) Adds a listener at the end of the listeners array for the specified event. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of event and listener will result in the listener being added multiple times. Returns emitter, so calls can be chained.
3	once(event, listener) Adds a one time listener to the event. This listener is invoked only the next time the event is fired, after which it is removed. Returns emitter, so calls can be chained.
4	removeListener(event, listener) Removes a listener from the listener array for the specified event. Caution − It changes the array indices in the listener array behind the listener. removeListener will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified event, then removeListener must be called multiple times to remove each instance. Returns emitter, so calls can be chained.
5	removeAllListeners([event]) Removes all listeners, or those of the specified event. It's not a good idea to remove listeners that were added elsewhere in the code, especially when it's on an emitter that you didn't create (e.g. sockets or file streams). Returns emitter, so calls can be chained.
6	setMaxListeners(n) By default, EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default which helps finding memory leaks. Obviously not all Emitters should be limited to 10. This function allows that to be increased. Set to zero for unlimited.
7	listeners(event) Returns an array of listeners for the specified event.
8	emit(event, [arg1], [arg2], [...]) Execute each of the listeners in order with the supplied arguments. Returns true if the event had listeners, false otherwise.

Class Methods

Sr.No.	Method & Description
1	listenerCount(emitter, event) Returns the number of listeners for a given event.

Events

Sr.No.	Events & Description
1	newListener event − String : the event name listener − Function : the event handler functionThis event is emitted any time a listener is added. When this event is triggered, the listener may not yet have been added to the array of listeners for the event.
2	removeListener event − String The event name listener − Function The event handler function This event is emitted any time someone removes a listener. When this event is triggered, the listener may not yet have been removed from the array of listeners for the event.

ex)

EventEmitter.js

var events = require('events');
var eventEmitter = new events.EventEmitter();

// listener #1
var listner1 = function listner1() {
   console.log('listner1 executed.');
}

// listener #2
var listner2 = function listner2() {
   console.log('listner2 executed.');
}

// Bind the connection event with the listner1 function
eventEmitter.addListener('connection', listner1);

// Bind the connection event with the listner2 function
eventEmitter.on('connection', listner2);

var eventListeners = require('events').EventEmitter.listenerCount
   (eventEmitter,'connection');
console.log(eventListeners + " Listner(s) listening to connection event");

// Fire the connection event
eventEmitter.emit('connection');

// Remove the binding of listner1 function
eventEmitter.removeListener('connection', listner1);
console.log("Listner1 will not listen now.");

// Fire the connection event
eventEmitter.emit('connection');

eventListeners = require('events').EventEmitter.listenerCount(eventEmitter,'connection');
console.log(eventListeners + " Listner(s) listening to connection event");

console.log("Program Ended.");

Result

$ node EventEmitter.js
2 Listner(s) listening to connection event
listner1 executed.
listner2 executed.
Listner1 will not listen now.
listner2 executed.
1 Listner(s) listening to connection event
Program Ended.