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Events and Emitters
in Node JS
const 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, listenerCb);
^---- callback function
v---- event name
emitter.on(event, listenerCb);
Alias
Emit an Event
emitter.emit(event);
Removing an Event Listener
emitter.removeListener(event, listener)
^---- event handle function
v---- event name
Creating an
Event Emitter
Define a class
class Timer {
constructor () {
super();
setInterval(() => {
// do something every 1000ms
}, 1000);
}
}Extend
EventEmitter
const EventEmitter = require('events');
class Timer extends EventEmitter {
constructor () {
super();
setInterval(() => {
// do something every 1000ms
}, 1000);
}
}Create a new instance
const myTimer = new Timer();
myTimer.addListener('tick', () => {
process.stdout.write('tick \n');
});and add an event listener
Emit the event
class Timer extends EventEmitter {
constructor () {
super();
setInterval(() => {
this.emit('tick');
}, 1000);
}
}named 'tick'
Send data back
class Timer extends EventEmitter {
constructor () {
super();
this.count = 0;
setInterval(() => {
this.emit('tick', { interval: this.count++ });
}, 1000);
}
}to the event handler
like callback arguments
use event argument
myTimer.addListener('tick', 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 ${event.interval}\n`);
if(event.interval == 5){
this.removeListener('tick', tickHandler);
}
}
myTimer.addListener('tick', tickHandler);class Timer extends EventEmitter {
constructor () {
super();
this.count = 0;
setInterval(() => {
this.emit('tick', { interval: this.count++ });
}, 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 DevLeague Coding Bootcamp
