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events-listener
Advanced tools
Node.js provides the events
modules, which provides EventEmitter
. This modules provides EventListener
,
an object that can listen for events.
This was heavily inspired by Backbone's listenTo()
and stopListening()
functions.
The idea is to make it easy to de-regster an event listener when you're done with it.
Supports Node.js 0.12 and up.
import EventEmitter from 'events';
class Widget() {
constructor(emitter) {
emitter.on('error', err => this.close());
}
...
}
let emitter = new EventEmitter();
let widget = new Widget(emitter);
// Do some stuff with `widget`. When we're done with it, clear the reference to
// `widget` so the garbage collector can free it.
widget = null;
The code above creates a pretty common memory leak in node.js apps; the problem in the above is that when we call
emitter.on('error', ...)
, we add the function we pass in to the EventEmitter's list of events to call, which
means EventEmitter has a reference to the handler function. But, the handler function is an arrow function,
which binds this
, which means the handler function has a reference to widget
. When we set widget = null
,
the Widget
object will never be garbage collected, because emitter
still has a reference to it.
One way to solve this problem is to call emitter.removeListener('error', handler)
, but note that the function
we passed to emitter.on(...)
was not this.close()
, so we can't call emitter.removeListener('error', this.close)
.
We need to keep a reference to the anonymous arrow function we passed in to emitter.on()
.
EventListener
's job is to keep track of these references for you:
import EventEmitter from 'events';
import EventListener from 'events-listener';
class Widget() {
constructor(emitter) {
this.listener = new EventListener();
this.listener.listenTo(emitter, 'error', err => this.close());
}
destroy() {
this.listener.stopListening();
}
...
}
let emitter = new EventEmitter();
let widget = new Widget(emitter);
// Do some stuff with `widget`.
widget.destroy();
widget = null;
Similar to calling emitter.on(event, handler)
.
Similar to calling emitter.once(event, handler)
.
Stop listening to some or all events that were registered with calls to listenTo()
or listenToOnce()
.
If all three arguments are passed, this is similar to calling emitter.removeListener(handler)
.
If no arguments are passed, then this will remove all listeners that have been registered on this listener.
If an emitter
is passed, this will remove all listeners that have been registered on the specific emitter.
If emitter
and event
are passed, then this will remove all listeners from the specific emitter that were
registered for the specific event.
FAQs
Listen to events from a Node.js EventEmitter.
The npm package events-listener receives a total of 340,347 weekly downloads. As such, events-listener popularity was classified as popular.
We found that events-listener demonstrated a not healthy version release cadence and project activity because the last version was released a year ago. It has 1 open source maintainer collaborating on the project.
Did you know?
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