events-async
v1.2.1
Published
A wrapper around the EventEmitter to handle promise based async events
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Events, but asynchronous
This module is the lightest wrapper I could come up with around the EventEmitter
to add support for async events with Promises. It only re-implements the emit
method. This now returns a Promise
which will be resolved when all the event handlers have completed.
Handler functions can return a Promise
to indicate that they are asynchronous which will then wait for resolution before emit resolves.
Install
$ npm install --save events-async
Usage
var EventEmitter = require('events-async');
var ee = new EventEmitter();
// Add an asyncronous listener
ee.on('evt', function () {
return new Promise(function (resolve, reject) {
// Do something async, for example:
process.nextTick(function () {
resolve();
});
});
});
// Emit an event and wait for it to complete
ee.emit('evt').then(function () {
// All of the listeners have run
});
With this module you can mix async listeners and sync listeners, so you are not tied to the promise interface when you don't need it. Listeners that don't return a promise are just run in the normal sync fashion.
This module also supports the same argument passing behavior as node core's EventEmitter
. It is actually as close to a direct port for that as possible, including the performance optimizations.
By default handler functions are run in parallel. If you need to run them in a series the first argument of the emit function needs to be an object with the series
option passed.
var EventEmitter = require('events-async');
var delay = require('delay');
var actual = [];
var ee = new EventEmitter();
// first
ee.on('evt', function () {
return delay(300).then(function () {
actual.push(300);
});
});
// second
ee.on('evt', function () {
return delay(200).then(function () {
actual.push(200);
});
});
// third
ee.on('evt', function () {
return delay(100).then(function () {
actual.push(100);
});
});
// Emit an event and wait for it to complete
ee.emit({ serial: true }, 'evt').then(function () {
// All of the listeners have run
console.log(actual);
//=> [ 300, 200, 100 ]
});
If you want to catch an error instead of it throwing a hard error then pass { catch: true }
into the emit options.