I am trying to design a scenario where on a particular event getting triggered, I wanted a few listeners to perform some task. Trying to also manage S.R.P of code, I want to have the listeners in a different source file. I want to know if this is possible using event Emitter. Does event emitter only work on a single source file?
var events = require('events');
var em = new events.EventEmitter();
exports.saveScheme = function (req, res) {
var dal = dalFactory.createDAL(constants.SCHEME);
return new Promise.resolve(dal.PromiseSave(req.body))
.then(function(data){
var schemeId = data._id;
em.addListener('FirstEvent', function (data) {
console.log('First subscriber: ' + data);
});
em.emit('FirstEvent', 'Test event emitter');
}).catch(function(error){
console.log(error);
});
};
My other source file is
var emitter = require('events').EventEmitter;
var em = new emitter();
//Subscribe FirstEvent
em.on('FirstEvent', function (data) {
console.log('First subscriber: ' + data);
});
Every eventEmitter object you create is a new instance so events fired from the first one won't be triggered in the second, so the answer to your question is - no, it's not possible. However, there are other solutions:
I think the best one is to create a centralized common eventEmitter, like so:
//firstFile.js
var common = require('./common');
var commonEmitter = common.commonEmitter;
exports.saveScheme = function (req, res) {
var dal = dalFactory.createDAL(constants.SCHEME);
return new Promise.resolve(dal.PromiseSave(req.body))
.then(function(data){
var schemeId = data._id;
commonEmitter.addListener('FirstEvent', function (data) {
console.log('First subscriber: ' + data);
});
commonEmitter.emit('FirstEvent', 'Test event emitter');
}).catch(function(error){
console.log(error);
});
};
//secondFile.js
var common = require('./common');
var commonEmitter = common.commonEmitter;
//Subscribe FirstEvent
commonEmitter.on('FirstEvent', function (data) {
console.log('First subscriber: ' + data);
});
//common.js
var events = require('events');
var em = new events.EventEmitter();
module.exports.commonEmitter = em;
But if you want the source file to "know" each other - You can do something like this:
//firstFile.js
var events = require('events');
var em = new events.EventEmitter();
exports.saveScheme = function (req, res) {
var dal = dalFactory.createDAL(constants.SCHEME);
return new Promise.resolve(dal.PromiseSave(req.body))
.then(function(data){
var schemeId = data._id;
em.addListener('FirstEvent', function (data) {
console.log('First subscriber: ' + data);
});
em.emit('FirstEvent', 'Test event emitter');
}).catch(function(error){
console.log(error);
});
};
exports.emitter = em;
//secondFile.js
var firstFile = require('./firstFile');
var firstFileEmitter = firstFile.emitter;
//Subscribe FirstEvent
firstFileEmitter.on('FirstEvent', function (data) {
console.log('First subscriber: ' + data);
});
The event listener and emitter can be in two different files. Please see a simple example below:
The first file, let's call it eventEx.js
var EventEmitter = require('events').EventEmitter
var myEmitter = new EventEmitter();
myEmitter.on('print', (arg) => {
console.log(arg);
})
exports.myEmitter = myEmitter
And the second file is as following:
var emitter = require('./eventEx.js')
var printEmitter = emitter.myEmitter
printEmitter.emit('print', 'how was Dunkirk?')
printEmitter.emit('print', 'Dunkirk was awesome!!')
So this is how I use in my testing. I use it with Class semantics
One caveat to note that is, it is always required to register a listener before an event is emitted, because when an event is emitted, it looks for already registered set of listeners and then emits the flow over there.
//emitter.js
// The core module events has EventEmitter class which we are going to make use of
// note that we are not going to use that explicitly but by means of another class
const EventEmitter = require('events');
Class EventEmitterClass extends EventEmitter{
emitterMethod(){
this.emit('testEventEmitted', {obj:'testString object'});
}
}
module.exports = EventEmitterClass; // we export the module with the objectInstance
//listener.js
// now import the emitter.js and we get back a class - EventEmitterClass
const EventEmitterClass = require('./emitter.js');//give the actual path of js
const eventEmitterObj = new EventEmitterClass();
//now it is mandatory to register the listener before we even think of calling the //emitter method
eventEmitterObj.addListener('testEventEmitted', res => {
console.log('this is the result from the emitted event:', res);
});
//now call the method that emits the event in the class
eventEmitterObj.emitterMethod();
Now run the listener.js -> node listerner.js
Apologies, if I have explained things way too elaborate
来源:https://stackoverflow.com/questions/37155249/node-js-event-listener-in-another-source-file