A brief analysis of Node's events module

In vue projects, sometimes we use the global event bus to manage communication between components. In the vue2 project, we can use$emit,$onand$offto encapsulate aneventHub; in vue3,$onand$offare removed, we can use mitt library or tiny-emitter library. In node, there is no need to be so troublesome. It has a built-in events module that can help us monitor and emit events.

Event monitoring and emission

First use CommonJS syntax to import theEventEmitterclass, and then generate an instanceemitter(EventEmitteris very important. For example, thestreamthat will be introduced in subsequent articles is an instance ofEventEmitter):

const EventEmitter = require('events') const emitter = new EventEmitter()

Then you can useemitter .on()Monitor the event. The first parameter passed in is the event name. The second parameter is the callback to be executed after listening to the event being emitted. If there are incoming parameters when emitting the event, it will be passed For the callback function, you can obtain it one by one, or you can use the remaining parameters of thefunction as follows: [Recommended related tutorials:nodejs video tutorial,Programming teaching】

// 监听事件 emitter.on('test', (...args) => { console.log(args) // [ 1, 2, 3 ] }) // 发射事件 emitter.emit('test', 1, 2, 3)

emitter.once():

emitter.once('test', () => { console.log('监听到了事件发射') }) emitter.emit('test') emitter.emit('test') // 本次发射不会触发打印

emitter.on('test', () => { console.log('监听到了事件发射，1') }) emitter.on('test', () => { console.log('监听到了事件发射，2') }) emitter.emit('test')

emitter.prependListener()(oremitter.prependOnceListener(), that is, listen in advance but only once):

emitter.on('test', () => { console.log('监听到了事件发射，1') }) emitter.prependListener('test', () => { console.log('监听到了事件发射，2') }) emitter.emit('test')

Remove event listening

emitter.off()(oremitter.removeListener ()) Remove the monitoring of events, but you need to pass in the corresponding event name and callback function, so our callback when monitoring cannot be directly defined inemitter.on()## as above # Internal, you need to define it externally and pass in a reference to the callback:

function handler(...args) { console.log(args) // [ 1, 2, 3 ] } emitter.on('test', handler) emitter.emit('test', 1, 2, 3) emitter.off('test', handler) emitter.emit('test', '无法被监听到')

Only one listener can be removed, and the listener callback must be passed in. If If you have multiple listeners and want to remove them all, you can useemitter.removeAllListeners():

emitter.on('test', handler) emitter.on('test', handler) emitter.on('test', handler) emitter.removeAllListeners()

If no parameters are passed in, then Removes all event listeners for all event names. It can also pass in the event name, and all event listeners corresponding to the event name will be removed.

Limit on the number of listeners1 EventEmitter object, a certain The maximum number of listeners for an event name defaults to 10, which can be verified by

:

console.log(emitter.getMaxListeners()) // 10

, an error will be reported, prompting us to useemitter.setMaxListeners()to increase the maximum limit:

If we If you want to know how many listeners there are for a certain event name on the current EventEmitter object and whether it exceeds the maximum limit, you can use

to pass in the event name to view:

console.log(emitter.listenerCount('test'))

Get event names and listenersUse

to get all event names registered on the current EventEmitter object, and the returned Array composed of event strings:

emitter.on('test1', handler) emitter.on('test2', handler) console.log(emitter.eventNames()) // [ 'test1', 'test2' ]

and pass in the event name:

function handler1() {} function handler2() {} emitter.on('test', handler1) emitter.on('test', handler2) console.log(emitter.listeners('test'))