class EventEmitter {
constructor(options = {}) {
this.events = {}
this.maxListener = options.maxListener || Infinity
}
emit(event, ...args) {
const cbs = this.events[event]
if (!cbs) return this
cbs.forEach(fn => fn.apply(this, args));
return this
}
on(event, cb) {
if (!this.events[event]) this.events[event] = []
if (this.maxListener !== Infinity && this.events[event].length >= this.maxListener) {
console.warn(`${event} has reached max listeners`);
return this
}
this.events[event].push(cb)
return this
}
once(event, cb) {
let self = this
function fn(...args) {
self.off(event, fn)
cb.apply(self, args)
}
this.on(event, fn)
return this
}
off(event, cb) {
if (!cb) {
this.events[event] = null
} else {
this.events[event] = this.events[event].filter(fn => fn !== cb)
}
return this
}
}
const add = (a,b) => console.log(a+b);
const log = (...arg) => console.log(...arg);
const event = new EventEmitter({maxListener: 3})
event.on('add', add)
event.on('add', add)
event.on('add', add)
event.on('add', add)
event.on('log', log)
event.emit('add', 1,2)
event.emit('log', 'hello~')
event.once('one', value => console.log(value))
event.emit('one', '第一次~')
event.emit('one', '第二次~')
