Observer

EventEmitter.js

class EventEmitter {
  constructor() {
    this.clientList = {}
  }

  on(key, fn) {
    if (!this.clientList[key]) {
      this.clientList[key] = []
    }
    this.clientList[key].push(fn)
  }

  emit(key, ...args) {
    let fns = this.clientList[key]
    if (!fns || !fns.length) {
      return false
    }

    for (let i = 0, fn; fn = fns[i++];) {
      fn.apply(this, args)
    }
  }

  removeListener(key, fn) {
    const fns = this.clientList[key]

    if (!fns || !fn) {
      return
    }

    for (let i = fns.length; i--;) {
      if (fns[i] === fn) {
        fns.splice(i, 1)
      }
    }
  }

  removeAllListeners(key) {
    if (key) {
      this.clientList[key].length = 0
    } else {
      this.clientList = {}
    }
  }

  once(key, fn) {
    function on(...args) {
      this.removeListener(key, on)
      fn.apply(this, args)
    }
    return this.on(key, on)
  }
}

const event = new EventEmitter()

event.once('key2', function(...args) {
  console.log('once-key2', args)
})


event.emit('key2', 'world')
event.emit('key2', 'world')
event.emit('key2', 'world')