简单的EventEmit(发布订阅者模式)

/**
 * simple eventEmit
 * @type {Object}
 */
export default class EventEmit {
  constructor () {
    this.handlers = {}
  }
  on (key, fn) {
    if (!this.handlers[key]) {
      this.handlers[key] = []
    }
    this.handlers[key].push(fn)
  }
  emit (...args) {
    if (!args || !args.length) return
    let key = args.shift()
    let fns = this.handlers[key]
    fns.forEach(fn => fn.apply(this, args))
  }
  off (key, fn) {
    let handlers = this.handlers[key]
    if (!handlers) return
    if (!fn) {
      handlers && (handlers.length=0)
    } else {
      const index = handlers.findIndex(item => item === fn)
      index && handlers.splice(index, 1)
    }
  }
}