javascript观察者模式简述

观察者模式又称发布订阅模式publish-subscribe，是存在普遍存在与js和大多数语言的一种消息机制。js的事件机制就是发布订阅模式，从而实现异步。

In software architecture, publish–subscribe is a messaging pattern where senders of messages, called publishers, do not program the messages to be sent directly to specific receivers, called subscribers, but instead categorize published messages into classes without knowledge of which subscribers, if any, there may be.

简单的实现如下

const _ = window._ //lodash
let Event = (function () {
  let p = {}
  let usid = -1
  let topics = {}
  p.publish = function (topic, args) {//发布
    if (!topics[topic]) { return }
    let sub = topics[topic]
    for (let i = 0; i < sub.length; i++) {
      sub[i].func(args, topic)
    }
    return this
  }
  p.subscribe = function (topic, func) {//订阅
    topics[topic] = topics[topic] || []
    let token = ++usid
    let sub = topics[topic]
    sub.push({
      token,
      func
    })
    return token
  }
  p.unsubscribe = function (topic, token) {//取消订阅
    if (!topics[topic]) { return }
    let sub = topics[topic]
    _.remove(sub, (n) => n.token === token)
    if (sub.length === 0) {
      delete topics[topic]
    }
    return this
  }
  return p
})()

export default Event