（四）观察者模式｜小册免费学

一、观察者模式的概念

观察者模式（又叫发布-订阅模式），它定义对象间的一种一对多的依赖关系，当一个对象的状 态发生改变时，所有依赖于它的对象都将得到通知。

二、结合应用场景深入理解

不论是在程序世界里还是现实生活中，观察者模式（发布-订阅模式）的应用都非常之广泛。

2.1 现实生活中的观察者模式

我们先看一个现实中的例子，售楼买房的案例。售楼处是发布者，买房者是订阅者，监听订阅楼房售卖的消息，中间需要一个记录楼房情况的花名册。理清楚思绪后，我们可以用JavaScript来实现，具体代码如下：

// 发布-订阅功能
var event = {
  clientList: [],
  listen: function(key, fn){
    if(!this.clientList[key]){
       this.clientList[key] = [];
    }
    this.clientList[key].push(fn);// 订阅的消息添加进缓存列表
  },
  trigger: function(){
    var key = Array.prototype.shift.call(arguments),
          fns = this.clientList[key];
     if(!fns || !fns.length) {
          return false;
     }
     for(var i = 0, fn;fn = fns[i++];) {
         fn.apply(this, arguments);
     }
  },
  remove: function(key, fn){// 取消订阅
    var fns = this.clientList[key];
    if(!fns) return false;// 如果key对应的消息没有被订阅，则直接返回
    if(!fn){// 如果没有传入具体的回调函数，表示需要取消 key 对应消息的所有订阅
      fns && (fns.length = 0)
    }else{
      for(var l = fns.length-1;l>=0;l--){// 反向遍历订阅的回调函数列表
        var _fn = fns[l];
        if(_fn === fn){
          fns.splice(l,1);// 删除订阅者的回调函数
        }
      }
    }
  }
}

// 给订阅对象，安装发布-订阅功能
var installEvent = function(obj){
  for(var i in event) {
    obj[i] = event[i];
  }
}

以上，我们就完成了基本发布订阅模式的设计与实现，其中event对象中定义消息缓存队列 clientList、监听订阅函数 listen、发布函数 trigger、取消订阅函数remove。

2.2 Vue双向绑定的实现原理

Vue双向绑定原理（又称响应式原理）是高频的面试题，其内部实现也属于观察者模式。今天我们就来剖析看看吧，先来看张官网介绍响应式原理的流程图： 从上图，我们不难看出Vue双向绑定实现逻辑里有有三个关键角色：

• observer（监听器）：在Vue数据双向绑定的角色结构里，所谓的 observer 不仅是一个数据监听器，它还需要对监听到的数据进行转发——也就是说它同时还是一个发布者。
• watcher（订阅者）：observer 把数据转发给了真正的订阅者——watcher对象。Watcher订阅者就是Observe和Compile之间的通信桥梁，主要做的事情是：
  a、在自身实例化时往属性订阅器(dep)里面添加自；
  b、自身必须有一个update()方法；
  c、待属性变动dep.notice()通知时，能调用自身的update()方法，并触发Compile中绑定的回调，然后功成身退。
• compile（编译器）：MVVM 框架特有的角色，负责对每个节点元素指令进行扫描和解析，指令的数据初始化、订阅者的创建这些“杂活”也归它管~

源码中可以找到具体的实现：
Observer: src/core/observer/index.js

export class Observer {
  value: any;
  dep: Dep;
  vmCount: number; // number of vms that have this object as root $data

  constructor (value: any) {
    this.value = value
    this.dep = new Dep()
    ...
    this.walk(value)
  }
  
  // 遍历obj的属性，执行defineReactive
  walk (obj: Object) {
    const keys = Object.keys(obj)
    for (let i = 0; i < keys.length; i++) {
      defineReactive(obj, keys[i])
    }
  }
  ...
}

export function defineReactive (
  obj: Object,
  key: string,
  val: any,
  customSetter?: ?Function,
  shallow?: boolean
) {
  const dep = new Dep()

  const property = Object.getOwnPropertyDescriptor(obj, key)
  if (property && property.configurable === false) {
    return
  }

  // cater for pre-defined getter/setters
  const getter = property && property.get
  const setter = property && property.set
  if ((!getter || setter) && arguments.length === 2) {
    val = obj[key]
  }

  let childOb = !shallow && observe(val)
  // 为当前属性安装监听器
  Object.defineProperty(obj, key, {
    // 可枚举
    enumerable: true,
    // 不可配置
    configurable: true,
    get: function reactiveGetter () {
      const value = getter ? getter.call(obj) : val
      if (Dep.target) {
        dep.depend()
        if (childOb) {
          childOb.dep.depend()
          if (Array.isArray(value)) {
            dependArray(value)
          }
        }
      }
      return value
    },
    set: function reactiveSetter (newVal) {
      const value = getter ? getter.call(obj) : val
      /* eslint-disable no-self-compare */
      if (newVal === value || (newVal !== newVal && value !== value)) {
        return
      }
      ...
      // #7981: for accessor properties without setter
      if (getter && !setter) return
      if (setter) {
        setter.call(obj, newVal)
      } else {
        val = newVal
      }
      childOb = !shallow && observe(newVal)
      dep.notify()
    }
  })
}

Watcher： src/core/observer/watcher.js

export default class Watcher {
  vm: Component;
  expression: string;
  cb: Function;
  id: number;
  deep: boolean;
  user: boolean;
  lazy: boolean;
  sync: boolean;
  dirty: boolean;
  active: boolean;
  deps: Array<Dep>;
  newDeps: Array<Dep>;
  depIds: SimpleSet;
  newDepIds: SimpleSet;
  before: ?Function;
  getter: Function;
  value: any;

  constructor (
    vm: Component,
    expOrFn: string | Function,
    cb: Function,
    options?: ?Object,
    isRenderWatcher?: boolean
  ) {
    this.vm = vm
    if (isRenderWatcher) {
      vm._watcher = this
    }
    vm._watchers.push(this)
    // options
    if (options) {
      this.deep = !!options.deep
      this.user = !!options.user
      this.lazy = !!options.lazy
      this.sync = !!options.sync
      this.before = options.before
    } else {
      this.deep = this.user = this.lazy = this.sync = false
    }
    this.cb = cb
    this.id = ++uid // uid for batching
    this.active = true
    this.dirty = this.lazy // for lazy watchers
    this.deps = []
    this.newDeps = []
    this.depIds = new Set()
    this.newDepIds = new Set()
    this.expression = process.env.NODE_ENV !== 'production'
      ? expOrFn.toString()
      : ''
    // parse expression for getter
    if (typeof expOrFn === 'function') {
      this.getter = expOrFn
    } else {
      this.getter = parsePath(expOrFn)
      if (!this.getter) {
        this.getter = noop
        process.env.NODE_ENV !== 'production' && warn(
          `Failed watching path: "${expOrFn}" ` +
          'Watcher only accepts simple dot-delimited paths. ' +
          'For full control, use a function instead.',
          vm
        )
      }
    }
    this.value = this.lazy
      ? undefined
      : this.get()
  }

  /**
   * Evaluate the getter, and re-collect dependencies.
   */
  get () {
    pushTarget(this)
    let value
    const vm = this.vm
    try {
      value = this.getter.call(vm, vm)
    } catch (e) {
      if (this.user) {
        handleError(e, vm, `getter for watcher "${this.expression}"`)
      } else {
        throw e
      }
    } finally {
      // "touch" every property so they are all tracked as
      // dependencies for deep watching
      if (this.deep) {
        traverse(value)
      }
      popTarget()
      this.cleanupDeps()
    }
    return value
  }

  /**
   * Add a dependency to this directive.
   */
  addDep (dep: Dep) {
    const id = dep.id
    if (!this.newDepIds.has(id)) {
      this.newDepIds.add(id)
      this.newDeps.push(dep)
      if (!this.depIds.has(id)) {
        dep.addSub(this)
      }
    }
  }

  /**
   * Clean up for dependency collection.
   */
  cleanupDeps () {
    let i = this.deps.length
    while (i--) {
      const dep = this.deps[i]
      if (!this.newDepIds.has(dep.id)) {
        dep.removeSub(this)
      }
    }
    let tmp = this.depIds
    this.depIds = this.newDepIds
    this.newDepIds = tmp
    this.newDepIds.clear()
    tmp = this.deps
    this.deps = this.newDeps
    this.newDeps = tmp
    this.newDeps.length = 0
  }

  /**
   * Subscriber interface.
   * Will be called when a dependency changes.
   */
  update () {
    /* istanbul ignore else */
    if (this.lazy) {
      this.dirty = true
    } else if (this.sync) {
      this.run()
    } else {
      queueWatcher(this)
    }
  }

  /**
   * Scheduler job interface.
   * Will be called by the scheduler.
   */
  run () {
    if (this.active) {
      const value = this.get()
      if (
        value !== this.value ||
        // Deep watchers and watchers on Object/Arrays should fire even
        // when the value is the same, because the value may
        // have mutated.
        isObject(value) ||
        this.deep
      ) {
        // set new value
        const oldValue = this.value
        this.value = value
        if (this.user) {
          try {
            this.cb.call(this.vm, value, oldValue)
          } catch (e) {
            handleError(e, this.vm, `callback for watcher "${this.expression}"`)
          }
        } else {
          this.cb.call(this.vm, value, oldValue)
        }
      }
    }
  }
  
  /**
   * Depend on all deps collected by this watcher.
   */
  depend () {
    let i = this.deps.length
    while (i--) {
      this.deps[i].depend()
    }
  }
  ...
 
}

Compiler： src/compiler/index.js

export const createCompiler = createCompilerCreator(function baseCompile (
  template: string,
  options: CompilerOptions
): CompiledResult {
  // 1. parse 解析成 AST 抽象语法树
  const ast = parse(template.trim(), options)
  if (options.optimize !== false) {
    // 2. optimize 优化
    optimize(ast, options)
  }
  // 3. generate 代码生成
  const code = generate(ast, options)
  return {
    ast,
    render: code.render,
    staticRenderFns: code.staticRenderFns
  }
})

三、总结

观察者模式是JavaScript设计模式最为重要的模式，也是面试的高频考题。通过结合上述案例分析，相信大家可以充分理解观察者这一设计模式。

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