深入解析vue响应式原理

摘要：本文主要通过结合vue官方文档及源码，对vue响应式原理进行深入分析。

1.定义

作为vue最独特的特性，响应式可以说是vue的灵魂了，表面上看就是数据发生变化后，对应的界面会重新渲染，那么响应式系统的底层细节到底是怎么一回事呢？

Tips:vue的响应式系统在vue2.0和vue3.0版本中的底层实现有所不同，简单了来说就是处理属性的getter/setter部分从Object.defineProperty替换成了Proxy(不过vue3也保留了Object.defineProperty方式用于支持IE浏览器)

1.1.vue2.0实现原理

当一个普通的javascript对象传入vue实例作为data选项时，vue将遍历data的所有属性，并使用Object.defineProperty重写这些属性的getter/setter方法,这些属性的getter/setter对于用户不可见，但是vue可以利用他们来追踪依赖，在属性值被访问和修改时通知变更。每个组件实例都对应一个watcher实例，它会在组件渲染的过程中访问过的属性设置为依赖。之后当属性的setter触发时，会通知watcher对关联的组件进行重新渲染。

1.2.vue3.0实现原理

当一个普通的javascript对象传入vue实例作为data选项时，vue会将其转化为Proxy。首次渲染后，组件将跟踪在渲染过程中被访问的属性，组件就成了这些属性的订阅者。当proxy拦截到set操作时，该属性将通知所有订阅了它的组件进行重新渲染。

2.源码解析

通过上面的定义可能对于响应式的原理还不够清楚，接下来通过对vue源码的分析进行深入理解。

2.1.vue2.0源码实现

在vue2.0中，vue的响应式系统是基于数据拦截+发布订阅的模式，包含了四个模块：

1. Observer：通过Object.defineProperty拦截data属性的setter/getter方法，从而使每个属性都拥有一个Dep，当触发getter时收集依赖（使用该属性的watcher），当触发setter时通知更新；
2. Dep：依赖收集器，用于维护依赖data属性的所有Watcher；
3. Watcher：将视图依赖的属性绑定到Dep中，当数据修改时触发setter，调用Dep的notify方法，通知所有依赖该属性的Watcher进行update更新视图，使属性值与视图绑定起来；
4. Compile：模板指令解析器，对模板每个元素节点的指令进行扫描解析，根据指令模板替换属性数据，同时注入Watcher更新数据的回调方法。

• Observer

import Dep from './dep'
import VNode from '../vdom/vnode'
import { arrayMethods } from './array'
import {
  def,
  warn,
  hasOwn,
  hasProto,
  isObject,
  isPlainObject,
  isPrimitive,
  isUndef,
  isValidArrayIndex,
  isServerRendering
} from '../util/index'

const arrayKeys = Object.getOwnPropertyNames(arrayMethods)

/**
 * In some cases we may want to disable observation inside a component's
 * update computation.
 */
export let shouldObserve: boolean = true

export function toggleObserving (value: boolean) {
  shouldObserve = value
}

/**
 * Observer class that is attached to each observed
 * object. Once attached, the observer converts the target
 * object's property keys into getter/setters that
 * collect dependencies and dispatch updates.
 */
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.vmCount = 0
    def(value, '__ob__', this)
    if (Array.isArray(value)) {
      if (hasProto) {
        protoAugment(value, arrayMethods)
      } else {
        copyAugment(value, arrayMethods, arrayKeys)
      }
      this.observeArray(value)
    } else {
      this.walk(value)
    }
  }

  /**
   * Walk through all properties and convert them into
   * getter/setters. This method should only be called when
   * value type is Object.
   */
  walk (obj: Object) {
    const keys = Object.keys(obj)
    for (let i = 0; i < keys.length; i++) {
      defineReactive(obj, keys[i])
    }
  }

  /**
   * Observe a list of Array items.
   */
  observeArray (items: Array) {
    for (let i = 0, l = items.length; i < l; i++) {
      observe(items[i])
    }
  }
}

// helpers

/**
 * Augment a target Object or Array by intercepting
 * the prototype chain using __proto__
 */
function protoAugment (target, src: Object) {
  /* eslint-disable no-proto */
  target.__proto__ = src
  /* eslint-enable no-proto */
}

/**
 * Augment a target Object or Array by defining
 * hidden properties.
 */
/* istanbul ignore next */
function copyAugment (target: Object, src: Object, keys: Array) {
  for (let i = 0, l = keys.length; i < l; i++) {
    const key = keys[i]
    def(target, key, src[key])
  }
}

/**
 * Attempt to create an observer instance for a value,
 * returns the new observer if successfully observed,
 * or the existing observer if the value already has one.
 */
export function observe (value: any, asRootData: ?boolean): Observer | void {
  if (!isObject(value) || value instanceof VNode) {
    return
  }
  let ob: Observer | void
  if (hasOwn(value, '__ob__') && value.__ob__ instanceof Observer) {
    ob = value.__ob__
  } else if (
    shouldObserve &&
    !isServerRendering() &&
    (Array.isArray(value) || isPlainObject(value)) &&
    Object.isExtensible(value) &&
    !value._isVue
  ) {
    ob = new Observer(value)
  }
  if (asRootData && ob) {
    ob.vmCount++
  }
  return ob
}

/**
 * Define a reactive property on an Object.
 */
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)
  Object.defineProperty(obj, key, {
    enumerable: true,
    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
      }
      /* eslint-enable no-self-compare */
      if (process.env.NODE_ENV !== 'production' && customSetter) {
        customSetter()
      }
      // #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()
    }
  })
}

/**
 * Set a property on an object. Adds the new property and
 * triggers change notification if the property doesn't
 * already exist.
 */
export function set (target: Array | Object, key: any, val: any): any {
  if (process.env.NODE_ENV !== 'production' &&
    (isUndef(target) || isPrimitive(target))
  ) {
    warn(`Cannot set reactive property on undefined, null, or primitive value: ${(target: any)}`)
  }
  if (Array.isArray(target) && isValidArrayIndex(key)) {
    target.length = Math.max(target.length, key)
    target.splice(key, 1, val)
    return val
  }
  if (key in target && !(key in Object.prototype)) {
    target[key] = val
    return val
  }
  const ob = (target: any).__ob__
  if (target._isVue || (ob && ob.vmCount)) {
    process.env.NODE_ENV !== 'production' && warn(
      'Avoid adding reactive properties to a Vue instance or its root $data ' +
      'at runtime - declare it upfront in the data option.'
    )
    return val
  }
  if (!ob) {
    target[key] = val
    return val
  }
  defineReactive(ob.value, key, val)
  ob.dep.notify()
  return val
}

/**
 * Delete a property and trigger change if necessary.
 */
export function del (target: Array | Object, key: any) {
  if (process.env.NODE_ENV !== 'production' &&
    (isUndef(target) || isPrimitive(target))
  ) {
    warn(`Cannot delete reactive property on undefined, null, or primitive value: ${(target: any)}`)
  }
  if (Array.isArray(target) && isValidArrayIndex(key)) {
    target.splice(key, 1)
    return
  }
  const ob = (target: any).__ob__
  if (target._isVue || (ob && ob.vmCount)) {
    process.env.NODE_ENV !== 'production' && warn(
      'Avoid deleting properties on a Vue instance or its root $data ' +
      '- just set it to null.'
    )
    return
  }
  if (!hasOwn(target, key)) {
    return
  }
  delete target[key]
  if (!ob) {
    return
  }
  ob.dep.notify()
}

/**
 * Collect dependencies on array elements when the array is touched, since
 * we cannot intercept array element access like property getters.
 */
function dependArray (value: Array) {
  for (let e, i = 0, l = value.length; i < l; i++) {
    e = value[i]
    e && e.__ob__ && e.__ob__.dep.depend()
    if (Array.isArray(e)) {
      dependArray(e)
    }
  }
}

总结：Observer通过重写data上各个属性的setter/getter方法，对每个属性都维护一个Dep，用于收集依赖该属性的所有Watcher，当该属性触发setter时，派发更新的通知。

• Dep

import type Watcher from './watcher'
import { remove } from '../util/index'
import config from '../config'

let uid = 0

/**
 * A dep is an observable that can have multiple
 * directives subscribing to it.
 */
export default class Dep {
  static target: ?Watcher;
  id: number;
  subs: Array;

  constructor () {
    this.id = uid++
    this.subs = []
  }

  addSub (sub: Watcher) {
    this.subs.push(sub)
  }

  removeSub (sub: Watcher) {
    remove(this.subs, sub)
  }

  depend () {
    if (Dep.target) {
      Dep.target.addDep(this)
    }
  }

  notify () {
    // stabilize the subscriber list first
    const subs = this.subs.slice()
    if (process.env.NODE_ENV !== 'production' && !config.async) {
      // subs aren't sorted in scheduler if not running async
      // we need to sort them now to make sure they fire in correct
      // order
      subs.sort((a, b) => a.id - b.id)
    }
    for (let i = 0, l = subs.length; i < l; i++) {
      subs[i].update()
    }
  }
}

// The current target watcher being evaluated.
// This is globally unique because only one watcher
// can be evaluated at a time.
Dep.target = null
const targetStack = []

export function pushTarget (target: ?Watcher) {
  targetStack.push(target)
  Dep.target = target
}

export function popTarget () {
  targetStack.pop()
  Dep.target = targetStack[targetStack.length - 1]
}

总结：Dep一方面用数组收集与属性相关的Watcher，另一方面遍历数组通知每个Watcher进行update。

• Watcher

import {
  warn,
  remove,
  isObject,
  parsePath,
  _Set as Set,
  handleError,
  noop
} from '../util/index'

import { traverse } from './traverse'
import { queueWatcher } from './scheduler'
import Dep, { pushTarget, popTarget } from './dep'

import type { SimpleSet } from '../util/index'

let uid = 0

/**
 * A watcher parses an expression, collects dependencies,
 * and fires callback when the expression value changes.
 * This is used for both the $watch() api and directives.
 */
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;
  newDeps: Array;
  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)
        }
      }
    }
  }

  /**
   * Evaluate the value of the watcher.
   * This only gets called for lazy watchers.
   */
  evaluate () {
    this.value = this.get()
    this.dirty = false
  }

  /**
   * Depend on all deps collected by this watcher.
   */
  depend () {
    let i = this.deps.length
    while (i--) {
      this.deps[i].depend()
    }
  }

  /**
   * Remove self from all dependencies' subscriber list.
   */
  teardown () {
    if (this.active) {
      // remove self from vm's watcher list
      // this is a somewhat expensive operation so we skip it
      // if the vm is being destroyed.
      if (!this.vm._isBeingDestroyed) {
        remove(this.vm._watchers, this)
      }
      let i = this.deps.length
      while (i--) {
        this.deps[i].removeSub(this)
      }
      this.active = false
    }
  }
}

总结：Watcher是一个依赖于数据的订阅者，当数据发生变化时，Dep调用notify方法，触发这些Watcher的update方法。

• Compile

class Complie {
  constructor(el, vm) {
    this.el = this.isElementNode(el) ? el : document.querySelector(el)
    this.vm = vm

    if (this.el) {
      // 1.获取文档碎片对象，放入内存中，会减少页面的回流和重绘
      let fragment = this.nodeFragment(this.el)

      // 2.编译模板
      this.complie(fragment)

      // 3.追加子元素到根元素上
      this.el.appendChild(fragment)
    }
  }

  isElementNode(node) {
    return node.nodeType === 1
  }
  nodeFragment(el) {
    el.firstChild
    // 创建一个内存碎片对象
    const fragment = document.createDocumentFragment()
    let firstChild
    while ((firstChild = el.firstChild)) {
      fragment.appendChild(firstChild)
    }
    return fragment
  }

  // 遍历获取并区分元素节点还是文本节点，然后进行相应的处理
  complie(fragment) {
    // 1.获取到每个子节点
    const childNodes = fragment.childNodes
    childNodes.forEach(child => {
      if (this.isElementNode(child)) {
        // 是元素节点
        // 编译元素节点
        this.complieElement(child)
      } else {
        // 文本节点
        // 编译文本节点
        this.complieText(child)
      }

      if (child.childNodes && child.childNodes.length) {
        this.complie(child)
      }
    })
  }
  // 编译元素
  complieElement(node) {
    const attributes = node.attributes
    Array.from(attributes).forEach(attr => {
      const { name, value } = attr
      if (this.isDirective(name)) {
        // 表明是一个指令
        const [, dirctive] = name.split('-')
        const [dirName, eventName] = dirctive.split(':') // text html model on

        /*
          node(整个节点)
          value(msg)
          this.vm(相当于整个 MVue实例对象)
          eventName(v-on:click='btnClick') 中的事件名btnClick
        */
        // 更新数据 数据驱动视图
        compileUtil[dirName](node, value, this.vm, eventName)

        // 删除有指令的标签上的属性
        node.removeAttribute('v-' + dirctive)
      } else if (this.isEventName(name)) {
        // @click='handleClick'
        let [, eventName] = name.split('@')
        compileUtil['on'](node, value, this.vm, eventName)
      }
    })
  }
  isEventName(eventName) {
    return eventName.startsWith('@')
  }

  // 检测字符串是否以 v- 开头
  isDirective(attrName) {
    return attrName.startsWith('v-')
  }

  // 编译文本
  complieText(node) {
    const content = node.textContent
    if (/\{\{(.+?)\}\}/.test(content)) {
      compileUtil['text'](node, content, this.vm)
    }
  }
}

blockquote { border-left: 3px solid rgba(128, 128, 128, 1) }
p { line-height: 1.8 }
h3 { margin-left: 0 !important; padding-left: 0 !important }
pre { color: rgba(255, 250, 250, 1); font-family: "Courier New"; background-color: rgba(45, 45, 45, 1); border: 1px solid rgba(128, 128, 128, 1) }