引言
vue 源码的响应式原理部分应该是比较重要的部分，无论是在我们学习的时候还是工作面试的时候都是比较中要的的一个点，这文章将重点归纳vue2.x响应式源码分析
源码部分
上篇文章分析到响应式原理的入口是initState(vm)
复制代码initState
src/core/instance/state.js
* 两件事：
* 数据响应式的入口：分别处理 props、methods、data、computed、watch
* 优先级：props、methods、data、computed 对象中的属性不能出现重复，优先级出顺序一致
* 其中 computed 中的 key 不能和 props、data 中的 key 重复，methods 不影响
*/
export function initState (vm: Component) {
vm._watchers = []
// 处理 props 对象，为 props 对象的每个属性设置响应式，并将其代理到 vm 实例上
const opts = vm.$options
if (opts.props) initProps(vm, opts.props)
// 处理 methods 对象，校验每个属性的值是否为函数、和 props 属性比对进行判重处理，最后得到 vm[key] = methods[key]
if (opts.methods) initMethods(vm, opts.methods)
/**
* 做了三件事
* 1、判重处理，data 对象上的属性不能和 props、methods 对象上的属性相同
* 2、代理 data 对象上的属性到 vm 实例
* 3、为 data 对象的上数据设置响应式
*/
if (opts.data) {
initData(vm)
} else {
observe(vm._data = {}, true /* asRootData */)
}
/**
* 三件事：
* 1、为 computed[key] 创建 watcher 实例，默认是懒执行
* 2、代理 computed[key] 到 vm 实例
* 3、判重，computed 中的 key 不能和 data、props 中的属性重复
*/
if (opts.computed) initComputed(vm, opts.computed)
/**
* 三件事：
* 1、处理 watch 对象
* 2、为 每个 watch.key 创建 watcher 实例，key 和 watcher 实例可能是 一对多 的关系
* 3、如果设置了 immediate，则立即执行 回调函数
*/
if (opts.watch && opts.watch !== nativeWatch) {
initWatch(vm, opts.watch)
}
}
// 处理 props 对象，为 props 对象的每个属性设置响应式，并将其代理到 vm 实例上
function initProps (vm: Component, propsOptions: Object) {
const propsData = vm.$options.propsData || {}
const props = vm._props = {}
// 缓存 props 的每个 key，性能优化
// cache prop keys so that future props updates can iterate using Array
// instead of dynamic object key enumeration.
const keys = vm.$options._propKeys = []
const isRoot = !vm.$parent
// root instance props should be converted
if (!isRoot) {
toggleObserving(false)
}
for (const key in propsOptions) {
keys.push(key)
// 获取 props[key] 的默认值
const value = validateProp(key, propsOptions, propsData, vm)
/* istanbul ignore else */
if (process.env.NODE_ENV !== 'production') {
const hyphenatedKey = hyphenate(key)
if (isReservedAttribute(hyphenatedKey) ||
config.isReservedAttr(hyphenatedKey)) {
warn(
`"${hyphenatedKey}" is a reserved attribute and cannot be used as component prop.`,
vm
)
}
defineReactive(props, key, value, () => {
if (!isRoot && !isUpdatingChildComponent) {
warn(
`Avoid mutating a prop directly since the value will be ` +
`overwritten whenever the parent component re-renders. ` +
`Instead, use a data or computed property based on the prop's ` +
`value. Prop being mutated: "${key}"`,
vm
)
}
})
} else {
defineReactive(props, key, value)
}
// static props are already proxied on the component's prototype
// during Vue.extend(). We only need to proxy props defined at
// instantiation here.
if (!(key in vm)) {
// 将propsKey代理到vm实例上
proxy(vm, `_props`, key)
}
}
toggleObserving(true)
}
复制代码initProps
src/core/instance/state.js
// 处理 props 对象，为 props 对象的每个属性设置响应式，并将其代理到 vm 实例上
function initProps (vm: Component, propsOptions: Object) {
const propsData = vm.$options.propsData || {}
const props = vm._props = {}
// 缓存 props 的每个 key，性能优化
// cache prop keys so that future props updates can iterate using Array
// instead of dynamic object key enumeration.
const keys = vm.$options._propKeys = []
const isRoot = !vm.$parent
// root instance props should be converted
if (!isRoot) {
toggleObserving(false)
}
for (const key in propsOptions) {
keys.push(key)
// 获取 props[key] 的默认值
const value = validateProp(key, propsOptions, propsData, vm)
/* istanbul ignore else */
if (process.env.NODE_ENV !== 'production') {
const hyphenatedKey = hyphenate(key)
if (isReservedAttribute(hyphenatedKey) ||
config.isReservedAttr(hyphenatedKey)) {
warn(
`"${hyphenatedKey}" is a reserved attribute and cannot be used as component prop.`,
vm
)
}
defineReactive(props, key, value, () => {
if (!isRoot && !isUpdatingChildComponent) {
warn(
`Avoid mutating a prop directly since the value will be ` +
`overwritten whenever the parent component re-renders. ` +
`Instead, use a data or computed property based on the prop's ` +
`value. Prop being mutated: "${key}"`,
vm
)
}
})
} else {
defineReactive(props, key, value)
}
// static props are already proxied on the component's prototype
// during Vue.extend(). We only need to proxy props defined at
// instantiation here.
if (!(key in vm)) {
// 将propsKey代理到vm实例上
proxy(vm, `_props`, key)
}
}
toggleObserving(true)
}
复制代码proxy
src/core/instance/state.js
/**
* 通过Object.defineProperty将key代理到target上
* 面试容易问到例如为什么我们在vue中可以同this.xxx访问到props,data的数据呢
*/
export function proxy (target: Object, sourceKey: string, key: string) {
sharedPropertyDefinition.get = function proxyGetter () {
return this[sourceKey][key]
}
sharedPropertyDefinition.set = function proxySetter (val) {
this[sourceKey][key] = val
}
Object.defineProperty(target, key, sharedPropertyDefinition)
}
复制代码initMethods
src/core/instance/state.js
* 做了以下三件事，其实最关键的就是第三件事情
* 1、校验 methoss[key]，必须是一个函数
* 2、判重
* methods 中的 key 不能和 props 中的 key 相同
* methos 中的 key 与 Vue 实例上已有的方法重叠，一般是一些内置方法，比如以 $ 和 _ 开头的方法
* 3、将 methods[key] 放到 vm 实例上，得到 vm[key] = methods[key]
*/
function initMethods (vm: Component, methods: Object) {
const props = vm.$options.props
for (const key in methods) {
if (process.env.NODE_ENV !== 'production') {
if (typeof methods[key] !== 'function') {
warn(
`Method "${key}" has type "${typeof methods[key]}" in the component definition. ` +
`Did you reference the function correctly?`,
vm
)
}
if (props && hasOwn(props, key)) {
warn(
`Method "${key}" has already been defined as a prop.`,
vm
)
}
if ((key in vm) && isReserved(key)) {
warn(
`Method "${key}" conflicts with an existing Vue instance method. ` +
`Avoid defining component methods that start with _ or $.`
)
}
}
vm[key] = typeof methods[key] !== 'function' ? noop : bind(methods[key], vm)
}
}
复制代码initData
src/core/instance/state.js
/**
* 做了三件事
* 1、判重处理，data 对象上的属性不能和 props、methods 对象上的属性相同
* 2、代理 data 对象上的属性到 vm 实例
* 3、为 data 对象的上数据设置响应式
*/
function initData (vm: Component) {
let data = vm.$options.data
data = vm._data = typeof data === 'function'
? getData(data, vm)
: data || {}
if (!isPlainObject(data)) {
data = {}
process.env.NODE_ENV !== 'production' && warn(
'data functions should return an object:\n' +
'https://vuejs.org/v2/guide/components.html#data-Must-Be-a-Function',
vm
)
}
// proxy data on instance
const keys = Object.keys(data)
const props = vm.$options.props
const methods = vm.$options.methods
let i = keys.length
while (i--) {
const key = keys[i]
if (process.env.NODE_ENV !== 'production') {
if (methods && hasOwn(methods, key)) {
warn(
`Method "${key}" has already been defined as a data property.`,
vm
)
}
}
if (props && hasOwn(props, key)) {
process.env.NODE_ENV !== 'production' && warn(
`The data property "${key}" is already declared as a prop. ` +
`Use prop default value instead.`,
vm
)
} else if (!isReserved(key)) {
proxy(vm, `_data`, key)
}
}
// observe data
// 为data对象上的数据设置为响应式
observe(data, true /* asRootData */)
}
复制代码initComputed
src/core/instance/state.js
* 三件事：
* 1、为 computed[key] 创建 watcher 实例，默认是懒执行
* 2、代理 computed[key] 到 vm 实例
* 3、判重，computed 中的 key 不能和 data、props 中的属性重复
* @param {*} computed = {
* key1: function() { return xx },
* key2: {
* get: function() { return xx },
* set: function(val) {}
* }
* }
*/
function initComputed (vm: Component, computed: Object) {
// $flow-disable-line
const watchers = vm._computedWatchers = Object.create(null)
// computed properties are just getters during SSR
const isSSR = isServerRendering()
for (const key in computed) {
const userDef = computed[key]
const getter = typeof userDef === 'function' ? userDef : userDef.get
if (process.env.NODE_ENV !== 'production' && getter == null) {
warn(
`Getter is missing for computed property "${key}".`,
vm
)
}
if (!isSSR) {
// create internal watcher for the computed property.
watchers[key] = new Watcher(
vm,
getter || noop,
noop,
computedWatcherOptions
)
}
// component-defined computed properties are already defined on the
// component prototype. We only need to define computed properties defined
// at instantiation here.
if (!(key in vm)) {
defineComputed(vm, key, userDef)
} else if (process.env.NODE_ENV !== 'production') {
if (key in vm.$data) {
warn(`The computed property "${key}" is already defined in data.`, vm)
} else if (vm.$options.props && key in vm.$options.props) {
warn(`The computed property "${key}" is already defined as a prop.`, vm)
} else if (vm.$options.methods && key in vm.$options.methods) {
warn(`The computed property "${key}" is already defined as a method.`, vm)
}
}
}
}
/**
* 代理 computed 对象中的 key 到 target（vm）上
*/
export function defineComputed (
target: any,
key: string,
userDef: Object | Function
) {
const shouldCache = !isServerRendering()
if (typeof userDef === 'function') {
sharedPropertyDefinition.get = shouldCache
? createComputedGetter(key)
: createGetterInvoker(userDef)
sharedPropertyDefinition.set = noop
} else {
sharedPropertyDefinition.get = userDef.get
? shouldCache && userDef.cache !== false
? createComputedGetter(key)
: createGetterInvoker(userDef.get)
: noop
sharedPropertyDefinition.set = userDef.set || noop
}
if (process.env.NODE_ENV !== 'production' &&
sharedPropertyDefinition.set === noop) {
sharedPropertyDefinition.set = function () {
warn(
`Computed property "${key}" was assigned to but it has no setter.`,
this
)
}
}
Object.defineProperty(target, key, sharedPropertyDefinition)
}
/**
* @returns 返回一个函数，这个函数在访问 vm.computedProperty 时会被执行，然后返回执行结果
*/
function createComputedGetter (key) {
return function computedGetter () {
const watcher = this._computedWatchers && this._computedWatchers[key]
if (watcher) {
if (watcher.dirty) {
watcher.evaluate()
}
if (Dep.target) {
watcher.depend()
}
return watcher.value
}
}
}
复制代码initWatch
src/core/instance/state.js
* 处理 watch 对象的入口，做了两件事：
* 1、遍历 watch 对象
* 2、调用 createWatcher 函数
* @param {*} watch = {
* 'key1': function(val, oldVal) {},
* 'key2': 'this.methodName',
* 'key3': {
* handler: function(val, oldVal) {},
* deep: true
* },
* 'key4': [
* 'this.methodNanme',
* function handler1() {},
* {
* handler: function() {},
* immediate: true
* }
* ],
* 'key.key5' { ... }
* }
*/
function initWatch (vm: Component, watch: Object) {
for (const key in watch) {
const handler = watch[key]
if (Array.isArray(handler)) {
for (let i = 0; i < handler.length; i++) {
createWatcher(vm, key, handler[i])
}
} else {
createWatcher(vm, key, handler)
}
}
}
/**
* 两件事：
* 1、兼容性处理，保证 handler 肯定是一个函数
* 2、调用 $watch
* @returns
*/
function createWatcher (
vm: Component,
expOrFn: string | Function,
handler: any,
options?: Object
) {
if (isPlainObject(handler)) {
options = handler
handler = handler.handler
}
if (typeof handler === 'string') {
handler = vm[handler]
}
return vm.$watch(expOrFn, handler, options)
}
export function stateMixin (Vue: Class<Component>) {
// flow somehow has problems with directly declared definition object
// when using Object.defineProperty, so we have to procedurally build up
// the object here.
const dataDef = {}
dataDef.get = function () { return this._data }
const propsDef = {}
propsDef.get = function () { return this._props }
if (process.env.NODE_ENV !== 'production') {
dataDef.set = function () {
warn(
'Avoid replacing instance root $data. ' +
'Use nested data properties instead.',
this
)
}
propsDef.set = function () {
warn(`$props is readonly.`, this)
}
}
Object.defineProperty(Vue.prototype, '$data', dataDef)
Object.defineProperty(Vue.prototype, '$props', propsDef)
Vue.prototype.$set = set
Vue.prototype.$delete = del
/**
* 创建 watcher，返回 unwatch，共完成如下 5 件事：
* 1、兼容性处理，保证最后 new Watcher 时的 cb 为函数
* 2、标示用户 watcher
* 3、创建 watcher 实例
* 4、如果设置了 immediate，则立即执行一次 cb
* 5、返回 unwatch
* @param {*} expOrFn key
* @param {*} cb 回调函数
* @param {*} options 配置项，用户直接调用 this.$watch 时可能会传递一个 配置项
* @returns 返回 unwatch 函数，用于取消 watch 监听
*/
Vue.prototype.$watch = function (
expOrFn: string | Function,
cb: any,
options?: Object
): Function {
const vm: Component = this
if (isPlainObject(cb)) {
return createWatcher(vm, expOrFn, cb, options)
}
options = options || {}
options.user = true
const watcher = new Watcher(vm, expOrFn, cb, options)
if (options.immediate) {
const info = `callback for immediate watcher "${watcher.expression}"`
pushTarget()
invokeWithErrorHandling(cb, vm, [watcher.value], vm, info)
popTarget()
}
return function unwatchFn () {
watcher.teardown()
}
}
}
复制代码observer
src/core/observer/index.js
在上面initData中调用了observe(data, true /* asRootData */)然后我们可以进入到一下函数
/**
* 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.
* * 响应式处理的真正入口
* 为对象创建观察者实例，如果对象已经被观察过，则返回已有的观察者实例，否则创建新的观察者实例
* @param {*} value 对象 => {}
*/
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
) {
// 创建观察者实例
ob = new Observer(value)
}
if (asRootData && ob) {
ob.vmCount++
}
return ob
}
复制代码Observer
src/core/Observer/index.js
/**
* 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.
* * 观察者类，会被附加到每个被观察的对象上，value.__ob__ = this
* 而对象的各个属性则会被转换成 getter/setter，并收集依赖和通知更新
*/
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
// 实例化一个Dep
this.dep = new Dep()
this.vmCount = 0
def(value, '__ob__', this)
/**
* value 为数组
* hasProto = '__proto__' in {}
* 用于判断对象是否存在 __proto__ 属性，通过 obj.__proto__ 可以访问对象的原型链
* 但由于 __proto__ 不是标准属性，所以有些浏览器不支持，比如 IE6-10，Opera10.1
* 为什么要判断，是因为一会儿要通过 __proto__ 操作数据的原型链
* 覆盖数组默认的七个原型方法，以实现数组响应式
*/
if (Array.isArray(value)) {
if (hasProto) {
protoAugment(value, arrayMethods)
} else {
copyAugment(value, arrayMethods, arrayKeys)
}
this.observeArray(value)
} else {
// value 为对象，为对象的每个属性（包括嵌套对象）设置响应式
this.walk(value)
}
}
/**
* Walk through all properties and convert them into
* getter/setters. This method should only be called when
* value type is Object.
* * 遍历对象上的每个 key，为每个 key 设置响应式
* 仅当值为对象时才会走这里
*/
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.
* * 遍历数组，为数组的每一项设置观察，处理数组元素为对象的情况
*
*/
observeArray (items: Array<any>) {
for (let i = 0, l = items.length; i < l; i++) {
observe(items[i])
}
}
}
复制代码defineReactive
src/core/observer/index.js
* Define a reactive property on an Object.
* * 拦截 obj[key] 的读取和设置操作：
* 1、在第一次读取时收集依赖，比如执行 render 函数生成虚拟 DOM 时会有读取操作
* 2、在更新时设置新值并通知依赖更新
*/
export function defineReactive (
obj: Object,
key: string,
val: any,
customSetter?: ?Function,
shallow?: boolean
) {
// 实例化 dep，一个 key 一个 dep
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,
/**
* Dep.target 为 Dep 类的一个静态属性，值为 watcher，在实例化 Watcher 时会被设置
* 实例化 Watcher 时会执行 new Watcher 时传递的回调函数（computed 除外，因为它懒执行）
* 而回调函数中如果有 vm.key 的读取行为，则会触发这里的 读取 拦截，进行依赖收集
* 回调函数执行完以后又会将 Dep.target 设置为 null，避免这里重复收集依赖
*/
get: function reactiveGetter () {
const value = getter ? getter.call(obj) : val
if (Dep.target) {
// 依赖收集，在 dep 中添加 watcher，也在 watcher 中添加 dep
dep.depend()
if (childOb) {
// childOb 表示对象中嵌套对象的观察者对象，如果存在也对其进行依赖收集
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
}
// 对新值进行观察，让新值也是响应式的
childOb = !shallow && observe(newVal)
// 依赖通知更新
dep.notify()
}
})
}
复制代码dependArray
src/core/observer/index.js
* Collect dependencies on array elements when the array is touched, since
* we cannot intercept array element access like property getters.
* * 遍历每个数组元素，递归处理数组项为对象的情况，为其添加依赖
* 因为前面的递归阶段无法为数组中的对象元素添加依赖
*/
function dependArray (value: Array<any>) {
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)
}
}
}
复制代码数组响应式
src/core/observer/array.js
* not type checking this file because flow doesn't play well with
* dynamically accessing methods on Array prototype
* * 定义 arrayMethods 对象，用于增强 Array.prototype
* 当访问 arrayMethods 对象上的那七个方法时会被拦截，以实现数组响应式
*/
import { def } from '../util/index'
const arrayProto = Array.prototype
export const arrayMethods = Object.create(arrayProto)
const methodsToPatch = [
'push',
'pop',
'shift',
'unshift',
'splice',
'sort',
'reverse'
]
/**
* Intercept mutating methods and emit events
*/
methodsToPatch.forEach(function (method) {
// cache original method
const original = arrayProto[method]
def(arrayMethods, method, function mutator (...args) {
const result = original.apply(this, args)
const ob = this.__ob__
let inserted
switch (method) {
case 'push':
case 'unshift':
inserted = args
break
case 'splice':
inserted = args.slice(2)
break
}
if (inserted) ob.observeArray(inserted)
// notify change
ob.dep.notify()
return result
})
})
复制代码Dep
src/core/observer
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.
* * 一个 dep 对应一个 obj.key
* 在读取响应式数据时，负责收集依赖，每个 dep（或者说 obj.key）依赖的 watcher 有哪些
* 在响应式数据更新时，负责通知 dep 中那些 watcher 去执行 update 方法
*/
export default class Dep {
static target: ?Watcher;
id: number;
subs: Array<Watcher>;
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.
/**
* 当前正在执行的 watcher，同一时间只会有一个 watcher 在执行
* Dep.target = 当前正在执行的 watcher
* 通过调用 pushTarget 方法完成赋值，调用 popTarget 方法完成重置（null)
*/
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]
}
复制代码watcher
src/core/observer/watcher.js
import {
warn,
remove,
isObject,
parsePath,
_Set as Set,
handleError,
invokeWithErrorHandling,
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 $watch() api and directives.
* * 一个组件一个 watcher（渲染 watcher）或者一个表达式一个 watcher（用户watcher）
* 当数据更新时 watcher 会被触发，访问 this.computedProperty 时也会触发 watcher
*/
export default class Watcher {
vm: Component;
expression: string;
cb: Function;
id: number;
deep: boolean;
user: boolean;
lazy: boolean;
sync: boolean;
dirty: boolean;
active: boolean;
deps: Array<Dep>;
newDeps: Array<Dep>;
depIds: SimpleSet;
newDepIds: SimpleSet;
before: ?Function;
getter: Function;
value: any;
constructor (
vm: Component,
expOrFn: string | Function,
cb: Function,
options?: ?Object,
isRenderWatcher?: boolean
) {
this.vm = vm
if (isRenderWatcher) {
vm._watcher = this
}
vm._watchers.push(this)
// options
if (options) {
this.deep = !!options.deep
this.user = !!options.user
this.lazy = !!options.lazy
this.sync = !!options.sync
this.before = options.before
} else {
this.deep = this.user = this.lazy = this.sync = false
}
this.cb = cb
this.id = ++uid // uid for batching
this.active = true
this.dirty = this.lazy // for lazy watchers
this.deps = []
this.newDeps = []
this.depIds = new Set()
this.newDepIds = new Set()
this.expression = process.env.NODE_ENV !== 'production'
? expOrFn.toString()
: ''
// parse expression for getter
if (typeof expOrFn === 'function') {
this.getter = expOrFn
} else {
this.getter = parsePath(expOrFn)
if (!this.getter) {
this.getter = noop
process.env.NODE_ENV !== 'production' && warn(
`Failed watching path: "${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.
* 执行 this.getter，并重新收集依赖
* this.getter 是实例化 watcher 时传递的第二个参数，一个函数或者字符串，比如：updateComponent 或者 parsePath 返回的读取 this.xx 属性值的函数
* 为什么要重新收集依赖？
* 因为触发更新说明有响应式数据被更新了，但是被更新的数据虽然已经经过 observe 观察了，但是却没有进行依赖收集，
* 所以，在更新页面时，会重新执行一次 render 函数，执行期间会触发读取操作，这时候进行依赖收集
*/
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 "${this.expression}"`)
} else {
throw e
}
} finally {
// "touch" every property so they are all tracked as
// dependencies for deep watching
if (this.deep) {
traverse(value)
}
popTarget()
this.cleanupDeps()
}
return value
}
/**
* Add a dependency to this directive.
* * 两件事：
* 1、添加 dep 给自己（watcher）
* 2、添加自己（watcher）到 dep
*/
addDep (dep: Dep) {
// 判重处理
const id = dep.id
if (!this.newDepIds.has(id)) {
this.newDepIds.add(id)
this.newDeps.push(dep)
if (!this.depIds.has(id)) {
dep.addSub(this)
}
}
}
/**
* Clean up for dependency collection.
*/
cleanupDeps () {
let i = this.deps.length
while (i--) {
const dep = this.deps[i]
if (!this.newDepIds.has(dep.id)) {
dep.removeSub(this)
}
}
let tmp = this.depIds
this.depIds = this.newDepIds
this.newDepIds = tmp
this.newDepIds.clear()
tmp = this.deps
this.deps = this.newDeps
this.newDeps = tmp
this.newDeps.length = 0
}
/**
* Subscriber interface.
* Will be called when a dependency changes.
* * 根据 watcher 配置项，决定接下来怎么走，一般是 queueWatcher
*/
update () {
/* istanbul ignore else */
if (this.lazy) {
// 懒执行时走这里，比如 computed
// 将 dirty 置为 true，可以让 computedGetter 执行时重新计算 computed 回调函数的执行结果
this.dirty = true
} else if (this.sync) {
// 同步执行，在使用 vm.$watch 或者 watch 选项时可以传一个 sync 选项，
// 当为 true 时在数据更新时该 watcher 就不走异步更新队列，直接执行 this.run
// 方法进行更新
// 这个属性在官方文档中没有出现
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) {
const info = `callback for watcher "${this.expression}"`
invokeWithErrorHandling(this.cb, this.vm, [value, oldValue], this.vm, info)
} else {
this.cb.call(this.vm, value, oldValue)
}
}
}
}
/**
* Evaluate the value of the watcher.
* This only gets called for lazy watchers.
* * 懒执行的 watcher 会调用该方法
* 比如：computed，在获取 vm.computedProperty 的值时会调用该方法
* 然后执行 this.get，即 watcher 的回调函数，得到返回值
* this.dirty 被置为 false，作用是页面在本次渲染中只会一次 computed.key 的回调函数，
* 这也是大家常说的 computed 和 methods 区别之一是 computed 有缓存的原理所在
* 而页面更新后会 this.dirty 会被重新置为 true，这一步是在 this.update 方法中完成的
*/
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
}
}
}
复制代码总结
关于Dep和Wathcer是比较难理解的点，vue2.x中每个组件对应一个渲染Watcher,然后在initComputed和initWatch中会生成用户Watcher,而Dep的话例如Data中的每个属性也会有一个Dep，Dep中存放了相关的Watcher,当解析模板的时候会进行依赖收集，调用Dep.depend(),向Dep中添加Wathcer,例如更改data中的某个值则调用Dep.notify(),通知Dep中的Watcher调用自身的update方法
还有一个就是比较常见的问题是watch和computed的区别，本质上它们没有什么区别，都是通过Watcher来实现的，至于使用场景，computed是适用于多对一的关系，例如一个值依赖多个值，当这多个值中的某个值发生变化时，这个值就会重新计算。watch是适用于一对多的关系，监听的某个值发生变化去触发相应的回调，也可以支持异步。
关于Vue响应式原理大概就是这些，平时多看多理解，多debugger打印，例如我们打印一个Vue实例，可以看到例如data中的值有对应一个Dep,打开Dep有Wathcher,类似的大家可以多观察一下。
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