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。
一、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)
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。
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，将通知所有订阅了该属性的组件进行重新渲染。
二、vue2.0源码实现
Tips：本节采用的源码是v2.6.11
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，主要包含三个部分：
- 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；
- Dep：依赖收集器，用于维护依赖data属性的所有Watcher，并分发更新；
- 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；
- 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。(本文不涉及Compile的源码部分)
Observer
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 (obj: Object) {
const keys = Object.keys(obj)
for (let i = 0; i < keys.length; i++) {
defineReactive(obj, keys[i])
}
}
observeArray (items: Array) {
for (let i = 0, l = items.length; i < l; i++) {
observe(items[i])
}
}
}
在数据处理方法中，vue实例的data会作为value参数传递给Observer构造函数，Observer对value参数主要做了如下处理：
- 当value为数组时，首先会将数组进行变异，在copyAugment方法中会将数组的'push','pop','shift','unshift','splice','sort','reverse'七个方法触发dep.notify更新（即通知关联Watcher重新渲染界面，对这块感兴趣的朋友可以看一下array.js的源码）。然后再遍历每个值执行observe方法；
- 当value为对象时，遍历每个属性执行defineReactive方法。
针对数组进行处理的boserve方法：
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
}
简单来说，observe方法主要用于判断数组元素类型，如果是数组或对象将再次传入Observer构造函数。
针对对象进行处理的defineReactive方法：
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()
}
})
}
该方法主要通过Object.defineProperty定义了value每个属性的getter/setter方法，在getter中收集依赖，将属性与Watcher实例相关联；在setter中获取数据变更，通知属性关联得Watcher重新渲染视图。同时也会将属性继续传入Observe方法，一层一层向下处理。
总结：Observer主要通过遍历data对象，并对data对象进行递归处理（这点体现在data对象中嵌套对象也是支持响应式的），遍历递归出来的属性，只针对数组和对象进行下一步处理。其中数组比较特殊，通过数组变异方法监听了数组的7个操作函数，用于通知Watcher重新渲染界面。如果是对象，将遍历该对象的属性，为每个属性定义getter/setter方法，用于绑定依赖关系和通知Wathcer更新界面。
Dep
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()
}
}
}
总结：从Observer中我们可以知道一个数据会对应一个Dep，Dep中的subs数组就是用于收集与该数据关联的Watcher，使两者绑定起来。里面还有个Dep.target是全局共享的，他表示目前正在收集依赖的那个Watcher，且同一时间有且只有一个。
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;
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)
}
}
}
}
}
总结：Watcher主要做了如下工作：
- 初始化属性值，主要是deps、newDeps、depIds、newDepIds，分别表示现有依赖和新一轮要收集的依赖，这里的依赖指的就是上面的Dep；
- 通过传入构造函数的expOrFn参数设置getter属性，如果expOrFn是函数，直接设置为getter，如果expOrFn为表达式，则将表达式传入parsePath设置为getter；
- 执行get方法，这里主要用于收集依赖，并获取属性的值，方法最后要调用cleanupDeps来清除依赖。这是因为数据更新之后依赖有可能发生改变，所以要清除后重新收集依赖；
- update方法是触发更新之后调用，这里分为三种情况（1.lazy延迟加载，暂不调用数据更新；2.sync立即调用更新；3.放入队列中，等待下一个事件循环后执行。）,数据更新的时候都会调用run方法，run方法首先会重新调用get收集依赖，然后使用this.cb.call更新模板或表达式的值。
三、vue3.0源码实现
Tips:本节采用的源码是vue3.0.5
vue3.0中响应式系统大致分为以下几个阶段
- 初始化阶段：通过组件初始化方法，将data对象转化为proxy对象，并创建一个负责渲染的effect实例；
- get依赖收集阶段：通过解析组件模板，替换对应的data属性数据，这个过程会触发属性的getter，通过stack方法将proxy对象、key和effect存入dep；
- set派发更新阶段：当data属性数据值变化时，触发属性的getter，通过trigger方法，根据proxy和key找到对应的dep，再调用effect执行界面渲染更新。
Effect
export function effect<T = any>(
fn: () => T,
options: ReactiveEffectOptions = EMPTY_OBJ
): ReactiveEffect<T> {
if (isEffect(fn)) {
fn = fn.raw
}
const effect = createReactiveEffect(fn, options)
if (!options.lazy) {
effect()
}
return effect
}
export function stop(effect: ReactiveEffect) {
if (effect.active) {
cleanup(effect)
if (effect.options.onStop) {
effect.options.onStop()
}
effect.active = false
}
}
let uid = 0
function createReactiveEffect<T = any>(
fn: () => T,
options: ReactiveEffectOptions
): ReactiveEffect<T> {
const effect = function reactiveEffect(): unknown {
if (!effect.active) {
return options.scheduler ? undefined : fn()
}
if (!effectStack.includes(effect)) {
cleanup(effect)
try {
enableTracking()
effectStack.push(effect)
activeEffect = effect
return fn()
} finally {
effectStack.pop()
resetTracking()
activeEffect = effectStack[effectStack.length - 1]
}
}
} as ReactiveEffect
effect.id = uid++
effect.allowRecurse = !!options.allowRecurse
effect._isEffect = true
effect.active = true
effect.raw = fn
effect.deps = []
effect.options = options
return effect
}
function cleanup(effect: ReactiveEffect) {
const { deps } = effect
if (deps.length) {
for (let i = 0; i < deps.length; i++) {
deps[i].delete(effect)
}
deps.length = 0
}
}
Track
export function track(target: object, type: TrackOpTypes, key: unknown) {
if (!shouldTrack || activeEffect === undefined) {
return
}
let depsMap = targetMap.get(target)
if (!depsMap) {
targetMap.set(target, (depsMap = new Map()))
}
let dep = depsMap.get(key)
if (!dep) {
depsMap.set(key, (dep = new Set()))
}
if (!dep.has(activeEffect)) {
dep.add(activeEffect)
activeEffect.deps.push(dep)
if (__DEV__ && activeEffect.options.onTrack) {
activeEffect.options.onTrack({
effect: activeEffect,
target,
type,
key
})
}
}
}
Trigger
export function trigger(
target: object,
type: TriggerOpTypes,
key?: unknown,
newValue?: unknown,
oldValue?: unknown,
oldTarget?: Map<unknown, unknown> | Set<unknown>
) {
const depsMap = targetMap.get(target)
if (!depsMap) {
// never been tracked
return
}
const effects = new Set<ReactiveEffect>()
const add = (effectsToAdd: Set<ReactiveEffect> | undefined) => {
if (effectsToAdd) {
effectsToAdd.forEach(effect => {
if (effect !== activeEffect || effect.allowRecurse) {
effects.add(effect)
}
})
}
}
if (type === TriggerOpTypes.CLEAR) {
// collection being cleared
// trigger all effects for target
depsMap.forEach(add)
} else if (key === 'length' && isArray(target)) {
depsMap.forEach((dep, key) => {
if (key === 'length' || key >= (newValue as number)) {
add(dep)
}
})
} else {
// schedule runs for SET | ADD | DELETE
if (key !== void 0) {
add(depsMap.get(key))
}
// also run for iteration key on ADD | DELETE | Map.SET
switch (type) {
case TriggerOpTypes.ADD:
if (!isArray(target)) {
add(depsMap.get(ITERATE_KEY))
if (isMap(target)) {
add(depsMap.get(MAP_KEY_ITERATE_KEY))
}
} else if (isIntegerKey(key)) {
// new index added to array -> length changes
add(depsMap.get('length'))
}
break
case TriggerOpTypes.DELETE:
if (!isArray(target)) {
add(depsMap.get(ITERATE_KEY))
if (isMap(target)) {
add(depsMap.get(MAP_KEY_ITERATE_KEY))
}
}
break
case TriggerOpTypes.SET:
if (isMap(target)) {
add(depsMap.get(ITERATE_KEY))
}
break
}
}
const run = (effect: ReactiveEffect) => {
if (__DEV__ && effect.options.onTrigger) {
effect.options.onTrigger({
effect,
target,
key,
type,
newValue,
oldValue,
oldTarget
})
}
if (effect.options.scheduler) {
effect.options.scheduler(effect)
} else {
effect()
}
}
effects.forEach(run)
}
未完待续...