I. Overview
Vue is a two-way binding achieved through data hijacking combined with the publisher-subscriber model.
First Observertraverse each object, then Object,defineProperty()bind each property getter/setterto complete data hijacking, and then gettercollect dependencies in , Watcheradd it to the subscriber by , which Depis a subscriber. When the data transmission changes, it will be triggered setter, in which will setterpass through each related dependency in and implement the update. Next, let's look at the source code:Dep.notify()notifyfor 循环 depupdate()
2. Source code part
1. Data hijackingObserver
(Extension: class Observer source code path core/observer/index.js)
walk()Traverse each property of the object in defineReactive binding, if the property value is an array , observeobserveArray() each member of the array in
class Observer {
value: any;
dep: Dep;
vmCount: number; // number of vms that has this object as root $data
constructor (value: any) {
this.value = value
this.dep = new Dep()
this.vmCount = 0
/*
将Observer实例绑定到data的__ob__属性上面去，之前说过observe的时候会先检测是否已经有__ob__对象存放Observer实例了，def方法定义可以参考https://github.com/vuejs/vue/blob/dev/src/core/util/lang.js#L16
*/
def(value, '__ob__', this)
if (Array.isArray(value)) {
/*
如果是数组，将修改后可以截获响应的数组方法替换掉该数组的原型中的原生方法，达到监听数组数据变化响应的效果。
这里如果当前浏览器支持__proto__属性，则直接覆盖当前数组对象原型上的原生数组方法，如果不支持该属性，则直接覆盖数组对象的原型。
*/
const augment = hasProto
? protoAugment /*直接覆盖原型的方法来修改目标对象*/
: copyAugment /*定义（覆盖）目标对象或数组的某一个方法*/
augment(value, arrayMethods, arrayKeys)
/*如果是数组则需要遍历数组的每一个成员进行observe*/
this.observeArray(value)
} else {
/*如果是对象则直接walk进行绑定*/
this.walk(value)
}
}
/**
* Walk through each property and convert them into
* getter/setters. This method should only be called when
* value type is Object.
*/
/*
遍历每一个对象并且在它们上面绑定getter与setter。这个方法只有在value的类型是对象的时候才能被调用
*/
walk (obj: Object) {
const keys = Object.keys(obj)
/*walk方法会遍历对象的每一个属性进行defineReactive绑定*/
for (let i = 0; i < keys.length; i++) {
defineReactive(obj, keys[i], obj[keys[i]])
}
}
/**
* Observe a list of Array items.
*/
/*对一个数组的每一个成员进行observe*/
observeArray (items: Array<any>) {
for (let i = 0, l = items.length; i < l; i++) {
/*数组需要遍历每一个成员进行observe*/
observe(items[i])
}
}
}
(1) observe(): (Extension: observe() source code path core/observer/index.js)
Attempts to create an Observerinstance ( __ob__), Observerreturning the new Observerinstance if the instance is successfully created, or the existing instance if Observerone already exists Observer.
function observe (value: any, asRootData: ?boolean): Observer | void {
if (!isObject(value)) {
return
}
let ob: Observer | void
/*这里用__ob__这个属性来判断是否已经有Observer实例，如果没有Observer实例则会新建一个Observer实例并赋值给__ob__这个属性，如果已有Observer实例则直接返回该Observer实例*/
if (hasOwn(value, '__ob__') && value.__ob__ instanceof Observer) {
ob = value.__ob__
} else if (
/*
这里的判断是为了确保value是单纯的对象，而不是函数或者是Regexp等情况。
而且该对象在shouldConvert的时候才会进行Observer。这是一个标识位，避免重复对value进行Observer
*/
observerState.shouldConvert &&
!isServerRendering() &&
(Array.isArray(value) || isPlainObject(value)) &&
Object.isExtensible(value) &&
!value._isVue
) {
ob = new Observer(value)
}
if (asRootData && ob) {
/*如果是根数据则计数，后面Observer中的observe的asRootData非true*/
ob.vmCount++
}
return ob
}
(2) defineReactive(): (Extension: defineReactive() source code path core/observer/index.js)
Dependency collection for object properties in dep.depend () (this method depends on collection, add an observer object when it exists Object.defineProperty, click for details below), if the object property value is an array, execute dependArray() , when data occurs When changing , first re- observe() the new value to ensure data responsiveness , and then dep.notify() (click to see the subscriber for details ) to notify all observers and trigger the view updategetDep.targetsetDepupdate()
function defineReactive (
obj: Object,
key: string,
val: any,
customSetter?: Function
) {
/*在闭包中定义一个dep对象*/
const dep = new Dep()
const property = Object.getOwnPropertyDescriptor(obj, key)
if (property && property.configurable === false) {
return
}
/*如果之前该对象已经预设了getter以及setter函数则将其取出来，新定义的getter/setter中会将其执行，保证不会覆盖之前已经定义的getter/setter。*/
// cater for pre-defined getter/setters
const getter = property && property.get
const setter = property && property.set
/*对象的子对象递归进行observe并返回子节点的Observer对象*/
let childOb = observe(val)
Object.defineProperty(obj, key, {
enumerable: true,
configurable: true,
get: function reactiveGetter () {
/*如果原本对象拥有getter方法则执行*/
const value = getter ? getter.call(obj) : val
if (Dep.target) {
/*进行依赖收集*/
dep.depend()
if (childOb) {
/*子对象进行依赖收集，其实就是将同一个watcher观察者实例放进了两个depend中，一个是正在本身闭包中的depend，另一个是子元素的depend*/
childOb.dep.depend()
}
if (Array.isArray(value)) {
/*是数组则需要对每一个成员都进行依赖收集，如果数组的成员还是数组，则递归。*/
dependArray(value)
}
}
return value
},
set: function reactiveSetter (newVal) {
/*通过getter方法获取当前值，与新值进行比较，一致则不需要执行下面的操作*/
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()
}
if (setter) {
/*如果原本对象拥有setter方法则执行setter*/
setter.call(obj, newVal)
} else {
val = newVal
}
/*新的值需要重新进行observe，保证数据响应式*/
childOb = observe(newVal)
/*dep对象通知所有的观察者*/
dep.notify()
}
})
}
(3) dependArray(): (Extension: dependArray() source code path core/observer/index.js)
function dependArray (value: Array<any>) {
for (let e, i = 0, l = value.length; i < l; i++) {
e = value[i]
/*通过对象上的观察者进行依赖收集*/
e && e.__ob__ && e.__ob__.dep.depend()
if (Array.isArray(e)) {
/*当数组成员还是数组的时候地柜执行该方法继续深层依赖收集，直到是对象为止。*/
dependArray(e)
}
}
}
2. SubscriberDep
(Extension: class Dep source path core/observer/dep.js)
Dependency collection, when Dep.target exists Dep.target.addDep ( addDep()see below) add observer object
class Dep {
static target: ?Watcher;
id: number;
subs: Array<Watcher>;
constructor () {
this.id = uid++
this.subs = []
}
/*添加一个观察者对象*/
addSub (sub: Watcher) {
this.subs.push(sub)
}
/*移除一个观察者对象*/
removeSub (sub: Watcher) {
remove(this.subs, sub)
}
/*依赖收集，当存在Dep.target的时候添加观察者对象*/
depend () {
if (Dep.target) {
Dep.target.addDep(this)
}
}
/*通知所有订阅者*/
notify () {
// stabilize the subscriber list first
const subs = this.subs.slice()
for (let i = 0, l = subs.length; i < l; i++) {
subs[i].update()
}
}
}
3. ObserverWatcher
(Extension: class Watcher source path core/observer/watcher.js)
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: ISet;
newDepIds: ISet;
getter: Function;
value: any;
constructor (
vm: Component,
expOrFn: string | Function,
cb: Function,
options?: Object
) {
this.vm = vm
/*_watchers存放订阅者实例*/
vm._watchers.push(this)
// options
if (options) {
this.deep = !!options.deep
this.user = !!options.user
this.lazy = !!options.lazy
this.sync = !!options.sync
} 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
/*把表达式expOrFn解析成getter*/
if (typeof expOrFn === 'function') {
this.getter = expOrFn
} else {
this.getter = parsePath(expOrFn)
if (!this.getter) {
this.getter = function () {
}
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.
*/
/*获得getter的值并且重新进行依赖收集*/
get () {
/*将自身watcher观察者实例设置给Dep.target，用以依赖收集。*/
pushTarget(this)
let value
const vm = this.vm
/*
执行了getter操作，看似执行了渲染操作，其实是执行了依赖收集。
在将Dep.target设置为自生观察者实例以后，执行getter操作。
譬如说现在的的data中可能有a、b、c三个数据，getter渲染需要依赖a跟c，
那么在执行getter的时候就会触发a跟c两个数据的getter函数，
在getter函数中即可判断Dep.target是否存在然后完成依赖收集，
将该观察者对象放入闭包中的Dep的subs中去。
*/
if (this.user) {
try {
value = this.getter.call(vm, vm)
} catch (e) {
handleError(e, vm, `getter for watcher "${
this.expression}"`)
}
} else {
value = this.getter.call(vm, vm)
}
// "touch" every property so they are all tracked as
// dependencies for deep watching
/*如果存在deep，则触发每个深层对象的依赖，追踪其变化*/
if (this.deep) {
/*递归每一个对象或者数组，触发它们的getter，使得对象或数组的每一个成员都被依赖收集，形成一个“深（deep）”依赖关系*/
traverse(value)
}
/*将观察者实例从target栈中取出并设置给Dep.target*/
popTarget()
this.cleanupDeps()
return value
}
/**
* Add a dependency to this directive.
*/
/*添加一个依赖关系到Deps集合中*/
addDep (dep: Dep) {
const id = dep.id
if (!this.newDepIds.has(id)) {
this.newDepIds.add(id)
this.newDeps.push(dep)
if (!this.depIds.has(id)) {
dep.addSub(this)
}
}
}
/**
* Clean up for dependency collection.
*/
/*清理依赖收集*/
cleanupDeps () {
/*移除所有观察者对象*/
let i = this.deps.length
while (i--) {
const dep = this.deps[i]
if (!this.newDepIds.has(dep.id)) {
dep.removeSub(this)
}
}
let tmp = this.depIds
this.depIds = this.newDepIds
this.newDepIds = tmp
this.newDepIds.clear()
tmp = this.deps
this.deps = this.newDeps
this.newDeps = tmp
this.newDeps.length = 0
}
/**
* Subscriber interface.
* Will be called when a dependency changes.
*/
/*
调度者接口，当依赖发生改变的时候进行回调。
*/
update () {
/* istanbul ignore else */
if (this.lazy) {
this.dirty = true
} else if (this.sync) {
/*同步则执行run直接渲染视图*/
this.run()
} else {
/*异步推送到观察者队列中，下一个tick时调用。*/
queueWatcher(this)
}
}
/**
* Scheduler job interface.
* Will be called by the scheduler.
*/
/*
调度者工作接口，将被调度者回调。
*/
run () {
if (this.active) {
/* get操作在获取value本身也会执行getter从而调用update更新视图 */
const value = this.get()
if (
value !== this.value ||
// Deep watchers and watchers on Object/Arrays should fire even
// when the value is the same, because the value may
// have mutated.
/*
即便值相同，拥有Deep属性的观察者以及在对象／数组上的观察者应该被触发更新，因为它们的值可能发生改变。
*/
isObject(value) ||
this.deep
) {
// set new value
const oldValue = this.value
/*设置新的值*/
this.value = value
/*触发回调*/
if (this.user) {
try {
this.cb.call(this.vm, value, oldValue)
} catch (e) {
handleError(e, this.vm, `callback for watcher "${
this.expression}"`)
}
} else {
this.cb.call(this.vm, value, oldValue)
}
}
}
}
/**
* Evaluate the value of the watcher.
* This only gets called for lazy watchers.
*/
/*获取观察者的值*/
evaluate () {
this.value = this.get()
this.dirty = false
}
/**
* Depend on all deps collected by this watcher.
*/
/*收集该watcher的所有deps依赖*/
depend () {
let i = this.deps.length
while (i--) {
this.deps[i].depend()
}
}
/**
* Remove self from all dependencies' subscriber list.
*/
/*将自身从所有依赖收集订阅列表删除*/
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.
/*从vm实例的观察者列表中将自身移除，由于该操作比较耗费资源，所以如果vm实例正在被销毁则跳过该步骤。*/
if (!this.vm._isBeingDestroyed) {
remove(this.vm._watchers, this)
}
let i = this.deps.length
while (i--) {
this.deps[i].removeSub(this)
}
this.active = false
}
}
}
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