摘要：本文主要通過結合vue官方文件及原始碼，對vue響應式原理進行深入分析。
1.定義
作為vue最獨特的特性，響應式可以說是vue的靈魂了，表面上看就是資料發生變化後，對應的介面會重新渲染，那麼響應式系統的底層細節到底是怎麼一回事呢？
Tips:vue的響應式系統在vue2.0和vue3.0版本中的底層實現有所不同，簡單了來說就是處理屬性的getter/setter部分從Object.defineProperty替換成了Proxy(不過vue3也保留了Object.defineProperty方式用於支援IE瀏覽器)
1.1.vue2.0實現原理
當一個普通的javascript物件傳入vue例項作為data選項時，vue將遍歷data的所有屬性，並使用Object.defineProperty重寫這些屬性的getter/setter方法,這些屬性的getter/setter對於使用者不可見，但是vue可以利用他們來追蹤依賴，在屬性值被訪問和修改時通知變更。每個元件例項都對應一個watcher例項，它會在元件渲染的過程中訪問過的屬性設定為依賴。之後當屬性的setter觸發時，會通知watcher對關聯的元件進行重新渲染。
1.2.vue3.0實現原理
當一個普通的javascript物件傳入vue例項作為data選項時，vue會將其轉化為Proxy。首次渲染後，元件將跟蹤在渲染過程中被訪問的屬性，元件就成了這些屬性的訂閱者。當proxy攔截到set操作時，該屬性將通知所有訂閱了它的元件進行重新渲染。
2.原始碼解析
通過上面的定義可能對於響應式的原理還不夠清楚，接下來通過對vue原始碼的分析進行深入理解。
2.1.vue2.0原始碼實現
在vue2.0中，vue的響應式系統是基於資料攔截+釋出訂閱的模式，包含了四個模組：
- Observer：通過Object.defineProperty攔截data屬性的setter/getter方法，從而使每個屬性都擁有一個Dep，當觸發getter時收集依賴（使用該屬性的watcher），當觸發setter時通知更新；
- Dep：依賴收集器，用於維護依賴data屬性的所有Watcher；
- Watcher：將檢視依賴的屬性繫結到Dep中，當資料修改時觸發setter，呼叫Dep的notify方法，通知所有依賴該屬性的Watcher進行update更新檢視，使屬性值與檢視繫結起來；
- Compile：模板指令解析器，對模板每個元素節點的指令進行掃描解析，根據指令模板替換屬性資料，同時注入Watcher更新資料的回撥方法。
- 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)
}
}
}
總結：Observer通過重寫data上各個屬性的setter/getter方法，對每個屬性都維護一個Dep，用於收集依賴該屬性的所有Watcher，當該屬性觸發setter時，派發更新的通知。
- 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]
}
總結：Dep一方面用陣列收集與屬性相關的Watcher，另一方面遍歷陣列通知每個Watcher進行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
}
}
}
總結：Watcher是一個依賴於資料的訂閱者，當資料發生變化時，Dep呼叫notify方法，觸發這些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.獲取文件碎片物件，放入記憶體中，會減少頁面的迴流和重繪
let fragment = this.nodeFragment(this.el)
// 2.編譯模板
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
// 建立一個記憶體碎片物件
const fragment = document.createDocumentFragment()
let firstChild
while ((firstChild = el.firstChild)) {
fragment.appendChild(firstChild)
}
return fragment
}
// 遍歷獲取並區分元素節點還是文字節點，然後進行相應的處理
complie(fragment) {
// 1.獲取到每個子節點
const childNodes = fragment.childNodes
childNodes.forEach(child => {
if (this.isElementNode(child)) {
// 是元素節點
// 編譯元素節點
this.complieElement(child)
} else {
// 文字節點
// 編譯文字節點
this.complieText(child)
}
if (child.childNodes && child.childNodes.length) {
this.complie(child)
}
})
}
// 編譯元素
complieElement(node) {
const attributes = node.attributes
Array.from(attributes).forEach(attr => {
const { name, value } = attr
if (this.isDirective(name)) {
// 表明是一個指令
const [, dirctive] = name.split('-')
const [dirName, eventName] = dirctive.split(':') // text html model on
/*
node(整個節點)
value(msg)
this.vm(相當於整個 MVue例項物件)
eventName(v-on:click='btnClick') 中的事件名btnClick
*/
// 更新資料 資料驅動檢視
compileUtil[dirName](node, value, this.vm, eventName)
// 刪除有指令的標籤上的屬性
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('@')
}
// 檢測字串是否以 v- 開頭
isDirective(attrName) {
return attrName.startsWith('v-')
}
// 編譯文字
complieText(node) {
const content = node.textContent
if (/\{\{(.+?)\}\}/.test(content)) {
compileUtil['text'](node, content, this.vm)
}
}
}