继上一篇:深入理解Vue的watch实现原理及其实现方式 继续讲解
Vue的computed实现相对于watch和data来说比较难以理解,要真正的理解computed的工作方式,你需要深入理解Vue的双向数据绑定原理和实现方式。
如果你还不是很理解推荐你先看此文章:
首先来看一波Vue中computed的使用方式:
var vm = new Vue({
data: { a: 1 },
computed: {
// 仅读取
aDouble: function () {
return this.a * 2
},
// 读取和设置
aPlus: {
get: function () {
return this.a + 1
},
set: function (v) {
this.a = v - 1
}
}
}
})
vm.aPlus // => 2
vm.aPlus = 3
vm.a // => 2
vm.aDouble // => 4
计算属性的主要应用场景是代替模板内的表达式,或者data值的任何复杂逻辑都应该使用computed来计算,它有两大优势:
1、逻辑清晰,方便于管理
2、计算值会被缓存,依赖的data值改变时才会从新计算
此文我们需要核心理解的是:
1、computed是如何初始化,初始化之后干了些什么
2、为何触发data值改变时computed会从新计算
3、computed值为什么说是被缓存的呢,如何做的
以下大部分代码摘自Vue源码。
如果你看到了这里,就当做你已经深入理解了Vue的MVVM原理及其实现方式。相关Vue的MVVM实现直接取自上一篇文章。
Dep代码的实现:
//标识当前的Dep id
let uidep = 0
class Dep{
constructor () {
this.id = uidep++
// 存放所有的监听watcher
this.subs = []
}
//添加一个观察者对象
addSub (Watcher) {
this.subs.push(Watcher)
}
//依赖收集
<span class="hljs-function"><span class="hljs-title">depend</span></span> () {
//Dep.target 作用只有需要的才会收集依赖
<span class="hljs-keyword">if</span> (Dep.target) {
Dep.target.addDep(this)
}
}
// 调用依赖收集的Watcher更新
<span class="hljs-function"><span class="hljs-title">notify</span></span> () {
const subs = this.subs.slice()
<span class="hljs-keyword">for</span> (<span class="hljs-built_in">let</span> i = 0, l = subs.length; i < l; i++) {
subs[i].update()
}
}
}
Dep.target = null
const targetStack = []
// 为Dep.target 赋值
function pushTarget (Watcher) {
if (Dep.target) targetStack.push(Dep.target)
Dep.target = Watcher
}
function popTarget () {
Dep.target = targetStack.pop()
}
Watcher代码的实现:
//去重 防止重复收集
let uid = 0
class Watcher{
constructor(vm,expOrFn,cb,options){
//传进来的对象 例如Vue
this.vm = vm
if (options) {
this.deep = !!options.deep
this.user = !!options.user
this.lazy = !!options.lazy
}else{
this.deep = this.user = this.lazy = false
}
this.dirty = this.lazy
//在Vue中cb是更新视图的核心,调用diff并更新视图的过程
this.cb = cb
this.id = ++uid
this.deps = []
this.newDeps = []
this.depIds = new Set()
this.newDepIds = new Set()
if (typeof expOrFn === ‘function’) {
//data依赖收集走此处
this.getter = expOrFn
} else {
//watch依赖走此处
this.getter = this.parsePath(expOrFn)
}
//设置Dep.target的值,依赖收集时的watcher对象
this.value = this.lazy ? undefined : this.get()
}
<span class="hljs-function"><span class="hljs-title">get</span></span>(){
//设置Dep.target值,用以依赖收集
pushTarget(this)
const vm = this.vm
//此处会进行依赖收集 会调用data数据的 get
<span class="hljs-built_in">let</span> value = this.getter.call(vm, vm)
popTarget()
<span class="hljs-built_in">return</span> value
}
//添加依赖
addDep (dep) {
//去重
const id = dep.id
<span class="hljs-keyword">if</span> (!this.newDepIds.has(id)) {
this.newDepIds.add(id)
this.newDeps.push(dep)
<span class="hljs-keyword">if</span> (!this.depIds.has(id)) {
//收集watcher 每次data数据 <span class="hljs-built_in">set</span>
//时会遍历收集的watcher依赖进行相应视图更新或执行watch监听函数等操作
dep.addSub(this)
}
}
}
//更新
<span class="hljs-function"><span class="hljs-title">update</span></span> () {
<span class="hljs-keyword">if</span> (this.lazy) {
this.dirty = <span class="hljs-literal">true</span>
}<span class="hljs-keyword">else</span>{
this.run()
}
}
//更新视图
<span class="hljs-function"><span class="hljs-title">run</span></span>(){
console.log(`这里会去执行Vue的diff相关方法,进而更新数据`)
const value = this.get()
const oldValue = this.value
this.value = value
<span class="hljs-keyword">if</span> (this.user) {
//watch 监听走此处
this.cb.call(this.vm, value, oldValue)
}<span class="hljs-keyword">else</span>{
//data 监听走此处
//这里只做简单的console.log 处理,在Vue中会调用diff过程从而更新视图
this.cb.call(this.vm, value, oldValue)
}
}
//如果计算熟悉依赖的data值发生变化时会调用
//案例中 当data.name值发生变化时会执行此方法
<span class="hljs-function"><span class="hljs-title">evaluate</span></span> () {
this.value = this.get()
this.dirty = <span class="hljs-literal">false</span>
}
//收集依赖
<span class="hljs-function"><span class="hljs-title">depend</span></span> () {
<span class="hljs-built_in">let</span> i = this.deps.length
<span class="hljs-keyword">while</span> (i--) {
this.deps[i].depend()
}
}
// 此方法获得每个watch中key在data中对应的value值
//使用split(<span class="hljs-string">'.'</span>)是为了得到 像<span class="hljs-string">'a.b.c'</span> 这样的监听值
parsePath (path){
const bailRE = /[^w.$]/
<span class="hljs-keyword">if</span> (bailRE.test(path)) <span class="hljs-built_in">return</span>
const segments = path.split(<span class="hljs-string">'.'</span>)
<span class="hljs-built_in">return</span> <span class="hljs-keyword">function</span> (obj) {
<span class="hljs-keyword">for</span> (<span class="hljs-built_in">let</span> i = 0; i < segments.length; i++) {
<span class="hljs-keyword">if</span> (!obj) <span class="hljs-built_in">return</span>
//此处为了兼容我的代码做了一点修改
//此处使用新获得的值覆盖传入的值 因此能够处理 <span class="hljs-string">'a.b.c'</span>这样的监听方式
<span class="hljs-keyword">if</span>(i==0){
obj = obj.data[segments[i]]
}<span class="hljs-keyword">else</span>{
obj = obj[segments[i]]
}
}
<span class="hljs-built_in">return</span> obj
}
}
}
在Watcher中对于computed来说核心注意点是以下方法:
//如果计算熟悉依赖的data值发生变化时会调用
//案例中 当data.name值发生变化时会执行此方法
evaluate () {
this.value = this.get()
this.dirty = false
}
当computed中用到的data值发生变化时,视图更新调用computed值时会从新执行,获得新的计算属性值。
Observer代码实现
class Observer{
constructor (value) {
this.value = value
// 增加dep属性(处理数组时可以直接调用)
this.dep = new Dep()
//将Observer实例绑定到data的__ob__属性上面去,后期如果oberve时直接使用,不需要从新Observer,
//处理数组是也可直接获取Observer对象
def(value, ‘ob’, this)
if (Array.isArray(value)) {
//这里只测试对象
} else {
//处理对象
this.walk(value)
}
}
walk (obj) {
const keys = Object.keys(obj)
<span class="hljs-keyword">for</span> (<span class="hljs-built_in">let</span> i = 0; i < keys.length; i++) {
//此处我做了拦截处理,防止死循环,Vue中在oberve函数中进行的处理
<span class="hljs-keyword">if</span>(keys[i]==<span class="hljs-string">'__ob__'</span>) <span class="hljs-built_in">return</span>;
defineReactive(obj, keys[i], obj[keys[i]])
}
}
}
//数据重复Observer
function observe(value){
if(typeof(value) != ‘object’ ) return;
let ob = new Observer(value)
return ob;
}
// 把对象属性改为getter/setter,并收集依赖
function defineReactive (obj,key,val) {
const dep = new Dep()
//处理children
let childOb = observe(val)
Object.defineProperty(obj, key, {
enumerable: true,
configurable: true,
get: function reactiveGetter () {
console.log(调用get获取值,值为<span class="hljs-variable">${val}</span>
)
const value = val
if (Dep.target) {
dep.depend()
if (childOb) {
childOb.dep.depend()
}
}
return value
},
set: function reactiveSetter (newVal) {
console.log(调用了<span class="hljs-built_in">set</span>,值为<span class="hljs-variable">${newVal}</span>
)
const value = val
val = newVal
//对新值进行observe
childOb = observe(newVal)
//通知dep调用,循环调用手机的Watcher依赖,进行视图的更新
dep.notify()
}
})
}
//辅助方法
function def (obj, key, val) {
Object.defineProperty(obj, key, {
value: val,
enumerable: true,
writable: true,
configurable: true
})
}
此文的重点Computed代码实现:
//空函数
const noop = ()=>{}
// computed初始化的Watcher传入lazy: true就会触发Watcher中的dirty值为true
const computedWatcherOptions = { lazy: true }
//Object.defineProperty 默认value参数
const sharedPropertyDefinition = {
enumerable: true,
configurable: true,
get: noop,
set: noop
}
// 初始化computed
class initComputed {
constructor(vm, computed){
//新建存储watcher对象,挂载在vm对象执行
const watchers = vm._computedWatchers = Object.create(null)
//遍历computed
for (const key in computed) {
const userDef = computed[key]
//getter值为computed中key的监听函数或对象的get值
let getter = typeof userDef === ‘function’ ? userDef : userDef.get
//新建computed的 watcher
watchers[key] = new Watcher(vm, getter, noop, computedWatcherOptions)
if (!(key in vm)) {
/定义计算属性/
this.defineComputed(vm, key, userDef)
}
}
}
//把计算属性的key挂载到vm对象下,并使用Object.defineProperty进行处理
//因此调用vm.somecomputed 就会触发get函数
defineComputed (target, key, userDef) {
if (typeof userDef === ‘function’) {
sharedPropertyDefinition.get = this.createComputedGetter(key)
sharedPropertyDefinition.set = noop
} else {
sharedPropertyDefinition.get = userDef.get
? userDef.cache !== false
? this.createComputedGetter(key)
: userDef.get
: noop
//如果有设置set方法则直接使用,否则赋值空函数
sharedPropertyDefinition.set = userDef.set
? userDef.set
: noop
}
Object.defineProperty(target, key, sharedPropertyDefinition)
}
//计算属性的getter 获取计算属性的值时会调用
createComputedGetter (key) {
<span class="hljs-built_in">return</span> <span class="hljs-keyword">function</span> <span class="hljs-function"><span class="hljs-title">computedGetter</span></span> () {
//获取到相应的watcher
const watcher = this._computedWatchers && this._computedWatchers[key]
<span class="hljs-keyword">if</span> (watcher) {
//watcher.dirty 参数决定了计算属性值是否需要重新计算,默认值为<span class="hljs-literal">true</span>,即第一次时会调用一次
<span class="hljs-keyword">if</span> (watcher.dirty) {
/*每次执行之后watcher.dirty会设置为<span class="hljs-literal">false</span>,只要依赖的data值改变时才会触发
watcher.dirty为<span class="hljs-literal">true</span>,从而获取值时从新计算*/
watcher.evaluate()
}
//获取依赖
<span class="hljs-keyword">if</span> (Dep.target) {
watcher.depend()
}
//返回计算属性的值
<span class="hljs-built_in">return</span> watcher.value
}
}
}
}
代码已经写完,完整代码如下:
//标识当前的Dep id
let uidep = 0
class Dep{
constructor () {
this.id = uidep++
// 存放所有的监听watcher
this.subs = []
}
//添加一个观察者对象
addSub (Watcher) {
this.subs.push(Watcher)
}
//依赖收集
<span class="hljs-function"><span class="hljs-title">depend</span></span> () {
//Dep.target 作用只有需要的才会收集依赖
<span class="hljs-keyword">if</span> (Dep.target) {
Dep.target.addDep(this)
}
}
// 调用依赖收集的Watcher更新
<span class="hljs-function"><span class="hljs-title">notify</span></span> () {
const subs = this.subs.slice()
<span class="hljs-keyword">for</span> (<span class="hljs-built_in">let</span> i = 0, l = subs.length; i < l; i++) {
subs[i].update()
}
}
}
Dep.target = null
const targetStack = []
// 为Dep.target 赋值
function pushTarget (Watcher) {
if (Dep.target) targetStack.push(Dep.target)
Dep.target = Watcher
}
function popTarget () {
Dep.target = targetStack.pop()
}
/----------------------------------------Watcher------------------------------------/
//去重 防止重复收集
let uid = 0
class Watcher{
constructor(vm,expOrFn,cb,options){
//传进来的对象 例如Vue
this.vm = vm
if (options) {
this.deep = !!options.deep
this.user = !!options.user
this.lazy = !!options.lazy
}else{
this.deep = this.user = this.lazy = false
}
this.dirty = this.lazy
//在Vue中cb是更新视图的核心,调用diff并更新视图的过程
this.cb = cb
this.id = ++uid
this.deps = []
this.newDeps = []
this.depIds = new Set()
this.newDepIds = new Set()
if (typeof expOrFn === ‘function’) {
//data依赖收集走此处
this.getter = expOrFn
} else {
//watch依赖走此处
this.getter = this.parsePath(expOrFn)
}
//设置Dep.target的值,依赖收集时的watcher对象
this.value = this.lazy ? undefined : this.get()
}
<span class="hljs-function"><span class="hljs-title">get</span></span>(){
//设置Dep.target值,用以依赖收集
pushTarget(this)
const vm = this.vm
//此处会进行依赖收集 会调用data数据的 get
<span class="hljs-built_in">let</span> value = this.getter.call(vm, vm)
popTarget()
<span class="hljs-built_in">return</span> value
}
//添加依赖
addDep (dep) {
//去重
const id = dep.id
<span class="hljs-keyword">if</span> (!this.newDepIds.has(id)) {
this.newDepIds.add(id)
this.newDeps.push(dep)
<span class="hljs-keyword">if</span> (!this.depIds.has(id)) {
//收集watcher 每次data数据 <span class="hljs-built_in">set</span>
//时会遍历收集的watcher依赖进行相应视图更新或执行watch监听函数等操作
dep.addSub(this)
}
}
}
//更新
<span class="hljs-function"><span class="hljs-title">update</span></span> () {
<span class="hljs-keyword">if</span> (this.lazy) {
this.dirty = <span class="hljs-literal">true</span>
}<span class="hljs-keyword">else</span>{
this.run()
}
}
//更新视图
<span class="hljs-function"><span class="hljs-title">run</span></span>(){
console.log(`这里会去执行Vue的diff相关方法,进而更新数据`)
const value = this.get()
const oldValue = this.value
this.value = value
<span class="hljs-keyword">if</span> (this.user) {
//watch 监听走此处
this.cb.call(this.vm, value, oldValue)
}<span class="hljs-keyword">else</span>{
//data 监听走此处
//这里只做简单的console.log 处理,在Vue中会调用diff过程从而更新视图
this.cb.call(this.vm, value, oldValue)
}
}
//如果计算熟悉依赖的data值发生变化时会调用
//案例中 当data.name值发生变化时会执行此方法
<span class="hljs-function"><span class="hljs-title">evaluate</span></span> () {
this.value = this.get()
this.dirty = <span class="hljs-literal">false</span>
}
//收集依赖
<span class="hljs-function"><span class="hljs-title">depend</span></span> () {
<span class="hljs-built_in">let</span> i = this.deps.length
<span class="hljs-keyword">while</span> (i--) {
this.deps[i].depend()
}
}
// 此方法获得每个watch中key在data中对应的value值
//使用split(<span class="hljs-string">'.'</span>)是为了得到 像<span class="hljs-string">'a.b.c'</span> 这样的监听值
parsePath (path){
const bailRE = /[^w.$]/
<span class="hljs-keyword">if</span> (bailRE.test(path)) <span class="hljs-built_in">return</span>
const segments = path.split(<span class="hljs-string">'.'</span>)
<span class="hljs-built_in">return</span> <span class="hljs-keyword">function</span> (obj) {
<span class="hljs-keyword">for</span> (<span class="hljs-built_in">let</span> i = 0; i < segments.length; i++) {
<span class="hljs-keyword">if</span> (!obj) <span class="hljs-built_in">return</span>
//此处为了兼容我的代码做了一点修改
//此处使用新获得的值覆盖传入的值 因此能够处理 <span class="hljs-string">'a.b.c'</span>这样的监听方式
<span class="hljs-keyword">if</span>(i==0){
obj = obj.data[segments[i]]
}<span class="hljs-keyword">else</span>{
obj = obj[segments[i]]
}
}
<span class="hljs-built_in">return</span> obj
}
}
}
/----------------------------------------Observer------------------------------------/
class Observer{
constructor (value) {
this.value = value
// 增加dep属性(处理数组时可以直接调用)
this.dep = new Dep()
//将Observer实例绑定到data的__ob__属性上面去,后期如果oberve时直接使用,不需要从新Observer,
//处理数组是也可直接获取Observer对象
def(value, ‘ob’, this)
if (Array.isArray(value)) {
//这里只测试对象
} else {
//处理对象
this.walk(value)
}
}
walk (obj) {
const keys = Object.keys(obj)
<span class="hljs-keyword">for</span> (<span class="hljs-built_in">let</span> i = 0; i < keys.length; i++) {
//此处我做了拦截处理,防止死循环,Vue中在oberve函数中进行的处理
<span class="hljs-keyword">if</span>(keys[i]==<span class="hljs-string">'__ob__'</span>) <span class="hljs-built_in">return</span>;
defineReactive(obj, keys[i], obj[keys[i]])
}
}
}
//数据重复Observer
function observe(value){
if(typeof(value) != ‘object’ ) return;
let ob = new Observer(value)
return ob;
}
// 把对象属性改为getter/setter,并收集依赖
function defineReactive (obj,key,val) {
const dep = new Dep()
//处理children
let childOb = observe(val)
Object.defineProperty(obj, key, {
enumerable: true,
configurable: true,
get: function reactiveGetter () {
console.log(调用get获取值,值为<span class="hljs-variable">${val}</span>
)
const value = val
if (Dep.target) {
dep.depend()
if (childOb) {
childOb.dep.depend()
}
}
return value
},
set: function reactiveSetter (newVal) {
console.log(调用了<span class="hljs-built_in">set</span>,值为<span class="hljs-variable">${newVal}</span>
)
const value = val
val = newVal
//对新值进行observe
childOb = observe(newVal)
//通知dep调用,循环调用手机的Watcher依赖,进行视图的更新
dep.notify()
}
})
}
//辅助方法
function def (obj, key, val) {
Object.defineProperty(obj, key, {
value: val,
enumerable: true,
writable: true,
configurable: true
})
}
/----------------------------------------初始化watch------------------------------------/
//空函数
const noop = ()=>{}
// computed初始化的Watcher传入lazy: true就会触发Watcher中的dirty值为true
const computedWatcherOptions = { lazy: true }
//Object.defineProperty 默认value参数
const sharedPropertyDefinition = {
enumerable: true,
configurable: true,
get: noop,
set: noop
}
// 初始化computed
class initComputed {
constructor(vm, computed){
//新建存储watcher对象,挂载在vm对象执行
const watchers = vm._computedWatchers = Object.create(null)
//遍历computed
for (const key in computed) {
const userDef = computed[key]
//getter值为computed中key的监听函数或对象的get值
let getter = typeof userDef === ‘function’ ? userDef : userDef.get
//新建computed的 watcher
watchers[key] = new Watcher(vm, getter, noop, computedWatcherOptions)
if (!(key in vm)) {
/定义计算属性/
this.defineComputed(vm, key, userDef)
}
}
}
//把计算属性的key挂载到vm对象下,并使用Object.defineProperty进行处理
//因此调用vm.somecomputed 就会触发get函数
defineComputed (target, key, userDef) {
if (typeof userDef === ‘function’) {
sharedPropertyDefinition.get = this.createComputedGetter(key)
sharedPropertyDefinition.set = noop
} else {
sharedPropertyDefinition.get = userDef.get
? userDef.cache !== false
? this.createComputedGetter(key)
: userDef.get
: noop
//如果有设置set方法则直接使用,否则赋值空函数
sharedPropertyDefinition.set = userDef.set
? userDef.set
: noop
}
Object.defineProperty(target, key, sharedPropertyDefinition)
}
//计算属性的getter 获取计算属性的值时会调用
createComputedGetter (key) {
<span class="hljs-built_in">return</span> <span class="hljs-keyword">function</span> <span class="hljs-function"><span class="hljs-title">computedGetter</span></span> () {
//获取到相应的watcher
const watcher = this._computedWatchers && this._computedWatchers[key]
<span class="hljs-keyword">if</span> (watcher) {
//watcher.dirty 参数决定了计算属性值是否需要重新计算,默认值为<span class="hljs-literal">true</span>,即第一次时会调用一次
<span class="hljs-keyword">if</span> (watcher.dirty) {
/*每次执行之后watcher.dirty会设置为<span class="hljs-literal">false</span>,只要依赖的data值改变时才会触发
watcher.dirty为<span class="hljs-literal">true</span>,从而获取值时从新计算*/
watcher.evaluate()
}
//获取依赖
<span class="hljs-keyword">if</span> (Dep.target) {
watcher.depend()
}
//返回计算属性的值
<span class="hljs-built_in">return</span> watcher.value
}
}
}
}
computed测试:
//1、首先来创建一个Vue构造函数:
function Vue(){
}
//2、设置data和computed的值:
let data={
name:‘Hello’,
}
let computed={
getfullname:function(){
console.log(’-----走了computed 之 getfullname------’)
console.log(‘新的值为:’+data.name + ’ - world’)
return data.name + ’ - world’
}
}
//3、实例化Vue并把data挂载到Vue上
let vue = new Vue()
vue.data = data
//4、创建Watcher对象
let updateComponent = (vm)=>{
// 收集依赖
data.name
}
let watcher1 = new Watcher(vue,updateComponent,()=>{})
//5、初始化Data并收集依赖
observe(data)
//6、初始化computed
let watcher2 = new initComputed(vue,computed)
在浏览器console中测试:
//首先获得一次getfullname
vue.getfullname
//第二次调用getfullname看看会有什么变化呢
vue.getfullname
//为data.name赋值
data.name = 'Hi’
//name值变更之后再次执行会是什么结果呢
vue.getfullname
//再执行一次
vue.getfullname
所有测试代码如下:
/----------------------------------------Vue------------------------------------/
function Vue(){
}
/----------------------------------------测试代码------------------------------------/
// 调用
let data={
name:‘Hello’,
}
let computed={
getfullname:function(){
console.log(’-----走了computed 之 getfullname------’)
console.log(‘新的值为:’+data.name + ’ - world’)
return data.name + ’ - world’
}
}
let vue = new Vue()
vue.data = data
let updateComponent = (vm)=>{
// 收集依赖
data.name
}
let watcher1 = new Watcher(vue,updateComponent,()=>{})
observe(data)
let watvher2 = new initComputed(vue,computed)
//测试 浏览器console中相继运行一下代码测试
vue.getfullname
vue.getfullname
data.name=‘Hi’
vue.getfullname
vue.getfullname
若有疑问欢迎交流。
版权声明:著作权归作者所有。商业转载请联系作者获得授权,非商业转载请注明出处。 作者: zane1 原文链接:https://juejin.im/post/6844903606676799501