引言
发布订阅者模式是最常见的模式之一，它是一种一对多的对应关系，当一个对象发生变化时会通知依赖他的对象，接受到通知的对象会根据情况执行自己的行为。
假设有财经报纸送报员financialDep，有报纸阅读爱好者a，b，c，那么a，b，c想订报纸就告诉financialDep，financialDep依次记录a，b，c这三个人的家庭地址，次日，送报员一大早把报纸送到a，b，c家门口的邮箱中，a，b，c收到报纸后都会认认真真的打开阅读。随着时间的推移，会有以下几种场景：
- 有新的订阅者加入： 有一天d也想订报纸了，那么找到financialDep，financialDep把d的家庭地址记录到a，b，c的后面，次日，为a，b，c，d分别送报纸。
- 有订阅者退出了：有一天a要去旅游了，提前给送报员financialDep打电话取消了订阅，如果不取消的话，积攒的报纸就会溢出小邮箱。
- 有新的报社开业：有一天镇子又开了家体育类的报馆，送报员是sportDep，b和d也是球类爱好者，于是在sportDep那里做了登记，sportDep的记录中就有了b和d。
从上面的例子中可以看出，刚开始送报员financialDep的记录中有a，b和c，先是d加进来后来是a离开，最终financialDep的记录中有b，c和d。体育类报馆开张的时候，b和d也订阅了报纸，sportDep的记录中就有了b和d。我们发现，c只订阅了财经类报刊，而b和d既订阅了财经类的报纸也定了财经类的报刊。
一、发布订阅者模式的特点
从以上例子可以发现特点：
- 发布者可以支持订阅者的加入
- 发布者可以支持订阅者的删除
- 一个发布者可以有多个订阅者，一个订阅者也可以订阅多个发布者的消息那可能会有疑问，有没有可能会有发布者的删除，答案是会，但是此时，发布者已消失，订阅者再也不会收到消息，也就不会与当前发布者相关的消息诱发的行为。好比体育类报馆关停了（发布者删除）那么b和d在也不会收到体育类报纸（消息），也就不会再阅读体育类报纸（行为）。
二、vue中的发布订阅者模式
以上的例子基本就是vue中发布订阅者的大体概况，vue中的发布者是啥时候定义的?
在new Vue实例化的过程中会执行this._init的初始化方法，_init方法中有方法initState：
export function initState (vm: Component) {
// ...
if (opts.data) {
initData(vm)
} else {
observe(vm._data = {}, true /* asRootData */)
}
//...
}
首先看initData对于data的初始化：
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
observe(data, true /* asRootData */)
}
这里首先获取data，如果data是函数又会执行getData方法。然后，获取methods和props中的key值，如果已经定义过则在开发环境进行控制台警告。其中，proxy的目的是让访问this[key]相当于访问this._data[key]。最后，对数据进行响应式处理 observe(data, true /* asRootData */)：
/**
* 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
}
如果不是对象或者当前值是VNode的实例直接返回。如果当前当前值上有属性__ob__并且value.__ob__是Observer的实例，那么说明该值已经被响应式处理过，直接将value.__ob__赋值给ob并在最后返回即可。如果满足else if中的条件，则可执行ob = new Observer(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])
}
}
}
Observer是构造函数，通过对value是否是数组的判断，分别执行observeArray和walk，observeArray会对数组中的元素执行observe(items[i])，即通过递归的方式对value树进行深度遍历，递归的最后都会执行到walk方法。再看walk中的defineReactive(obj, keys[i])方法：
/**
* 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()
}
})
}
这里就是vue响应式原理、watcher订阅者收集、数据变化时发布者dep通知subs中订阅者watcher进行相应操作的主要流程，new Dep()实例化、Object.defineProperty方法、dep.depend()订阅者收集和dep.notify()是主要的功能。先看发布者Dep的实例化：
1、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]
}
这里的dep就相当于财经或者体育报馆，其中定义了属性id和subs,subs相当于送报员financialDep手中的笔记本，用来是用来记录订阅者的数组。发布者的消息如何发给订阅者，就需要借助Object.defineProperty:
2、Object.defineProperty
对于一个对象的属性进行访问或者设置的时候可以为其设置get和set方法，在其中进行相应的操作，这也是vue响应式原理的本质，也是IE低版本浏览器不支持vue框架的原因，因为IE低版本浏览器不支持Object.defineProperty方法。
Object.defineProperty(obj, key, {
enumerable: true,
configurable: true,
get: function reactiveGetter () {
// 当访问属性的时候，进行订阅者的收集
},
set: function reactiveSetter (newVal) {
// 当修改属性的时候，收到发布者消息的时候进行相应的操作
}
})
在vue中订阅者有computer watcher计算属性、watch watcher侦听器和render watcher渲染watcher。这里先介绍渲染watcher:
3、watcher
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
}
// watcher还有很多其他自定义方法，用的时候再列举
}
Watcher实例化的最后会执行this.value = this.lazy ? undefined : this.get()方法，默认this.lazy=false，满足条件执行Watcher实例的回调this.get()方法。 pushTarget(this)定义在dep.js文件中，为全局targetStack中推入当前订阅者，是一种栈的组织方式。Dep.target = target表示当前订阅者是正在计算中的订阅者，全局同一时间点有且只有一个。 然后执行value = this.getter.call(vm, vm)，这里的this.getter就是
updateComponent = () => {
vm._update(vm._render(), hydrating)
}
进行当前vue实例的渲染，在渲染过程中会创建vNode，进而访问数据data中的属性，进入到get方法中，触发dep.depend()。
4、dep.depend
dep.depend()是在访问obj[key]的时候进行执行的，在渲染过程中Dep.target就是渲染watcher，条件满足，执行Dep.target.addDep(this)，即执行watcher中的
/**
* 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)
}
}
}
newDepIds和depIds分别表示当前订阅者依赖的当前发布者和旧发布者id的Set集合，newDeps表示当前发布者实例的数组列表。首次渲染时this.newDepIds中不包含id，this.newDepIds添加了发布者的id，this.newDeps中添加了dep实例。同时，this.depIds中不包含id，继而执行到dep.addSub(this)。
addSub (sub: Watcher) {
this.subs.push(sub)
}
这个动作就表示订阅者watcher订阅了发布者dep发布的消息，当前发布者的subs数组中订阅者数量+1，等下次数据变化时发布者就通过dep.notify()的方式进行消息通知。
5、dep.notify
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()
}
}
const subs = this.subs.slice()对订阅者进行浅拷贝，subs.sort((a, b) => a.id - b.id)按照订阅者的id进行排序，最后循环订阅者，订阅者触发update方法：
/**
* 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)
}
}
this.dirty表示计算属性，这里是false，this.sync表示同步，这里是false，最后会走到queueWatcher(this):
/**
* Push a watcher into the watcher queue.
* Jobs with duplicate IDs will be skipped unless it's
* pushed when the queue is being flushed.
*/
export function queueWatcher (watcher: Watcher) {
const id = watcher.id
if (has[id] == null) {
has[id] = true
if (!flushing) {
queue.push(watcher)
} else {
// if already flushing, splice the watcher based on its id
// if already past its id, it will be run next immediately.
let i = queue.length - 1
while (i > index && queue[i].id > watcher.id) {
i--
}
queue.splice(i + 1, 0, watcher)
}
// queue the flush
if (!waiting) {
waiting = true
if (process.env.NODE_ENV !== 'production' && !config.async) {
flushSchedulerQueue()
return
}
nextTick(flushSchedulerQueue)
}
}
}
这里未刷新状态flushing === false时会在队列queue中推入订阅者watcher，如果没有在等待状态waiting===false时执行nextTick将flushSchedulerQueue的执行推入异步队列中，等待所有的同步操作执行完毕再去按照次序执行异步的flushSchedulerQueue。需要了解nextTick原理请移步：https://www.jb51.net/article/261842.htm
/**
* Flush both queues and run the watchers.
*/
function flushSchedulerQueue () {
currentFlushTimestamp = getNow()
flushing = true
let watcher, id
// Sort queue before flush.
// This ensures that:
// 1. Components are updated from parent to child. (because parent is always
// created before the child)
// 2. A component's user watchers are run before its render watcher (because
// user watchers are created before the render watcher)
// 3. If a component is destroyed during a parent component's watcher run,
// its watchers can be skipped.
queue.sort((a, b) => a.id - b.id)
// do not cache length because more watchers might be pushed
// as we run existing watchers
for (index = 0; index < queue.length; index++) {
watcher = queue[index]
if (watcher.before) {
watcher.before()
}
id = watcher.id
has[id] = null
watcher.run()
// in dev build, check and stop circular updates.
if (process.env.NODE_ENV !== 'production' && has[id] != null) {
circular[id] = (circular[id] || 0) + 1
if (circular[id] > MAX_UPDATE_COUNT) {
warn(
'You may have an infinite update loop ' + (
watcher.user
? `in watcher with expression "${watcher.expression}"`
: `in a component render function.`
),
watcher.vm
)
break
}
}
}
// keep copies of post queues before resetting state
const activatedQueue = activatedChildren.slice()
const updatedQueue = queue.slice()
resetSchedulerState()
// call component updated and activated hooks
callActivatedHooks(activatedQueue)
callUpdatedHooks(updatedQueue)
// devtool hook
/* istanbul ignore if */
if (devtools && config.devtools) {
devtools.emit('flush')
}
}
function callUpdatedHooks (queue) {
let i = queue.length
while (i--) {
const watcher = queue[i]
const vm = watcher.vm
if (vm._watcher === watcher && vm._isMounted && !vm._isDestroyed) {
callHook(vm, 'updated')
}
}
}
这里主要做了四件事：
- 对队列
queue进行排序 - 遍历执行
watcher的run方法 resetSchedulerState进行重置，清空queue，并且waiting = flushing = false进行状态重置callUpdatedHooks执行callHook(vm, 'updated')生命周期钩子函数 这里的run是在Watcher的时候定义的：
/**
* 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)
}
}
}
}
active默认为true，执行到const value = this.get()就开始了数据变化后的渲染的操作，好比订阅者收到报纸后认真读报一样。get方法中，value = this.getter.call(vm, vm)渲染执行完以后，会通过popTarget把targetStack栈顶的元素移除，并且通过Dep.target = targetStack[targetStack.length - 1]修改当前执行的元素。最后执行this.cleanupDeps:
6、订阅者取消订阅
/**
* 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
}
首先通过while的方式循环旧的this.deps发布者的数组，如果当前订阅者所依赖的发布者this.newDepIds中没有包含旧的发布者，那么，就让发布者在this.subs中移除订阅者，这样就不会让发布者dep进行额外的通知，这种额外的通知可能会引起未订阅者的行为(可能消耗内存资源或引起不必要的计算)。后面的逻辑就是让新旧发布者id和dep进行交换，方便下次发布者发布消息后的清除操作。
小结
vue中的发布订阅者是在借助Object.defineProperty将数据变成响应式的过程中定义了dep，在get过程中dep对于订阅者的加入进行处理，在set修改数据的过程中dep通知订阅者进行相应的操作。
以上就是vue2从数据变化到视图变化发布订阅模式详解的详细内容，更多关于vue2数据视图变化发布订阅模式的资料请关注脚本之家其它相关文章！