vue2从数据变化到视图变化发布订阅模式详解
目录
- 引言
- 一、发布订阅者模式的特点
- 二、vue中的发布订阅者模式
- 1、dep
- 2、Object.defineProperty
- 3、watcher
- 4、dep.depend
- 5、dep.notify
- 6、订阅者取消订阅
- 小结
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<any>) { 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<Watcher>; 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 $watch() api and directives. */ export default class Watcher { vm: Component; expression: string; cb: Function; id: number; deep: boolean; user: boolean; lazy: boolean; sync: boolean; dirty: boolean; active: boolean; deps: Array<Dep>; newDeps: Array<Dep>; depIds: SimpleSet; newDepIds: SimpleSet; before: ?Function; getter: Function; value: any; constructor ( vm: Component, expOrFn: string | Function, cb: Function, options?: ?Object, isRenderWatcher?: boolean ) { this.vm = vm if (isRenderWatcher) { vm._watcher = this } vm._watchers.push(this) // options if (options) { this.deep = !!options.deep this.user = !!options.user this.lazy = !!options.lazy this.sync = !!options.sync this.before = options.before } else { this.deep = this.user = this.lazy = this.sync = false } this.cb = cb this.id = ++uid // uid for batching this.active = true this.dirty = this.lazy // for lazy watchers this.deps = [] this.newDeps = [] this.depIds = new Set() this.newDepIds = new Set() this.expression = process.env.NODE_ENV !== 'production' ? expOrFn.toString() : '' // parse expression for getter if (typeof expOrFn === 'function') { this.getter = expOrFn } else { this.getter = parsePath(expOrFn) if (!this.getter) { this.getter = noop process.env.NODE_ENV !== 'production' && warn( `Failed watching path: "${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我们其它相关文章!