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Vue源码解读-观察者模式

2019-10-08 15:05 295 查看

本文将讲解Vue的响应式原理，结合Vue的源码进行分析。响应式原理，也就是数据改变，页面展示也将重新渲染，实现页面无刷新更新。这里使用的是前端开发模式中的观察者模式：Vue中每个组件实例都对应一个 watcher 实例，它会在组件渲染的过程中把“接触”过的数据属性记录为依赖。之后当依赖项的 setter 触发时，会通知 watcher，从而使它关联的组件重新渲染。

了解了观察者模式后，Vue具体是怎么实现的呢，下面开始上源码。

入口在src/core/observer/index/js中的defineReactive方法

[code]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()
}
})
}

方法接收五个参数，obj为要实现响应式的对象，key为具体响应的obj的key，val为obj中对应key的value，customSetter为一个函数，可以让用户自定义一个setter，shallow为是否为浅响应的bool值。

方法的开始就new了一个Dep，Dep是订阅者Watcher对应的数据依赖，来看下Dep的定义，在同级目录的dep.js。

[code]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()
}
}
}

首先这里维护了一个全局的uid，他是唯一的，新增一个Dep，就会自增，以此来标识这个Dep的唯一性。Dep维护了三个属性，target、id和subs。target是Watcher对象，是当前Dep的目标，subs是Dep订阅的Watcher数组。方法有新增和删除依赖，用以操作subs数组。depend方法是把Dep对象添加进Watcher中。notify便是遍历subs数组，按id顺序调用subs里每个Watcher的update方法。

看到这里肯定就会想知道Watcher又是怎样的一个对象，在同级目录的watcher.js中。

[code]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
}

/**
* 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
}
}
}

代码片段较长，这里一个function一个function的解读。

Watcher是什么，注释写的很清楚，大概意思就是观察者解析表达式、收集依赖、当数据改变时调用回调函数。他被用于$watch()的api和指令。和Dep一样，也维护了一个uid。可以看到Watcher的属性有很多，能从字面上理解的只有一部分：vm就是组件对象，expression是表达式，deep是深观察的标记位，lazy是否为懒加载，active是否启用，deps是数据依赖Dep的数组，newDeps为新增的数据依赖Dep数组，depIds和newDepIds分别为对应的Dep的id集合，getter和value自不必说。

先来看构造函数：

[code]  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()
}

乍一看写了挺多，其实没啥。构造函数接收5个参数，vm为组件，expOrFn为表达式或者函数，用作Watcher的getter，cb是个函数，暂时还不知道是个啥，options为参数对象，isRenderWatcher标记位看代码的作用是确认该Watcher是否为组件的唯一Watcher。下面就是将接收的参数设置到Watcher的属性中。

接着来看get的定义。

[code]/**
* 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
}

这里涉及到pushTarget、popTarget、traverse和cleanupDeps。

[code]const targetStack = []

export function pushTarget (target: ?Watcher) {
targetStack.push(target)
Dep.target = target
}

export function popTarget () {
targetStack.pop()
Dep.target = targetStack[targetStack.length - 1]
}

这两个方法在observer/dep.js中，一个是给Dep的target赋值Watcher并在targetStack数组中push，一个是pop掉targetStack并让target指向stack里的前一个。

[code]const seenObjects = new Set()

/**
* Recursively traverse an object to evoke all converted
* getters, so that every nested property inside the object
* is collected as a "deep" dependency.
*/
export function traverse (val: any) {
_traverse(val, seenObjects)
seenObjects.clear()
}

function _traverse (val: any, seen: SimpleSet) {
let i, keys
const isA = Array.isArray(val)
if ((!isA && !isObject(val)) || Object.isFrozen(val) || val instanceof VNode) {
return
}
if (val.__ob__) {
const depId = val.__ob__.dep.id
if (seen.has(depId)) {
return
}
seen.add(depId)
}
if (isA) {
i = val.length
while (i--) _traverse(val[i], seen)
} else {
keys = Object.keys(val)
i = keys.length
while (i--) _traverse(val[keys[i]], seen)
}
}

traverse在observer/traverse.js中，其实就是递归遍历val，这里用到val.__ob__的判断，只要val被观察了，它就会有__ob__属性，这里其实就是判断val是否被观察过了。

[code]  /**
* 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
}

cleanupDeps为watcher的一个方法，就是清除所有依赖，并更新了相关属性。

这样看来get方法就是收集对象的依赖，添加到依赖中，实现订阅。

接下来看Watcher的addDep方法。

[code]  /**
* 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)
}
}
}

该方法用于向当前指令增加一个数据依赖，这里用到了Dep的id去判断是否已存在，避免重复添加。

接着就是当数据更新时调用的update方法了：

[code]  /**
* 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)
}
}

如果是懒加载，就把dirty设为true，在下一个tick时再更新，如果是同步的，就调用run方法，立即执行更新，异步就调用queueWatcher方法，把其添加到队列中，下一个tick时一起更新。

先看run方法：

[code]  /**
* 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)
}
}
}
}

如果Watcher是启用状态，就更新数据，并且调用cb方法，此时就明白了，cb方法就是更新组件触发重新渲染的方法。

再看queueWatcher方法，此方法在scheduler.js中：

[code]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)
}
}
}

queue就是一个Watcher的数组，如果还没刷新，就添加进去，如果已经刷新了，就等到下一个tick再去做。

接着看Watcher的evaluate方法：

[code]  /**
* Evaluate the value of the watcher.
* This only gets called for lazy watchers.
*/
evaluate () {
this.value = this.get()
this.dirty = false
}

就是对于懒加载来说，用户可能希望立即更新，就可以调用这个方法，立即获得最新值，并且更新并渲染。

[code]  /**
* Depend on all deps collected by this watcher.
*/
depend () {
let i = this.deps.length
while (i--) {
this.deps[i].depend()
}
}

depend就是调用deps数组里所有Dep的depend方法。

最后一个Watcher的方法tearDown：

[code]  /**
* 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
}
}
}

应该是用于销毁所有的数据依赖订阅列表，然后把active设为false。这样就不会再触发更新渲染了。

至此，所有与响应式原理相关的Vue源码解读完毕。

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