寫在前面
我們在開發過程中可能會存在這樣的疑問：
- Vue.js把哪些物件變成了響應式物件？
- Vue.js究竟是如何響應式修改資料的？
- 上面這幅圖的下半部分是怎樣一個執行流程？
- 為什麼資料有時是延時的（即什麼情況下要用到nextTick）？
實現一個簡易版的響應式系統
響應式系統核心的程式碼定義在src/core/observer中：
/**
* Dep是資料和Watcher之間的橋樑，主要實現了以下兩個功能：
* 1.用 addSub 方法可以在目前的 Dep 物件中增加一個 Watcher 的訂閱操作；
* 2.用 notify 方法通知目前 Dep 物件的 subs 中的所有 Watcher 物件觸發更新操作。
*/
class Dep {
constructor () {
// 用來存放Watcher物件的陣列
this.subs = [];
}
addSub (sub) {
// 往subs中新增Watcher物件
this.subs.push(sub);
}
// 通知所有Watcher物件更新檢視
notify () {
this.subs.forEach((sub) => {
sub.update();
})
}
}
// 觀察者物件
class Watcher {
constructor () {
// Dep.target表示當前全域性正在計算的Watcher（當前的Watcher物件），在get中會用到
Dep.target = this;
}
// 更新檢視
update () {
console.log("檢視更新啦");
}
}
Dep.target = null;
class Vue {
// Vue構造類
constructor(options) {
this._data = options.data;
this.observer(this._data);
// 例項化Watcher觀察者物件，這時候Dep.target會指向這個Watcher物件
new Watcher();
console.log('render', this._data.message);
}
// 對Object.defineProperty進行封裝，給物件動態新增setter和getter
defineReactive (obj, key, val) {
const dep = new Dep();
Object.defineProperty(obj, key, {
enumerable: true,
configurable: true,
get: function reactiveGetter () {
// 往dep中新增Dep.target（當前正在進行的Watcher物件）
dep.addSub(Dep.target);
return val;
},
set: function reactiveSetter (newVal) {
if (newVal === val) return;
// 在set的時候通知dep的notify方法來通知所有的Wacther物件更新檢視
dep.notify();
}
});
}
// 對傳進來的物件進行遍歷執行defineReactive
observer (value) {
if (!value || (typeof value !== 'object')) {
return;
}
Object.keys(value).forEach((key) => {
this.defineReactive(value, key, value[key]);
});
}
}
let obj = new Vue({
el: "#app",
data: {
message: 'test'
}
})
obj._data.message = 'update'
複製程式碼執行以上程式碼，列印出來的資訊為：
render test
檢視更新啦
複製程式碼下面結合Vue.js原始碼來分析它的流程：
Object.defineProperty()
我們都知道響應式的核心是利用來ES5的Object.defineProperty()方法，這也是Vue.js不支援IE9一下的原因，而且現在也沒有什麼好的補丁來修復這個問題。具體的可以參考MDN文件。這是它的使用方法：
/*
obj: 目標物件
prop: 需要操作的目標物件的屬性名
descriptor: 描述符
return value 傳入物件
*/
Object.defineProperty(obj, prop, descriptor)
複製程式碼其中descriptor有兩個非常核心的屬性：get和set。在我們訪問一個屬性的時候會觸發getter方法，當我們對一個屬性做修改的時候會觸發setter方法。當一個物件擁有來getter方法和setter方法，我們可以稱這個物件為響應式物件。
從new Vue()開始
Vue實際上是一個用Function實現的類，定義在src/core/instance/index.js中：
Observer
接下來看Observer的定義，在/src/core/observer/index.js中：
/**
* 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 has 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)) {
const augment = hasProto
? protoAugment
: copyAugment
augment(value, arrayMethods, arrayKeys)
this.observeArray(value)
} else {
this.walk(value)
}
}
/**
* Walk through each property 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])
}
}
}
複製程式碼注意看英文註釋，尤大把晦澀難懂的地方都已經用英文註釋寫出來。Observer它的作用就是給物件的屬性新增getter和setter，用來依賴收集和派發更新。walk方法就是把傳進來的物件的屬性遍歷進行defineReactive繫結，observeArray方法就是把傳進來的陣列遍歷進行observe。
defineReactive
接下來看一下defineReative方法，定義在src/core/observer/index.js中：
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()
}
if (setter) {
setter.call(obj, newVal)
} else {
val = newVal
}
childOb = !shallow && observe(newVal)
dep.notify()
}
})
複製程式碼物件的子物件遞迴進行observe並返回子節點的Observer物件：
childOb = !shallow && observe(val)
複製程式碼如果存在當前的Watcher物件，對其進行依賴收集，並對其子物件進行依賴收集，如果是陣列，則對陣列進行依賴收集，如果陣列的子成員還是陣列，則對其遍歷：
if (Dep.target) {
dep.depend()
if (childOb) {
childOb.dep.depend()
if (Array.isArray(value)) {
dependArray(value)
}
}
}
複製程式碼執行set方法的時候，新的值需要observe，保證新的值是響應式的：
childOb = !shallow && observe(newVal)
複製程式碼dep物件會執行notify方法通知所有的Watcher觀察者物件：
dep.notify()
複製程式碼Dep
export default class Dep {
static target: ?Watcher;
id: number;
subs: Array<Watcher>;
constructor () {
this.id = uid++
this.subs = []
}
// 新增一個觀察者
addSub (sub: Watcher) {
this.subs.push(sub)
}
// 移除一個觀察者
removeSub (sub: Watcher) {
remove(this.subs, sub)
}
// 依賴收集，當存在Dep.target的時候新增Watcher觀察者物件
depend () {
if (Dep.target) {
Dep.target.addDep(this)
}
}
// 通知所有訂閱者
notify () {
// stabilize the subscriber list first
const subs = this.subs.slice()
for (let i = 0, l = subs.length; i < l; i++) {
subs[i].update()
}
}
}
// the current target watcher being evaluated.
// this is globally unique because there could be only one
// watcher being evaluated at any time.
Dep.target = null
// 收集完依賴之後，將Dep.target設定為null，防止繼續收集依賴
複製程式碼Watcher
Watcher是一個觀察者物件，依賴收集以後Watcher物件會被儲存在Deps中，資料變動的時候會由Deps通知Watcher例項。定義在/src/core/observer/watcher.js中：
/**
* A watcher parses an expression, collects dependencies,
* and fires callback when the expression value changes.
* This is used for both the $watch() api and directives.
*/
export default class Watcher {
vm: Component;
expression: string;
cb: Function;
id: number;
deep: boolean;
user: boolean;
computed: boolean;
sync: boolean;
dirty: boolean;
active: boolean;
dep: Dep;
deps: Array<Dep>;
newDeps: Array<Dep>;
depIds: SimpleSet;
newDepIds: SimpleSet;
before: ?Function;
getter: Function;
value: any;
constructor (
vm: Component,
expOrFn: string | Function,
cb: Function,
options?: ?Object,
isRenderWatcher?: boolean
) {
this.vm = vm
if (isRenderWatcher) {
vm._watcher = this
}
vm._watchers.push(this)
// options
if (options) {
this.deep = !!options.deep
this.user = !!options.user
this.computed = !!options.computed
this.sync = !!options.sync
this.before = options.before
} else {
this.deep = this.user = this.computed = this.sync = false
}
this.cb = cb
this.id = ++uid // uid for batching
this.active = true
this.dirty = this.computed // for computed watchers
this.deps = []
this.newDeps = []
this.depIds = new Set()
this.newDepIds = new Set()
this.expression = process.env.NODE_ENV !== 'production'
? expOrFn.toString()
: ''
// parse expression for getter
if (typeof expOrFn === 'function') {
this.getter = expOrFn
} else {
this.getter = parsePath(expOrFn)
if (!this.getter) {
this.getter = function () {}
process.env.NODE_ENV !== 'production' && warn(
`Failed watching path: "${expOrFn}" ` +
'Watcher only accepts simple dot-delimited paths. ' +
'For full control, use a function instead.',
vm
)
}
}
if (this.computed) {
this.value = undefined
this.dep = new Dep()
} else {
this.value = 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.computed) {
// A computed property watcher has two modes: lazy and activated.
// It initializes as lazy by default, and only becomes activated when
// it is depended on by at least one subscriber, which is typically
// another computed property or a component's render function.
if (this.dep.subs.length === 0) {
// In lazy mode, we don't want to perform computations until necessary,
// so we simply mark the watcher as dirty. The actual computation is
// performed just-in-time in this.evaluate() when the computed property
// is accessed.
this.dirty = true
} else {
// In activated mode, we want to proactively perform the computation
// but only notify our subscribers when the value has indeed changed.
this.getAndInvoke(() => {
this.dep.notify()
})
}
} else if (this.sync) {
this.run()
} else {
queueWatcher(this)
}
}
/**
* Scheduler job interface.
* Will be called by the scheduler.
*/
run () {
if (this.active) {
this.getAndInvoke(this.cb)
}
}
getAndInvoke (cb: Function) {
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
this.dirty = false
if (this.user) {
try {
cb.call(this.vm, value, oldValue)
} catch (e) {
handleError(e, this.vm, `callback for watcher "${this.expression}"`)
}
} else {
cb.call(this.vm, value, oldValue)
}
}
}
/**
* Evaluate and return the value of the watcher.
* This only gets called for computed property watchers.
*/
evaluate () {
if (this.dirty) {
this.value = this.get()
this.dirty = false
}
return this.value
}
/**
* Depend on this watcher. Only for computed property watchers.
*/
depend () {
if (this.dep && Dep.target) {
this.dep.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
}
}
}
複製程式碼