1、传入options，调用Vue初始函数，如：
new Vue({
data(){
return {},
}
}) ，时会调用Vue._init（options）方法。
2、_init方法会完成对生命周期、时间、渲染函数、state初始化，然后将vm挂载到el上，源码如下：
Step1: /vue/scr/core/instance/index.js
function Vue (options) {
if (process.env.NODE_ENV !== 'production' &&
!(this instanceof Vue)
) {
warn('Vue is a constructor and should be called with the `new` keyword')
}
this._init(options)
}
Step2: /vue/src/core/instance/init.js
...
import { initState } from './state'
...
Vue.prototype._init = function(options:Object){
const vm:Component = this; //将vm指向this
//此处省去一些代码，对options做了一些判断
vm._self = vm
initLifecycle(vm) //初始化生命周期
initEvents(vm) //初始化事件
initRender(vm) //初始化渲染函数
callHook(vm, 'beforeCreate') //执行beforeCreate 钩子
initInjections(vm) // resolve injections before data/props
initState(vm) //初始化数据观测
initProvide(vm) // resolve provide after data/props
callHook(vm, 'created') //执行created钩子
if (vm.$options.el) {
vm.$mount(vm.$options.el) //判定options中是否有el选项，如有则将vue挂载
}
}
vue响应性是其最核心的功能，其中很重要的一个部分是实现对数据的监听，其中vue2.X是运用ES5 Object.defineProperty特性来进行数据监听的，为了便于理解，我们看如下范例：
var object = { name: "wh", age: "18" };
function defineReactive(object,key){
var val = object[key];
Object.defineProperty(object, key, {
get() {
console.log("set" + key);
const value = val;
return value;
},
set(newVal) {
console.log("set" + key);
val = newVal;
},
});
}
var keys = Object.keys(object);
for (var i=0;i<keys.length;i++) {
defineReactive(object,keys[i]);
}
object.name = "kongzhi";
console.log(object.name);
object.age = "20";
console.log(object.age);
console.log(object.name);
运行后结果为：
setname
kongzhi
setage
20
kongzhi
通过上述代码运行，我们可以发现，运用Object.defineProperty对数据进行劫持后，每当调用数据则会触发get方法，而每当改变数据则会调用set方法。
VUE是通过对监听器、订阅器和订阅者的实现来实现响应性的，源码如下：
next Step1：/vue/src/core/instance/state.js
import {
set,
del,
observe,
defineReactive,
toggleObserving
} from /vue/src/core/observer/index
export function initState (vm: Component) {
//vm 即为vue实例
vm._watchers = []
const opts = vm.$options
if (opts.props) initProps(vm, opts.props)
if (opts.methods) initMethods(vm, opts.methods)
if (opts.data) {
//若存在data 选项则执行initDate
initData(vm)
} else {
observe(vm._data = {}, true /* asRootData */)
}
if (opts.computed) initComputed(vm, opts.computed)
if (opts.watch && opts.watch !== nativeWatch) {
initWatch(vm, opts.watch)
}
}
function initData (vm: Component) {
let data = vm.$options.data
//data选项类型是否为function，禁止订阅器订阅
data = vm._data = typeof data === 'function'
? getData(data, vm)
: data || {}
if (!isPlainObject(data)) {
data = {}
process.env.NODE_ENV !== 'production' && warn(
'data functions should return an object:\n' +
'https://vuejs.org/v2/guide/components.html#data-Must-Be-a-Function',
vm
)
}
// proxy data on instance
const keys = Object.keys(data)
const props = vm.$options.props
const methods = vm.$options.methods
let i = keys.length
//判定data 和 methods、props 选项是否重名，若存在重名则抛出警告。
while (i--) {
const key = keys[i]
if (process.env.NODE_ENV !== 'production') {
if (methods && hasOwn(methods, key)) {
warn(
`Method "${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)
}
}
// 重点：调用observe函数对data进行监听
observe(data, true /* asRootData */)
}
next Step2：/vue/src/coreobserver/index.js
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
) {
//若data未被监听，则添加监听器
ob = new Observer(value)
}
if (asRootData && ob) {
ob.vmCount++
}
return ob
}
//监听器类
export class Observer {
value: any;
//监听器
dep: Dep;
vmCount: number; // number of vms that have this object as root $data
constructor (value: any) {
this.value = value
//初始化观测器
this.dep = new Dep()
this.vmCount = 0
//为data添加__ob__属性
def(value, '__ob__', this)
//实现数组监听
if (Array.isArray(value)) {
if (hasProto) {
protoAugment(value, arrayMethods)
} else {
copyAugment(value, arrayMethods, arrayKeys)
}
this.observeArray(value)
} else {
//实现对象监听
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++) {
//遍历data对象属性，添加监听器
defineReactive(obj, keys[i])
}
}
}
/**
* Define a reactive property on an Object.
*/
export function defineReactive (
obj: Object,
key: string,
val: any,
customSetter?: ?Function,
shallow?: boolean
) {
//初始化订阅器
const dep = new Dep()
//...省略一堆代码
//若值为对象则递归添加监听器
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) {
//若数据被调用，则订阅者者收集订阅器
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
}
//若数据被修改，则对新数据重新添加观测器
childOb = !shallow && observe(newVal)
//调用监听器notify方法
dep.notify()
}
})
}
next Step2：/vue/src/core/observer/dep.js
export default class Dep {
static target: ?Watcher;
id: number;
subs: Array<Watcher>;
constructor () {
this.id = uid++
this.subs = []
}
//监听器可以实现对监听者添加和删除
addSub (sub: Watcher) {
this.subs.push(sub)
}
removeSub (sub: Watcher) {
remove(this.subs, sub)
}
depend () {
if (Dep.target) {
Dep.target.addDep(this)
}
}
//当数据被修改，调用notify方法
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++) {
//循环遍历监听者，调用监听者update()方法
subs[i].update()
}
}
}
next Step3：/vue/src/core/observer/watcher.js
update () {
/* istanbul ignore else */
if (this.lazy) {
this.dirty = true
} else if (this.sync) {
//若为异步则调用run方法
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 {
//触发监听者回调函数更新视图
this.cb.call(this.vm, value, oldValue)
} catch (e) {
handleError(e, this.vm, `callback for watcher "${this.expression}"`)
}
} else {
//触发监听者回调函数更新视图
this.cb.call(this.vm, value, oldValue)
}
}
}
}
next Step4：/vue/src/core/observer/scheduler.js
import { queueWatcher } from './scheduler'
export const MAX_UPDATE_COUNT = 100
const queue: Array<Watcher> = []
const activatedChildren: Array<Component> = []
let has: { [key: number]: ?true } = {}
let circular: { [key: number]: number } = {}
let waiting = false
let flushing = false
let index = 0
export function queueWatcher (watcher: Watcher) {
const id = watcher.id
if (has[id] == null) {
//判断监听者id是否存在，若不存在，则加入has
has[id] = true
if (!flushing) {
//若还未刷新则将监听者加入队列
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--
}
将最后一个监听者替换此前监听者
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)
}
}
}
