要了解new Vue()，发生了什么其实就是要找到vue 是在哪里定义的：
src/core/index.js

import Vue from './instance/index'

// 1.初始化全局API：component/filter/directive/use/mixin/util/extend
initGlobalAPI(Vue)

src/core/instance/index.js

import { initMixin } from './init'
import { stateMixin } from './state'
import { renderMixin } from './render'
import { eventsMixin } from './events'
import { lifecycleMixin } from './lifecycle'
import { warn } from '../util/index'

// 1.声明构造函数
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)
}

// 2.实例属性，实例方法：
initMixin(Vue) // _init()
stateMixin(Vue) // $data/$props/$set()/$delete()/$watch()
eventsMixin(Vue) // $emit()/$on/$off()/$once()
lifecycleMixin(Vue) // _update()/$forceUpdate()/$destroy()
renderMixin(Vue) // $nextTick()/_render()

export default Vue

/* @flow */

import config from '../config'
import { initProxy } from './proxy'
import { initState } from './state'
import { initRender } from './render'
import { initEvents } from './events'
import { mark, measure } from '../util/perf'
import { initLifecycle, callHook } from './lifecycle'
import { initProvide, initInjections } from './inject'
import { extend, mergeOptions, formatComponentName } from '../util/index'

let uid = 0

export function initMixin (Vue: Class<Component>) {
  // 实现_init初始化方法
  Vue.prototype._init = function (options?: Object) {
    const vm: Component = this
    // a uid
    vm._uid = uid++

    // a flag to avoid this being observed
    vm._isVue = true

    // 1.合并选项
    // merge options
    if (options && options._isComponent) {
      initInternalComponent(vm, options)
    } else {
      vm.$options = mergeOptions(
        resolveConstructorOptions(vm.constructor),
        options || {},
        vm
      )
    }
    /* istanbul ignore else */
    if (process.env.NODE_ENV !== 'production') {
      initProxy(vm)
    } else {
      vm._renderProxy = vm
    }
    // expose real self

    // 初始化核心逻辑
    vm._self = vm
    initLifecycle(vm) // $parent/$root/...
    initEvents(vm) // 自定义事件监听
    initRender(vm) // $slots/$createElement
    callHook(vm, 'beforeCreate') 
    initInjections(vm) // resolve injections before data/props
    initState(vm) // props/methods/data/computed/watch
    initProvide(vm) // resolve provide after data/props
    callHook(vm, 'created')

    // 当设置了el选项时，自动调用$mount
    if (vm.$options.el) {
      vm.$mount(vm.$options.el)
    }
  }
}

看到这里其实vue的生命周期图也就理解了

new()，依次实现以下：

1. 初始化全局API：component/filter/directive/use/mixin/util/extend
2. 实例属性，实例方法：

initMixin(Vue) // _init()
stateMixin(Vue) // $data/$props/$set()/$delete()/$watch()
eventsMixin(Vue) // $emit()/$on/$off()/$once()
lifecycleMixin(Vue) // _update()/$forceUpdate()/$destroy()
renderMixin(Vue) // $nextTick()/_render()

3. _init核心逻辑

// 初始化核心逻辑
    vm._self = vm
    initLifecycle(vm) // $parent/$root/...
    initEvents(vm) // 自定义事件监听
    initRender(vm) // $slots/$createElement
    callHook(vm, 'beforeCreate') 
    initInjections(vm) // resolve injections before data/props
    initState(vm) // props/methods/data/computed/watch
    initProvide(vm) // resolve provide after data/props
    callHook(vm, 'created')

    // 当设置了el选项时，自动调用$mount
    if (vm.$options.el) {
      vm.$mount(vm.$options.el)
    }