Vue实例挂载的过程发生了什么

 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)
 }

initMixin(Vue);     // 定义 _init
stateMixin(Vue);    // 定义 $set $get $delete $watch 等
eventsMixin(Vue);   // 定义事件  $on  $once $off $emit
lifecycleMixin(Vue);// 定义 _update  $forceUpdate  $destroy
renderMixin(Vue);   // 定义 _render 返回虚拟dom

 Vue.prototype._init = function (options?: Object) {
 const vm: Component = this
 // a uid
     vm._uid = uid++
let startTag, endTag
/* istanbul ignore if */
if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
      startTag = `vue-perf-start:${vm._uid}`
      endTag = `vue-perf-end:${vm._uid}`
 mark(startTag)
     }
 // a flag to avoid this being observed
     vm._isVue = true
 // merge options
 // 合并属性，判断初始化的是否是组件，这里合并主要是 mixins 或 extends 的方法
 if (options && options._isComponent) {
 // optimize internal component instantiation
 // since dynamic options merging is pretty slow, and none of the
 // internal component options needs special treatment.
 initInternalComponent(vm, options)
     } else { // 合并vue属性
       vm.$options = mergeOptions(
 resolveConstructorOptions(vm.constructor),
         options || {},
         vm
       )
     }
 /* istanbul ignore else */
 if (process.env.NODE_ENV !== 'production') {
 // 初始化proxy拦截器
 initProxy(vm)
     } else {
       vm._renderProxy = vm
     }
 // expose real self
     vm._self = vm
 // 初始化组件生命周期标志位
 initLifecycle(vm)
 // 初始化组件事件侦听
 initEvents(vm)
 // 初始化渲染方法
 initRender(vm)
 callHook(vm, 'beforeCreate')
 // 初始化依赖注入内容，在初始化data、props之前
 initInjections(vm) // resolve injections before data/props
 // 初始化props/data/method/watch/methods
 initState(vm)
 initProvide(vm) // resolve provide after data/props
 callHook(vm, 'created')
 /* istanbul ignore if */
 if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
       vm._name = formatComponentName(vm, false)
 mark(endTag)
 measure(`vue ${vm._name} init`, startTag, endTag)
     }
 // 挂载元素
 if (vm.$options.el) {
       vm.$mount(vm.$options.el)
     }
   }

 export function initState (vm: Component) {
 // 初始化组件的watcher列表
   vm._watchers = []
 const opts = vm.$options
 // 初始化props
 if (opts.props) initProps(vm, opts.props)
// 初始化methods方法
if (opts.methods) initMethods(vm, opts.methods)
if (opts.data) {
 // 初始化data  
 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.$options.data
 // 获取到组件上的data
   data = vm._data = typeof data === 'function'
     ? getData(data, vm)
     : data || {}
 if (!isPlainObject(data)) {
     data = {}
    process.env.NODE_ENV !== 'production' && warn(
 'data functions should return an object:\n' +
 'https://vuejs.org/v2/guide/components.html#data-Must-Be-a-Function',
       vm
     )
   }
 // proxy data on instance
 const keys = Object.keys(data)
 const props = vm.$options.props
 const methods = vm.$options.methods
 let i = keys.length
 while (i--) {
 const key = keys[i]
 if (process.env.NODE_ENV !== 'production') {
 // 属性名不能与方法名重复
 if (methods && hasOwn(methods, key)) {
 warn(
 `Method "${key}" has already been defined as a data property.`,
           vm
         )
       }
     }
 // 属性名不能与state名称重复
 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)) { // 验证key值的合法性
 // 将_data中的数据挂载到组件vm上,这样就可以通过this.xxx访问到组件上的数据
 proxy(vm, `_data`, key)
     }
   }
 // observe data
 // 响应式监听data是数据的变化
 observe(data, true /* asRootData */)
 }

 Vue.prototype.$mount = function (
   el?: string | Element,
   hydrating?: boolean
 ): Component {
 // 获取或查询元素
   el = el && query(el)
/* istanbul ignore if */
// vue 不允许直接挂载到body或页面文档上
 if (el === document.body || el === document.documentElement) {
     process.env.NODE_ENV !== 'production' && warn(
 `Do not mount Vue to <html> or <body> - mount to normal elements instead.`
     )
 return this
   }
 const options = this.$options
 // resolve template/el and convert to render function
 if (!options.render) {
 let template = options.template
 // 存在template模板，解析vue模板文件
 if (template) {
 if (typeof template === 'string') {
 if (template.charAt(0) === '#') {
           template = idToTemplate(template)
 /* istanbul ignore if */
 if (process.env.NODE_ENV !== 'production' && !template) {
 warn(
 `Template element not found or is empty: ${options.template}`,
 this
             )
           }
         }
       } else if (template.nodeType) {
         template = template.innerHTML
       } else {
 if (process.env.NODE_ENV !== 'production') {
 warn('invalid template option:' + template, this)
         }
 return this
       }
     } else if (el) {
 // 通过选择器获取元素内容
       template = getOuterHTML(el)
     }
 if (template) {
 /* istanbul ignore if */
 if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
 mark('compile')
       }
 /**
        *  1.将temmplate解析ast tree
        *  2.将ast tree转换成render语法字符串
        *  3.生成render方法
        */
 const { render, staticRenderFns } = compileToFunctions(template, {
 outputSourceRange: process.env.NODE_ENV !== 'production',
         shouldDecodeNewlines,
         shouldDecodeNewlinesForHref,
 delimiters: options.delimiters,
 comments: options.comments
       }, this)
       options.render = render
       options.staticRenderFns = staticRenderFns
 /* istanbul ignore if */
 if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
 mark('compile end')
 measure(`vue ${this._name} compile`, 'compile', 'compile end')
       }
     }
   }
 return mount.call(this, el, hydrating)
 }

 // public mount method
 Vue.prototype.$mount = function (
   el?: string | Element,
   hydrating?: boolean
 ): Component {
   el = el && inBrowser ? query(el) : undefined
 // 渲染组件
 return mountComponent(this, el, hydrating)
 }

export function mountComponent (
vm: Component,
el: ?Element,
  hydrating?: boolean
): Component {
  vm.$el = el
// 如果没有获取解析的render函数，则会抛出警告
// render是解析模板文件生成的
if (!vm.$options.render) {
     vm.$options.render = createEmptyVNode
 if (process.env.NODE_ENV !== 'production') {
 /* istanbul ignore if */
 if ((vm.$options.template && vm.$options.template.charAt(0) !== '#') ||
         vm.$options.el || el) {
 warn(
 'You are using the runtime-only build of Vue where the template ' +
 'compiler is not available. Either pre-compile the templates into ' +
 'render functions, or use the compiler-included build.',
           vm
         )
       } else {
 // 没有获取到vue的模板文件
 warn(
 'Failed to mount component: template or render function not defined.',
           vm
         )
       }
     }
   }
 // 执行beforeMount钩子
 callHook(vm, 'beforeMount')
 let updateComponent
 /* istanbul ignore if */
 if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
     updateComponent = () => {
 const name = vm._name
 const id = vm._uid
 const startTag = `vue-perf-start:${id}`
 const endTag = `vue-perf-end:${id}`
 mark(startTag)
 const vnode = vm._render()
 mark(endTag)
 measure(`vue ${name} render`, startTag, endTag)
 mark(startTag)
       vm._update(vnode, hydrating)
 mark(endTag)
 measure(`vue ${name} patch`, startTag, endTag)
     }
   } else {
 // 定义更新函数
     updateComponent = () => {
 // 实际调⽤是在lifeCycleMixin中定义的_update和renderMixin中定义的_render
       vm._update(vm._render(), hydrating)
     }
   }
 // we set this to vm._watcher inside the watcher's constructor
 // since the watcher's initial patch may call $forceUpdate (e.g. inside child
 // component's mounted hook), which relies on vm._watcher being already defined
 // 监听当前组件状态，当有数据变化时，更新组件
 new Watcher(vm, updateComponent, noop, {
     before () {
 if (vm._isMounted && !vm._isDestroyed) {
 // 数据更新引发的组件更新
 callHook(vm, 'beforeUpdate')
       }
     }
   }, true /* isRenderWatcher */)
   hydrating = false
 // manually mounted instance, call mounted on self
 // mounted is called for render-created child components in its inserted hook
 if (vm.$vnode == null) {
     vm._isMounted = true
 callHook(vm, 'mounted')
   }
 return vm
 }

// 定义vue 原型上的render方法
Vue.prototype._render = function (): VNode {
const vm: Component = this
// render函数来自于组件的option
const { render, _parentVnode } = vm.$options
if (_parentVnode) {
        vm.$scopedSlots = normalizeScopedSlots(
            _parentVnode.data.scopedSlots,
             vm.$slots,
             vm.$scopedSlots
         )
     }
 // set parent vnode. this allows render functions to have access
 // to the data on the placeholder node.
     vm.$vnode = _parentVnode
 // render self
 let vnode
 try {
 // There's no need to maintain a stack because all render fns are called
 // separately from one another. Nested component's render fns are called
 // when parent component is patched.
         currentRenderingInstance = vm
 // 调用render方法，自己的独特的render方法， 传入createElement参数，生成vNode
         vnode = render.call(vm._renderProxy, vm.$createElement)
     } catch (e) {
 handleError(e, vm, `render`)
 // return error render result,
 // or previous vnode to prevent render error causing blank component
 /* istanbul ignore else */
 if (process.env.NODE_ENV !== 'production' && vm.$options.renderError) {
 try {
                 vnode = vm.$options.renderError.call(vm._renderProxy, vm.$createElement, e)
             } catch (e) {
 handleError(e, vm, `renderError`)
                 vnode = vm._vnode
             }
         } else {
             vnode = vm._vnode
         }
     } finally {
         currentRenderingInstance = null
     }
 // if the returned array contains only a single node, allow it
 if (Array.isArray(vnode) && vnode.length === 1) {
         vnode = vnode[0]
     }
 // return empty vnode in case the render function errored out
 if (!(vnode instanceof VNode)) {
 if (process.env.NODE_ENV !== 'production' && Array.isArray(vnode)) {
 warn(
 'Multiple root nodes returned from render function. Render function ' +
 'should return a single root node.',
                 vm
             )
         }
         vnode = createEmptyVNode()
     }
 // set parent
     vnode.parent = _parentVnode
 return vnode
 }

Vue.prototype._update = function (vnode: VNode, hydrating?: boolean) {
const vm: Component = this
const prevEl = vm.$el
const prevVnode = vm._vnode
// 设置当前激活的作用域
const restoreActiveInstance = setActiveInstance(vm)
    vm._vnode = vnode
// Vue.prototype.__patch__ is injected in entry points
// based on the rendering backend used.
 if (!prevVnode) {
 // initial render
 // 执行具体的挂载逻辑
       vm.$el = vm.__patch__(vm.$el, vnode, hydrating, false /* removeOnly */)
     } else {
 // updates
       vm.$el = vm.__patch__(prevVnode, vnode)
     }
 restoreActiveInstance()
 // update __vue__ reference
 if (prevEl) {
       prevEl.__vue__ = null
     }
 if (vm.$el) {
       vm.$el.__vue__ = vm
     }
 // if parent is an HOC, update its $el as well
 if (vm.$vnode && vm.$parent && vm.$vnode === vm.$parent._vnode) {
       vm.$parent.$el = vm.$el
     }
 // updated hook is called by the scheduler to ensure that children are
 // updated in a parent's updated hook.
   }