Vue 源码解析 #

npm i flow-bin -g

flow init

// @flow

function split (str) {
  return str.split(' ')
}

split(11);

// @flow

function add (x, y) {
  return x + y
}

add('Hello', 11);

// @flow

function add (x: number, y: number): number {
  return x + y
}

add('Hello', 11);

// @flow

var arr: Array<number> = [1, 2, 3];

arr.push('Hello');

// @flow

class Bar {
  x: string;           // x 是字符串
  y: string | number;  // y 可以是字符串或者数字
  z: boolean;

  constructor (x: string, y: string | number) {
    this.x = x;
    this.y = y;
    this.z = false;
  }
}

var bar: Bar = new Bar('hello', 4);

// @flow

var obj: { a: string, b: number, c: Array<string>, d: Bar } = {
  a: 'hello',
  b: 11,
  c: ['hello', 'world'],
  d: new Bar('hello', 3)
}

// @flow

var foo: ?string = null;

[ignore]
.*/node_modules/.*
.*/test/.*
.*/scripts/.*
.*/examples/.*
.*/benchmarks/.*

[include]

[libs]
flow

[options]
unsafe.enable_getters_and_setters=true
module.name_mapper='^compiler/\(.*\)$' -> '<PROJECT_ROOT>/src/compiler/\1'
module.name_mapper='^core/\(.*\)$' -> '<PROJECT_ROOT>/src/core/\1'
module.name_mapper='^shared/\(.*\)$' -> '<PROJECT_ROOT>/src/shared/\1'
module.name_mapper='^web/\(.*\)$' -> '<PROJECT_ROOT>/src/platforms/web/\1'
module.name_mapper='^weex/\(.*\)$' -> '<PROJECT_ROOT>/src/platforms/weex/\1'
module.name_mapper='^server/\(.*\)$' -> '<PROJECT_ROOT>/src/server/\1'
module.name_mapper='^entries/\(.*\)$' -> '<PROJECT_ROOT>/src/entries/\1'
module.name_mapper='^sfc/\(.*\)$' -> '<PROJECT_ROOT>/src/sfc/\1'
suppress_comment= \\(.\\|\n\\)*\\$flow-disable-line

{
"name": "vue",
"version": "2.6.12",
"description": "Reactive, component-oriented view layer for modern web interfaces.",
"main": "dist/vue.runtime.common.js",
"module": "dist/vue.runtime.esm.js",
"unpkg": "dist/vue.js",
"jsdelivr": "dist/vue.js",
"typings": "types/index.d.ts",
"files": [
	"src",
	"dist/*.js",
	"types/*.d.ts"
],
  "sideEffects": false,
  "scripts": {
    "dev": "rollup -w -c scripts/config.js --environment TARGET:web-full-dev",
    "dev:cjs": "rollup -w -c scripts/config.js --environment TARGET:web-runtime-cjs-dev",
    "dev:esm": "rollup -w -c scripts/config.js --environment TARGET:web-runtime-esm",
    "dev:test": "karma start test/unit/karma.dev.config.js",
    "dev:ssr": "rollup -w -c scripts/config.js --environment TARGET:web-server-renderer",
    "dev:compiler": "rollup -w -c scripts/config.js --environment TARGET:web-compiler ",
    "dev:weex": "rollup -w -c scripts/config.js --environment TARGET:weex-framework",
    "dev:weex:factory": "rollup -w -c scripts/config.js --environment TARGET:weex-factory",
    "dev:weex:compiler": "rollup -w -c scripts/config.js --environment TARGET:weex-compiler ",
    "build": "node scripts/build.js",
    "build:ssr": "npm run build -- web-runtime-cjs,web-server-renderer",
    "build:weex": "npm run build -- weex",
    "test": "npm run lint && flow check && npm run test:types && npm run test:cover && npm run test:e2e -- --env phantomjs && npm run test:ssr && npm run test:weex",
    "test:unit": "karma start test/unit/karma.unit.config.js",
    "test:cover": "karma start test/unit/karma.cover.config.js",
    "test:e2e": "npm run build -- web-full-prod,web-server-basic-renderer && node test/e2e/runner.js",
    "test:weex": "npm run build:weex && jasmine JASMINE_CONFIG_PATH=test/weex/jasmine.js",
    "test:ssr": "npm run build:ssr && jasmine JASMINE_CONFIG_PATH=test/ssr/jasmine.js",
    "test:sauce": "npm run sauce -- 0 && npm run sauce -- 1 && npm run sauce -- 2",
    "test:types": "tsc -p ./types/test/tsconfig.json",
    "lint": "eslint src scripts test",
    "flow": "flow check",
    "sauce": "karma start test/unit/karma.sauce.config.js",
    "bench:ssr": "npm run build:ssr && node benchmarks/ssr/renderToString.js && node benchmarks/ssr/renderToStream.js",
    "release": "bash scripts/release.sh",
    "release:weex": "bash scripts/release-weex.sh",
    "release:note": "node scripts/gen-release-note.js",
    "commit": "git-cz"
  },
  "gitHooks": {
    "pre-commit": "lint-staged",
    "commit-msg": "node scripts/verify-commit-msg.js"
  },
  "lint-staged": {
    "*.js": [
      "eslint --fix",
      "git add"
    ]
  },
  "author": "Evan You",
  "license": "MIT",
  "bugs": {
    "url": "https://github.com/vuejs/vue/issues"
  },
  "homepage": "https://github.com/vuejs/vue#readme",
  "config": {
    "commitizen": {
      "path": "./node_modules/cz-conventional-changelog"
    }
  }
}

"scripts": {
	"build": "node scripts/build.js",
	"build:ssr": "npm run build -- web-runtime-cjs,web-server-renderer",
	"build:weex": "npm run build -- weex",
}

let builds = require('./config').getAllBuilds()

// filter builds via command line arg
if (process.argv[2]) {
  const filters = process.argv[2].split(',')
  builds = builds.filter(b => {
    return filters.some(f => b.output.file.indexOf(f) > -1 || b._name.indexOf(f) > -1)
  })
} else {
  // filter out weex builds by default
  builds = builds.filter(b => {
    return b.output.file.indexOf('weex') === -1
  })
}

build(builds)

const builds = {
  // Runtime only (CommonJS). Used by bundlers e.g. Webpack & Browserify
  'web-runtime-cjs-dev': {
    entry: resolve('web/entry-runtime.js'),
    dest: resolve('dist/vue.runtime.common.dev.js'),
    format: 'cjs',
    env: 'development',
    banner
  },
  'web-runtime-cjs-prod': {
    entry: resolve('web/entry-runtime.js'),
    dest: resolve('dist/vue.runtime.common.prod.js'),
    format: 'cjs',
    env: 'production',
    banner
  },
  // Runtime+compiler CommonJS build (CommonJS)
  'web-full-cjs-dev': {
    entry: resolve('web/entry-runtime-with-compiler.js'),
    dest: resolve('dist/vue.common.dev.js'),
    format: 'cjs',
    env: 'development',
    alias: { he: './entity-decoder' },
    banner
  },
  'web-full-cjs-prod': {
    entry: resolve('web/entry-runtime-with-compiler.js'),
    dest: resolve('dist/vue.common.prod.js'),
    format: 'cjs',
    env: 'production',
    alias: { he: './entity-decoder' },
    banner
  },
  // Runtime only ES modules build (for bundlers)
  'web-runtime-esm': {
    entry: resolve('web/entry-runtime.js'),
    dest: resolve('dist/vue.runtime.esm.js'),
    format: 'es',
    banner
  },
  // Runtime+compiler ES modules build (for bundlers)
  'web-full-esm': {
    entry: resolve('web/entry-runtime-with-compiler.js'),
    dest: resolve('dist/vue.esm.js'),
    format: 'es',
    alias: { he: './entity-decoder' },
    banner
  },
  // Runtime+compiler ES modules build (for direct import in browser)
  'web-full-esm-browser-dev': {
    entry: resolve('web/entry-runtime-with-compiler.js'),
    dest: resolve('dist/vue.esm.browser.js'),
    format: 'es',
    transpile: false,
    env: 'development',
    alias: { he: './entity-decoder' },
    banner
  },
  // Runtime+compiler ES modules build (for direct import in browser)
  'web-full-esm-browser-prod': {
    entry: resolve('web/entry-runtime-with-compiler.js'),
    dest: resolve('dist/vue.esm.browser.min.js'),
    format: 'es',
    transpile: false,
    env: 'production',
    alias: { he: './entity-decoder' },
    banner
  },
  // runtime-only build (Browser)
  'web-runtime-dev': {
    entry: resolve('web/entry-runtime.js'),
    dest: resolve('dist/vue.runtime.js'),
    format: 'umd',
    env: 'development',
    banner
  },
  // runtime-only production build (Browser)
  'web-runtime-prod': {
    entry: resolve('web/entry-runtime.js'),
    dest: resolve('dist/vue.runtime.min.js'),
    format: 'umd',
    env: 'production',
    banner
  },
  // Runtime+compiler development build (Browser)
  'web-full-dev': {
    entry: resolve('web/entry-runtime-with-compiler.js'),
    dest: resolve('dist/vue.js'),
    format: 'umd',
    env: 'development',
    alias: { he: './entity-decoder' },
    banner
  },
  // Runtime+compiler production build  (Browser)
  'web-full-prod': {
    entry: resolve('web/entry-runtime-with-compiler.js'),
    dest: resolve('dist/vue.min.js'),
    format: 'umd',
    env: 'production',
    alias: { he: './entity-decoder' },
    banner
  },
  // ...
}

const resolve = p => {
  const base = p.split('/')[0]
  if (aliases[base]) {
    return path.resolve(aliases[base], p.slice(base.length + 1))
  } else {
    return path.resolve(__dirname, '../', p)
  }
}

const path = require('path')

const resolve = p => path.resolve(__dirname, '../', p)

module.exports = {
  vue: resolve('src/platforms/web/entry-runtime-with-compiler'),
  compiler: resolve('src/compiler'),
  core: resolve('src/core'),
  shared: resolve('src/shared'),
  web: resolve('src/platforms/web'),
  weex: resolve('src/platforms/weex'),
  server: resolve('src/server'),
  sfc: resolve('src/sfc')
}

/* @flow */

import config from 'core/config'
import { warn, cached } from 'core/util/index'
import { mark, measure } from 'core/util/perf'

import Vue from './runtime/index'
import { query } from './util/index'
import { compileToFunctions } from './compiler/index'
import { shouldDecodeNewlines, shouldDecodeNewlinesForHref } from './util/compat'

const idToTemplate = cached(id => {
  const el = query(id)
  return el && el.innerHTML
})

const mount = Vue.prototype.$mount
Vue.prototype.$mount = function (
  el?: string | Element,
  hydrating?: boolean
): Component {
  el = el && query(el)

  /* istanbul ignore if */
  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
    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) {
      template = getOuterHTML(el)
    }
    if (template) {
      /* istanbul ignore if */
      if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
        mark('compile')
      }

      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)
}

/**
 * Get outerHTML of elements, taking care
 * of SVG elements in IE as well.
 */
function getOuterHTML (el: Element): string {
  if (el.outerHTML) {
    return el.outerHTML
  } else {
    const container = document.createElement('div')
    container.appendChild(el.cloneNode(true))
    return container.innerHTML
  }
}

Vue.compile = compileToFunctions

export default Vue

/* @flow */

import Vue from 'core/index'
import config from 'core/config'
import { extend, noop } from 'shared/util'
import { mountComponent } from 'core/instance/lifecycle'
import { devtools, inBrowser } from 'core/util/index'

import {
  query,
  mustUseProp,
  isReservedTag,
  isReservedAttr,
  getTagNamespace,
  isUnknownElement
} from 'web/util/index'

import { patch } from './patch'
import platformDirectives from './directives/index'
import platformComponents from './components/index'

// install platform specific utils
Vue.config.mustUseProp = mustUseProp
Vue.config.isReservedTag = isReservedTag
Vue.config.isReservedAttr = isReservedAttr
Vue.config.getTagNamespace = getTagNamespace
Vue.config.isUnknownElement = isUnknownElement

// install platform runtime directives & components
extend(Vue.options.directives, platformDirectives)
extend(Vue.options.components, platformComponents)

// install platform patch function
Vue.prototype.__patch__ = inBrowser ? patch : noop

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

import Vue from './instance/index'
import { initGlobalAPI } from './global-api/index'
import { isServerRendering } from 'core/util/env'
import { FunctionalRenderContext } from 'core/vdom/create-functional-component'

initGlobalAPI(Vue)

Object.defineProperty(Vue.prototype, '$isServer', {
  get: isServerRendering
})

Object.defineProperty(Vue.prototype, '$ssrContext', {
  get () {
    /* istanbul ignore next */
    return this.$vnode && this.$vnode.ssrContext
  }
})

// expose FunctionalRenderContext for ssr runtime helper installation
Object.defineProperty(Vue, 'FunctionalRenderContext', {
  value: FunctionalRenderContext
})

Vue.version = '__VERSION__'

export default Vue

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'

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)
stateMixin(Vue)
eventsMixin(Vue)
lifecycleMixin(Vue)
renderMixin(Vue)

export default Vue

/* @flow */

import config from '../config'
import { initUse } from './use'
import { initMixin } from './mixin'
import { initExtend } from './extend'
import { initAssetRegisters } from './assets'
import { set, del } from '../observer/index'
import { ASSET_TYPES } from 'shared/constants'
import builtInComponents from '../components/index'
import { observe } from 'core/observer/index'

import {
  warn,
  extend,
  nextTick,
  mergeOptions,
  defineReactive
} from '../util/index'

export function initGlobalAPI (Vue: GlobalAPI) {
  // config
  const configDef = {}
  configDef.get = () => config
  if (process.env.NODE_ENV !== 'production') {
    configDef.set = () => {
      warn(
        'Do not replace the Vue.config object, set individual fields instead.'
      )
    }
  }
  Object.defineProperty(Vue, 'config', configDef)

  // exposed util methods.
  // NOTE: these are not considered part of the public API - avoid relying on
  // them unless you are aware of the risk.
  Vue.util = {
    warn,
    extend,
    mergeOptions,
    defineReactive
  }

  Vue.set = set
  Vue.delete = del
  Vue.nextTick = nextTick

  // 2.6 explicit observable API
  Vue.observable = <T>(obj: T): T => {
    observe(obj)
    return obj
  }

  Vue.options = Object.create(null)
  ASSET_TYPES.forEach(type => {
    Vue.options[type + 's'] = Object.create(null)
  })

  // this is used to identify the "base" constructor to extend all plain-object
  // components with in Weex's multi-instance scenarios.
  Vue.options._base = Vue

  extend(Vue.options.components, builtInComponents)

  initUse(Vue)
  initMixin(Vue)
  initExtend(Vue)
  initAssetRegisters(Vue)
}

<div id="app">
  {{ message }}
</div>

var app = new Vue({
  el: '#app',
  data: {
    message: 'Hello Vue!'
  }
});

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)
}

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
	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 {
		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
	vm._self = vm
	initLifecycle(vm)
	initEvents(vm)
	initRender(vm)
	callHook(vm, 'beforeCreate')
	initInjections(vm) // resolve injections before data/props
	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)
	}

	if (vm.$options.el) {
        // $mount 挂载
		vm.$mount(vm.$options.el)
	}
}

export function initState (vm: Component) {
  vm._watchers = []
  const opts = vm.$options
  if (opts.props) initProps(vm, opts.props)
  if (opts.methods) initMethods(vm, opts.methods)
  if (opts.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.$options.data
  // 获取数据赋值给 data 和 vm._data
  data = vm._data = typeof data === 'function'
    ? getData(data, vm)
    : data || {}
  if (!isPlainObject(data)) {
    data = {}
    process.env.NODE_ENV !== 'production' && warn(
      'data functions should return an object:\n' +
      'https://vuejs.org/v2/guide/components.html#data-Must-Be-a-Function',
      vm
    )
  }
  // proxy data on instance
  const keys = Object.keys(data)
  const props = vm.$options.props
  const methods = vm.$options.methods
  // 如果 data 中定义过某项值，在 methods 和 props 中就不能使用，否则会报出警告
  let i = keys.length
  while (i--) {
    const key = keys[i]
    if (process.env.NODE_ENV !== 'production') {
      if (methods && hasOwn(methods, key)) {
        warn(
          `Method "${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)) {
      // 数据代理
      proxy(vm, `_data`, key)
    }
  }
  // observe data 
  // 响应式处理
  observe(data, true /* asRootData */)
}

const sharedPropertyDefinition = {
  enumerable: true,
  configurable: true,
  get: noop,
  set: noop
}

export function proxy (target: Object, sourceKey: string, key: string) {
  sharedPropertyDefinition.get = function proxyGetter () {
    return this[sourceKey][key]
  }
  sharedPropertyDefinition.set = function proxySetter (val) {
    this[sourceKey][key] = val
  }
  Object.defineProperty(target, key, sharedPropertyDefinition)
}

vm.key => vm[source][key] // 数据代理
vm.key => vm['_data'][key] // 数据代理

const mount = Vue.prototype.$mount
Vue.prototype.$mount = function (
  el?: string | Element,
  hydrating?: boolean
): Component {
  el = el && query(el)

  /* istanbul ignore if */
  // vue 不可以挂载到 body 和 documentElement 元素上
  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
    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) {
      template = getOuterHTML(el)
    }
    if (template) {
      /* istanbul ignore if */
      if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
        mark('compile')
      }

      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)
}

/**
 * Query an element selector if it's not an element already.
 */
export function query (el: string | Element): Element {
  if (typeof el === 'string') {
    const selected = document.querySelector(el)
    if (!selected) {
      process.env.NODE_ENV !== 'production' && warn(
        'Cannot find element: ' + el
      )
      return document.createElement('div')
    }
    return selected
  } else {
    return el
  }
}

/**
 * Get outerHTML of elements, taking care
 * of SVG elements in IE as well.
 */
function getOuterHTML (el: Element): string {
  if (el.outerHTML) {
    return el.outerHTML
  } else {
    const container = document.createElement('div')
    container.appendChild(el.cloneNode(true))
    return container.innerHTML
  }
}

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

export function mountComponent (
  vm: Component,
  el: ?Element,
  hydrating?: boolean
): Component {
  vm.$el = el
  // 如果不存在 render 或者 template 并没有正确转化 render 函数
  if (!vm.$options.render) {
    vm.$options.render = createEmptyVNode
    // 开发环境报警告
    if (process.env.NODE_ENV !== 'production') {
      /* istanbul ignore if */
       // 如果使用了 runtime-only 版本，但是有没有写 render 函数，写了 template
      if ((vm.$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 {
        // 既没有 template 和 render 
        warn(
          'Failed to mount component: template or render function not defined.',
          vm
        )
      }
    }
  }
  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 {
    // 渲染 不仅是首次渲染，更新时也会调用此方法
    updateComponent = () => {
      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
  // 渲染 watcher（观察者模式）
  new Watcher(vm, updateComponent, noop, {
    before () {
      if (vm._isMounted && !vm._isDestroyed) {
        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
}

/**
 * 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
    // 如果是渲染 watcher， vm._watcher = this。
    if (isRenderWatcher) {
      vm._watcher = this
    }
    vm._watchers.push(this)
    // options
    // 处理 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
    // 如果 expOrFn 是 function，直接赋值给 getter。
    // 否则调用 parsePath 进行转化赋值给 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 () {
    // 依赖收集相关
    pushTarget(this)
    let value
    const vm = this.vm
    try {
      // 对于首次渲染来说，getter 就是 updateComponent 。
      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
  }
  // ...
}