Vue 全局 API 初始化：

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, // ./debug
  extend, // shared/util
  nextTick, // ./next-tick
  mergeOptions, // ./options
  defineReactive // ../observer/index
} from '../util/index'
import type { GlobalAPI } from 'types/global-api'

function initGlobalAPI(Vue: GlobalAPI) {
  // config
  const configDef: Record<string, any> = {}
  configDef.get = () => config
  if (__DEV__) {
    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 = obj => {
    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)
}

1.Vue.config

在前文中，可以看到 Vue.config 利用了 Object.defineProperty 来定义，get 方法返回 config 对象，set 方法返回警告信息。

以下为 config 的默认配置：

export default {
  /**
   * Option merge strategies (used in core/util/options)
   */
  // $flow-disable-line
  optionMergeStrategies: Object.create(null),

  /**
   * Whether to suppress warnings.
   */
  silent: false,

  /**
   * Show production mode tip message on boot?
   */
  productionTip: __DEV__,

  /**
   * Whether to enable devtools
   */
  devtools: __DEV__,

  /**
   * Whether to record perf
   */
  performance: false,

  /**
   * Error handler for watcher errors
   */
  errorHandler: null,

  /**
   * Warn handler for watcher warns
   */
  warnHandler: null,

  /**
   * Ignore certain custom elements
   */
  ignoredElements: [],

  /**
   * Custom user key aliases for v-on
   */
  // $flow-disable-line
  keyCodes: Object.create(null),

  /**
   * Check if a tag is reserved so that it cannot be registered as a
   * component. This is platform-dependent and may be overwritten.
   */
  isReservedTag: no,

  /**
   * Check if an attribute is reserved so that it cannot be used as a component
   * prop. This is platform-dependent and may be overwritten.
   */
  isReservedAttr: no,

  /**
   * Check if a tag is an unknown element.
   * Platform-dependent.
   */
  isUnknownElement: no,

  /**
   * Get the namespace of an element
   */
  getTagNamespace: noop,

  /**
   * Parse the real tag name for the specific platform.
   */
  parsePlatformTagName: identity,

  /**
   * Check if an attribute must be bound using property, e.g. value
   * Platform-dependent.
   */
  mustUseProp: no,

  /**
   * Perform updates asynchronously. Intended to be used by Vue Test Utils
   * This will significantly reduce performance if set to false.
   */
  async: true,

  /**
   * Exposed for legacy reasons
   */
  _lifecycleHooks: LIFECYCLE_HOOKS
}

2.Vue.util

2.1 Vue.util.warn

warn 方法：

const warn = (msg, vm = currentInstance) => {
  const trace = vm ? generateComponentTrace(vm) : ''

  if (config.warnHandler) {
    config.warnHandler.call(null, msg, vm, trace)
  } else if (hasConsole && !config.silent) {
    console.error(`[Vue warn]: ${msg}${trace}`)
  }
}

2.2 Vue.util.extend

extend 方法：

function extend(
  to,
  _from
) {
  for (const key in _from) {
    to[key] = _from[key]
  }
  return to
}

2.3 Vue.util.mergeOptions

mergeOptions

2.4 Vue.util.defineReactive

TODO: 需要详细分析，涉及到 Vue 的响应式原理。

3.Vue.set

TODO: 同上 Vue.util.defineReactive

4.Vue.delete

TODO: 同上 Vue.util.defineReactive

5.Vue.nextTick

TODO:

6.Vue.observable

TODO: 同上 Vue.util.defineReactive

7.Vue.options

Vue.options 选项存储了以下内容：

1. components 全局组件
2. directives 全局指令
3. filters 全局过滤器
4. _base 基础构造函数，保存 Vue 基础构造函数，用以保证当存在 Vue 多实例时，正确继承。

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

Vue.options._base = Vue

extend(Vue.options.components, builtInComponents)

8.initUse

initUse 用来补充 Vue.use 方法：

function initUse(Vue: GlobalAPI) {
  Vue.use = function (plugin: Function | any) {
    const installedPlugins =
      this._installedPlugins || (this._installedPlugins = [])
    if (installedPlugins.indexOf(plugin) > -1) {
      return this
    }

    // additional parameters
    const args = toArray(arguments, 1)
    args.unshift(this)
    if (isFunction(plugin.install)) {
      plugin.install.apply(plugin, args)
    } else if (isFunction(plugin)) {
      plugin.apply(null, args)
    }
    installedPlugins.push(plugin)
    return this
  }
}

9.initMixin

initMixin 用来补充 Vue.mixin 方法：

function initMixin(Vue: GlobalAPI) {
  Vue.mixin = function (mixin: Object) {
    this.options = mergeOptions(this.options, mixin)
    return this
  }
}

10.initExtend

initExtend 用来补充 Vue.extend 方法。

Vue.extend 方法用于创建一个子类，继承自父类。

function initExtend(Vue: GlobalAPI) {
  /**
   * Each instance constructor, including Vue, has a unique
   * cid. This enables us to create wrapped "child
   * constructors" for prototypal inheritance and cache them.
   */
  Vue.cid = 0
  let cid = 1

  /**
   * Class inheritance
   */
  Vue.extend = function (extendOptions: any): typeof Component {
    extendOptions = extendOptions || {}
    const Super = this
    const SuperId = Super.cid
    const cachedCtors = extendOptions._Ctor || (extendOptions._Ctor = {})
    if (cachedCtors[SuperId]) {
      return cachedCtors[SuperId]
    }

    const name =
      getComponentName(extendOptions) || getComponentName(Super.options)
    if (__DEV__ && name) {
      validateComponentName(name)
    }

    const Sub = function VueComponent(this: any, options: any) {
      this._init(options)
    } as unknown as typeof Component
    Sub.prototype = Object.create(Super.prototype)
    Sub.prototype.constructor = Sub
    Sub.cid = cid++
    Sub.options = mergeOptions(Super.options, extendOptions)
    Sub['super'] = Super

    // For props and computed properties, we define the proxy getters on
    // the Vue instances at extension time, on the extended prototype. This
    // avoids Object.defineProperty calls for each instance created.
    if (Sub.options.props) {
      initProps(Sub)
    }
    if (Sub.options.computed) {
      initComputed(Sub)
    }

    // allow further extension/mixin/plugin usage
    Sub.extend = Super.extend
    Sub.mixin = Super.mixin
    Sub.use = Super.use

    // create asset registers, so extended classes
    // can have their private assets too.
    ASSET_TYPES.forEach(function (type) {
      Sub[type] = Super[type]
    })
    // enable recursive self-lookup
    if (name) {
      Sub.options.components[name] = Sub
    }

    // keep a reference to the super options at extension time.
    // later at instantiation we can check if Super's options have
    // been updated.
    Sub.superOptions = Super.options
    Sub.extendOptions = extendOptions
    Sub.sealedOptions = extend({}, Sub.options)

    // cache constructor
    cachedCtors[SuperId] = Sub
    return Sub
  }
}

function initProps(Comp: typeof Component) {
  const props = Comp.options.props
  for (const key in props) {
    proxy(Comp.prototype, `_props`, key)
  }
}

function initComputed(Comp: typeof Component) {
  const computed = Comp.options.computed
  for (const key in computed) {
    defineComputed(Comp.prototype, key, computed[key])
  }
}

11.initAssetRegisters

initAssetRegisters 用来补充 Vue.component、Vue.directive 和 Vue.filter 方法：

const ASSET_TYPES = ['component', 'directive', 'filter']

function initAssetRegisters(Vue: GlobalAPI) {
  /**
   * Create asset registration methods.
   */
  ASSET_TYPES.forEach(type => {
    // @ts-expect-error function is not exact same type
    Vue[type] = function (
      id: string,
      definition?: Function | Object
    ): Function | Object | void {
      if (!definition) {
        return this.options[type + 's'][id]
      } else {
        /* istanbul ignore if */
        if (__DEV__ && type === 'component') {
          validateComponentName(id)
        }
        if (type === 'component' && isPlainObject(definition)) {
          // @ts-expect-error
          definition.name = definition.name || id
          definition = this.options._base.extend(definition)
        }
        if (type === 'directive' && isFunction(definition)) {
          definition = { bind: definition, update: definition }
        }
        this.options[type + 's'][id] = definition
        return definition
      }
    }
  })
}