Vue.extend源码分析

整体思路比较简单，就直接写在代码里边了

Vue.extend = function (extendOptions: Object): Function {
    extendOptions = extendOptions || {}
    const Super = this // 这里指向function Vue() {}
    const SuperId = Super.cid
    const cachedCtors = extendOptions._Ctor || (extendOptions._Ctor = {})
    if (cachedCtors[SuperId]) {
      return cachedCtors[SuperId]
    }

    const name = extendOptions.name || Super.options.name
    if (process.env.NODE_ENV !== 'production' && name) {
      validateComponentName(name) // 进行校验名字是否合法
    }

    const Sub = function VueComponent (options) {
      this._init(options)
    }
    Sub.prototype = Object.create(Super.prototype) //原型的继承
    Sub.prototype.constructor = Sub // 此时Sub.prototype.constructor指向的是Super，所以要进行修改
    Sub.cid = cid++
    // 选项合并
    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
    // 就是一些静态方法的继承
    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 // 进行缓存构造函数
    return Sub // 返回的是一个构造函数
  }
}