vue中定义的data为什么是函数

new Vue({
  el: "#app",
  // 方式一：对象
  data: {
    obj: {
      name: "qb",
    }
  },
  // 方式二：工厂函数
  // data () {
  //   return {
  //     obj: {
  //       name: "qb",
  //     }
  //   }
  // },
  template: `<div>{{obj.name}}</div>`
});

Vue.component("countComponent", {
  data() {
    return {
      count: 1
    };
  },
  template: `<div>
    <button @click='changeCount'>递增</button>
    <span>{{count}}</span>
  </div>`,
  methods: {
    changeCount() {
      this.count++;
    }
  }
});

new Vue({
  el: "#app",
  template: `<div>
    <countComponent></countComponent>
    <countComponent></countComponent>
  </div>`
});

// ASSET_TYPES定义在文件shared/constants.js文件中
export const ASSET_TYPES = [
  'component',
  'directive',
  'filter'
]

// 以下ASSET_TYPES遍历绑定方法的定义在initGlobalAPI(Vue)全局方法挂载阶段完成
import { ASSET_TYPES } from 'shared/constants'
import { isPlainObject, validateComponentName } from '../util/index'
export function initAssetRegisters (Vue: GlobalAPI) {
  /**
   * Create asset registration methods.
   */
  ASSET_TYPES.forEach(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 (process.env.NODE_ENV !== 'production' && type === 'component') {
          validateComponentName(id)
        }
        if (type === 'component' && isPlainObject(definition)) {
          definition.name = definition.name || id
          definition = this.options._base.extend(definition)
        }
        if (type === 'directive' && typeof definition === 'function') {
          definition = { bind: definition, update: definition }
        }
        this.options[type + 's'][id] = definition
        return definition
      }
    }
  })
}

//  Vue.extend 方法的定义在initGlobalAPI(Vue)全局方法挂载阶段完成
export function initExtend (Vue: GlobalAPI) {
  Vue.extend = function (extendOptions: Object): Function {
    extendOptions = extendOptions || {}
    const Super = this
    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)
    }

    const Sub = function VueComponent (options) {
      this._init(options)
    }
    Sub.prototype = Object.create(Super.prototype)
    Sub.prototype.constructor = Sub
    Sub.cid = cid++
    Sub.options = mergeOptions(
      Super.options,
      extendOptions
    )
    // ...
  }
}

/**
 * Merge two option objects into a new one.
 * Core utility used in both instantiation and inheritance.
 */
export function mergeOptions (
  parent: Object,
  child: Object,
  vm?: Component
): Object {
  // ...
  const options = {}
  let key
  for (key in parent) {
    mergeField(key)
  }
  for (key in child) {
    if (!hasOwn(parent, key)) {
      mergeField(key)
    }
  }
  function mergeField (key) {
    const strat = strats[key] || defaultStrat
    options[key] = strat(parent[key], child[key], vm, key)
  }
  return options
}

strats.data = function (
  parentVal,
  childVal,
  vm
) {
  if (!vm) {
    if (childVal && typeof childVal !== 'function') {
      process.env.NODE_ENV !== 'production' && warn(
        'The "data" option should be a function ' +
        'that returns a per-instance value in component ' +
        'definitions.',
        vm
      );

      return parentVal
    }
    return mergeDataOrFn(parentVal, childVal)
  }

  return mergeDataOrFn(parentVal, childVal, vm)
};