export default {
  props: {
    P: Object
  },
  data () {
    return {
      A: {
        a: this.methodA
      },
      B: this.P
    };
  },
  methods: {
    resetData () { // 更新时调用
      Object.assign(this.$data, this.$options.data()); // 有问题！！！
    },
    methodA () {
      // do sth.
    },
    methodB () { // 通过用户操作调用
      this.A.a && this.A.a(); // this.A.a is undefined, this.B is undefined！！！
    }
  }
}

resetData () { // 更新时调用
  Object.assign(this.$data, this.$options.data.call(this));
}

// 创建一个vue实例
const options = {
  customOption: 'foo',
  data () {
    A: this.methodA
  },
  methods: {
    methodA () {}
  },
  created: function () {
    console.log(this.$options.customOption) // => 'foo'
  }
};
new Vue(options);
 
// src/core/instance/init.js
initMixin (Vue: Class<Component>) {
  const vm: Component = this
  // ...
  vm.$options = mergeOptions(
    resolveConstructorOptions(vm.constructor),
    options || {},
    vm
  )
  // ...
}

// src/core/instance/state.js
initState (vm: Component) {
  // ...
  const opts = vm.$options
  if (opts.props) initProps(vm, opts.props)
  if (opts.methods) initMethods(vm, opts.methods)
  if (opts.data) {
    initData(vm) // 里面通过getData(data, vm)改变this
  }
  // ...
}
 
getData (data: Function, vm: Component): any {
  // ...
  try {
    return data.call(vm, vm) // this替换为vm
  }
  // ...
}

// src/core/instance/state.js
proxy(vm, `_data`, key);
 
proxy (target: Object, sourceKey: string, key: string) {
  sharedPropertyDefinition.proxyget = function proxyGetter () {
    return this[sourceKey][key]
  }
  sharedPropertyDefinition.set = function proxySetter (val) {
    this[sourceKey][key] = val
  }
  Object.defineProperty(target, key, sharedPropertyDefinition)
}