源码揭秘为什么 Vue2 this 能够直接获取到 data 和 methods

目录

• 1. 示例：this 能够直接获取到 data 和 methods
• 2. 准备环境调试源码一探究竟

• 2.1 Vue 构造函数
• 2.2 _init 初始化函数
• 2.3 initState 初始化状态
• 2.4 initMethods 初始化方法

• 2.4.1 bind 返回一个函数，修改 this 指向

• 2.5.1 getData 获取数据
• 2.5.2 proxy 代理
• 2.5.3 Object.defineProperty 定义对象属性

• 2.6.1 hasOwn 是否是对象本身拥有的属性
• 2.6.2 isReserved 是否是内部私有保留的字符串$  和 _ 开头

const vm = new Vue({
    data: {
        name: '我是若川',
    },
    methods: {
        sayName(){
            console.log(this.name);
        }
    },
});
console.log(vm.name); // 我是若川
console.log(vm.sayName()); // 我是若川

function Person(options){
}
const p = new Person({
    data: {
        name: '若川'
    },
    methods: {
        sayName(){
            console.log(this.name);
        }
    }
});
console.log(p.name);
// undefined
console.log(p.sayName());
// Uncaught TypeError: p.sayName is not a function

<script src="https://unpkg.com/vue@2.6.14/dist/vue.js"></script>
<script>
    const vm = new Vue({
        data: {
            name: '我是若川',
        },
        methods: {
            sayName(){
                console.log(this.name);
            }
        },
    });
    console.log(vm.name);
    console.log(vm.sayName());
</script>

npm i -g http-server
cd examples
http-server .
// 如果碰到端口被占用，也可以指定端口
http-server -p 8081 .

function Vue (options) {
    if (!(this instanceof Vue)
    ) {
        warn('Vue is a constructor and should be called with the `new` keyword');
    }
    this._init(options);
}
// 初始化
initMixin(Vue);
stateMixin(Vue);
eventsMixin(Vue);
lifecycleMixin(Vue);
renderMixin(Vue);

jQuery = function( selector, context ) {
  // 返回new之后的对象
  return new jQuery.fn.init( selector, context );
};

// 代码有删减
function initMixin (Vue) {
    Vue.prototype._init = function (options) {
      var vm = this;
      // a uid
      vm._uid = uid$3++;
      // a flag to avoid this being observed
      vm._isVue = true;
      // merge options
      if (options && options._isComponent) {
        // optimize internal component instantiation
        // since dynamic options merging is pretty slow, and none of the
        // internal component options needs special treatment.
        initInternalComponent(vm, options);
      } else {
        vm.$options = mergeOptions(
          resolveConstructorOptions(vm.constructor),
          options || {},
          vm
        );
      }
      // expose real self
      vm._self = vm;
      initLifecycle(vm);
      initEvents(vm);
      initRender(vm);
      callHook(vm, 'beforeCreate');
      initInjections(vm); // resolve injections before data/props
      //  初始化状态
      initState(vm);
      initProvide(vm); // resolve provide after data/props
      callHook(vm, 'created');
    };
}

function initState (vm) {
    vm._watchers = [];
    var opts = vm.$options;
    if (opts.props) { initProps(vm, opts.props); }
    // 有传入 methods，初始化方法
    if (opts.methods) { initMethods(vm, opts.methods); }
    // 有传入 data，初始化 data
    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 initMethods (vm, methods) {
    var props = vm.$options.props;
    for (var key in methods) {
      {
        if (typeof methods[key] !== 'function') {
          warn(
            "Method \"" + key + "\" has type \"" + (typeof methods[key]) + "\" in the component definition. " +
            "Did you reference the function correctly?",
            vm
          );
        }
        if (props && hasOwn(props, key)) {
          warn(
            ("Method \"" + key + "\" has already been defined as a prop."),
            vm
          );
        }
        if ((key in vm) && isReserved(key)) {
          warn(
            "Method \"" + key + "\" conflicts with an existing Vue instance method. " +
            "Avoid defining component methods that start with _ or $."
          );
        }
      }
      vm[key] = typeof methods[key] !== 'function' ? noop : bind(methods[key], vm);
    }
}

function polyfillBind (fn, ctx) {
    function boundFn (a) {
      var l = arguments.length;
      return l
        ? l > 1
          ? fn.apply(ctx, arguments)
          : fn.call(ctx, a)
        : fn.call(ctx)
    }
    boundFn._length = fn.length;
    return boundFn
}
function nativeBind (fn, ctx) {
  return fn.bind(ctx)
}
var bind = Function.prototype.bind
  ? nativeBind
  : polyfillBind;

function initData (vm) {
    var data = vm.$options.data;
    data = vm._data = typeof data === 'function'
      ? getData(data, vm)
      : data || {};
    if (!isPlainObject(data)) {
      data = {};
      warn(
        'data functions should return an object:\n' +
        'https://vuejs.org/v2/guide/components.html#data-Must-Be-a-Function',
        vm
      );
    }
    // proxy data on instance
    var keys = Object.keys(data);
    var props = vm.$options.props;
    var methods = vm.$options.methods;
    var i = keys.length;
    while (i--) {
      var key = keys[i];
      {
        if (methods && hasOwn(methods, key)) {
          warn(
            ("Method \"" + key + "\" has already been defined as a data property."),
            vm
          );
        }
      }
      if (props && hasOwn(props, key)) {
        warn(
          "The data property \"" + key + "\" is already declared as a prop. " +
          "Use prop default value instead.",
          vm
        );
      } else if (!isReserved(key)) {
        proxy(vm, "_data", key);
      }
    }
    // observe data
    observe(data, true /* asRootData */);
}

function getData (data, vm) {
    // #7573 disable dep collection when invoking data getters
    pushTarget();
    try {
      return data.call(vm, vm)
    } catch (e) {
      handleError(e, vm, "data()");
      return {}
    } finally {
      popTarget();
    }
}

/**
   * Perform no operation.
   * Stubbing args to make Flow happy without leaving useless transpiled code
   * with ...rest (https://flow.org/blog/2017/05/07/Strict-Function-Call-Arity/).
   */
function noop (a, b, c) {}
var sharedPropertyDefinition = {
    enumerable: true,
    configurable: true,
    get: noop,
    set: noop
};
function proxy (target, sourceKey, key) {
    sharedPropertyDefinition.get = function proxyGetter () {
      return this[sourceKey][key]
    };
    sharedPropertyDefinition.set = function proxySetter (val) {
      this[sourceKey][key] = val;
    };
    Object.defineProperty(target, key, sharedPropertyDefinition);
}

调试模式下，按alt键，把鼠标移到方法名上，可以看到函数定义的地方。点击可以跳转。
/**
   * Check whether an object has the property.
   */
var hasOwnProperty = Object.prototype.hasOwnProperty;
function hasOwn (obj, key) {
  return hasOwnProperty.call(obj, key)
}
hasOwn({ a: undefined }, 'a') // true
hasOwn({}, 'a') // false
hasOwn({}, 'hasOwnProperty') // false
hasOwn({}, 'toString') // false
// 是自己的本身拥有的属性，不是通过原型链向上查找的。

/**
   * Check if a string starts with $ or _
   */
function isReserved (str) {
  var c = (str + '').charCodeAt(0);
  return c === 0x24 || c === 0x5F
}
isReserved('_data'); // true
isReserved('$options'); // true
isReserved('data'); // false
isReserved('options'); // false

function noop (a, b, c) {}
var sharedPropertyDefinition = {
    enumerable: true,
    configurable: true,
    get: noop,
    set: noop
};
function proxy (target, sourceKey, key) {
    sharedPropertyDefinition.get = function proxyGetter () {
      return this[sourceKey][key]
    };
    sharedPropertyDefinition.set = function proxySetter (val) {
      this[sourceKey][key] = val;
    };
    Object.defineProperty(target, key, sharedPropertyDefinition);
}
function initData(vm){
  const data = vm._data = vm.$options.data;
  const keys = Object.keys(data);
  var i = keys.length;
  while (i--) {
    var key = keys[i];
    proxy(vm, '_data', key);
  }
}
function initMethods(vm, methods){
  for (var key in methods) {
    vm[key] = typeof methods[key] !== 'function' ? noop : methods[key].bind(vm);
  } 
}
function Person(options){
  let vm = this;
  vm.$options = options;
  var opts = vm.$options;
  if(opts.data){
    initData(vm);
  }
  if(opts.methods){
    initMethods(vm, opts.methods)
  }
}
const p = new Person({
    data: {
        name: '若川'
    },
    methods: {
        sayName(){
            console.log(this.name);
        }
    }
});
console.log(p.name);
// 未实现前： undefined
// '若川'
console.log(p.sayName());
// 未实现前：Uncaught TypeError: p.sayName is not a function
// '若川'