为什么 Vue2 this 能够直接获取到 props、 data 、computed和 methods

前言

• 本文参加了由公众号@若川视野 发起的每周源码共读活动，点击了解详情一起参与。
• 这是源码共读的第23期，链接：第23期 | 为什么 Vue2 this 能够直接获取到 data 和 methods

this指向

js 的this指向可以通过call、aplly、bind来处理，其中bind需要处理旧版浏览器不支持问题，需要加垫片或者自己实现

vue 里 bind 的实现

  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;

代码很简单，就是根据入参长度调用 apply 或者 call，this 的指向问题搞明白了其实就简单了，剩下的就是把需要改写 this的函数改写为当前实例即可

代码结构

function Vue (options) {
  if (process.env.NODE_ENV !== 'production' &&
    !(this instanceof Vue)
  ) {
    warn('Vue is a constructor and should be called with the `new` keyword')
  }
  this._init(options)
}

initMixin(Vue)
stateMixin(Vue)
eventsMixin(Vue)
lifecycleMixin(Vue)
renderMixin(Vue)

export default Vue

第一个 initMixin 是把后面stateMixin、eventsMixin、lifecycleMixin、renderMixin用到的函数和属性都挂载到 vue上;在initMixin中会调用initState 初始化props 、methods 、data 、computed 、watch,本文核心关注的是 initState 和 stateMixin 的相关内容

defineReactive

export function defineReactive (
  obj: Object,
  key: string,
  val: any,
  customSetter?: ?Function,
  shallow?: boolean
) {
  const dep = new Dep()

  const property = Object.getOwnPropertyDescriptor(obj, key)
  if (property && property.configurable === false) {
    return
  }

  // cater for pre-defined getter/setters
  const getter = property && property.get
  const setter = property && property.set
  if ((!getter || setter) && arguments.length === 2) {
    val = obj[key]
  }

  let childOb = !shallow && observe(val)
  Object.defineProperty(obj, key, {
    enumerable: true,
    configurable: true,
    get: function reactiveGetter () {
      const value = getter ? getter.call(obj) : val
      if (Dep.target) {
        dep.depend()
        if (childOb) {
          childOb.dep.depend()
          if (Array.isArray(value)) {
            dependArray(value)
          }
        }
      }
      return value
    },
    set: function reactiveSetter (newVal) {
      const value = getter ? getter.call(obj) : val
      /* eslint-disable no-self-compare */
      if (newVal === value || (newVal !== newVal && value !== value)) {
        return
      }
      /* eslint-enable no-self-compare */
      if (process.env.NODE_ENV !== 'production' && customSetter) {
        customSetter()
      }
      // #7981: for accessor properties without setter
      if (getter && !setter) return
      if (setter) {
        setter.call(obj, newVal)
      } else {
        val = newVal
      }
      childOb = !shallow && observe(newVal)
      dep.notify()
    }
  })
}

该函数就是对属性做响应式的地方，下文会用到，其中 Dep 是负责收集和发布属性变化事件的

proxy 代理实现

  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);
  }

就是使用 Object.defineProperty api 实现代理

initState

 function initState (vm) {
    vm._watchers = [];
    var opts = vm.$options;
    if (opts.props) { initProps(vm, opts.props); }
    if (opts.methods) { initMethods(vm, opts.methods); }
    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);
    }
  }

从代码可以看出 vue 实例对象的属性包含了 props 、methods 、data 、computed 等内容

initProps

function initProps (vm, propsOptions) {
  var propsData = vm.$options.propsData || {};
  var props = vm._props = {};
  // cache prop keys so that future props updates can iterate using Array
  // instead of dynamic object key enumeration.
  var keys = vm.$options._propKeys = [];
  var isRoot = !vm.$parent;
  // root instance props should be converted
  if (!isRoot) {
    toggleObserving(false);
  }
  var loop = function ( key ) {
    keys.push(key);
    var value = validateProp(key, propsOptions, propsData, vm);
    /* istanbul ignore else */
    if (process.env.NODE_ENV !== 'production') {
      var hyphenatedKey = hyphenate(key);
      if (isReservedAttribute(hyphenatedKey) ||
          config.isReservedAttr(hyphenatedKey)) {
        warn(
          ("\"" + hyphenatedKey + "\" is a reserved attribute and cannot be used as component prop."),
          vm
        );
      }
      defineReactive$$1(props, key, value, function () {
        if (!isRoot && !isUpdatingChildComponent) {
          warn(
            "Avoid mutating a prop directly since the value will be " +
            "overwritten whenever the parent component re-renders. " +
            "Instead, use a data or computed property based on the prop's " +
            "value. Prop being mutated: \"" + key + "\"",
            vm
          );
        }
      });
    } else {
      defineReactive$$1(props, key, value);
    }
    // static props are already proxied on the component's prototype
    // during Vue.extend(). We only need to proxy props defined at
    // instantiation here.
    if (!(key in vm)) {
      proxy(vm, "_props", key);
    }
  };

  for (var key in propsOptions) loop( key );
  toggleObserving(true);
}

1. vm 属性上声明新的 _props 用来承接 props内容代理对象
2. 通过 defineReactive 把 props 的属性逐个代理到 vm._props 上
3. 再通过 proxy 把第二部收集的属性代理到 vm 上，这样就可以通过 this.xxx 获取到 vm._props.xxx的内容了

initMethods

  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);
    }
  }

这里就直接把 methods 的函数都直接声明了一个相同名称的属性到 vm 上，methods里的函数都使用的了 bind而不是call或者aplly，因为bind只会返回改写了this指向的函数，并不直接调用。

initData

function initData (vm) {
  var data = vm.$options.data;
  data = vm._data = typeof data === 'function'
    ? getData(data, vm)
    : data || {};
  if (!isPlainObject(data)) {
    data = {};
    process.env.NODE_ENV !== 'production' && warn(
      'data functions should return an object:\n' +
      'https://vuejs.org/v2/guide/components.html#data-Must-Be-a-Function',
      vm
    );
  }
  // proxy data on instance
  var keys = Object.keys(data);
  var props = vm.$options.props;
  var methods = vm.$options.methods;
  var i = keys.length;
  while (i--) {
    var key = keys[i];
    if (process.env.NODE_ENV !== 'production') {
      if (methods && hasOwn(methods, key)) {
        warn(
          ("Method \"" + key + "\" has already been defined as a data property."),
          vm
        );
      }
    }
    if (props && hasOwn(props, key)) {
      process.env.NODE_ENV !== 'production' && 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 */);
}
export function getData (data: Function, vm: Component): any {
  // #7573 disable dep collection when invoking data getters
  pushTarget()
  try {
    return data.call(vm, vm)
  } catch (e) {
    handleError(e, vm, `data()`)
    return {}
  } finally {
    popTarget()
  }
}

1. vm 属性上声明新的 _data 用来承接 data内容代理对象
2. 如果data是函数则调用 getData 获取数据赋值给 vm._data，其中 getData 调用data函数时使用 call把 this 指向 vm；不是函数且有值直接把 data 赋值给 vm._data
3. 再通过 proxy 把data的属性代理到 vm 上，这样就可以通过 this.xxx 获取到 vm._data.xxx的内容了
4. observe 是对 data 做数据监听的

initComputed

function initComputed (vm, computed) {
  // $flow-disable-line
  var watchers = vm._computedWatchers = Object.create(null);
  // computed properties are just getters during SSR
  var isSSR = isServerRendering();

  for (var key in computed) {
    var userDef = computed[key];
    var getter = typeof userDef === 'function' ? userDef : userDef.get;
    if (process.env.NODE_ENV !== 'production' && getter == null) {
      warn(
        ("Getter is missing for computed property \"" + key + "\"."),
        vm
      );
    }

    if (!isSSR) {
      // create internal watcher for the computed property.
      watchers[key] = new Watcher(
        vm,
        getter || noop,
        noop,
        computedWatcherOptions
      );
    }

    // component-defined computed properties are already defined on the
    // component prototype. We only need to define computed properties defined
    // at instantiation here.
    if (!(key in vm)) {
      defineComputed(vm, key, userDef);
    } else if (process.env.NODE_ENV !== 'production') {
      if (key in vm.$data) {
        warn(("The computed property \"" + key + "\" is already defined in data."), vm);
      } else if (vm.$options.props && key in vm.$options.props) {
        warn(("The computed property \"" + key + "\" is already defined as a prop."), vm);
      }
    }
  }
}
export function defineComputed (
  target: any,
  key: string,
  userDef: Object | Function
) {
  const shouldCache = !isServerRendering()
  if (typeof userDef === 'function') {
    sharedPropertyDefinition.get = shouldCache
      ? createComputedGetter(key)
      : createGetterInvoker(userDef)
    sharedPropertyDefinition.set = noop
  } else {
    sharedPropertyDefinition.get = userDef.get
      ? shouldCache && userDef.cache !== false
        ? createComputedGetter(key)
        : createGetterInvoker(userDef.get)
      : noop
    sharedPropertyDefinition.set = userDef.set || noop
  }
  if (process.env.NODE_ENV !== 'production' &&
      sharedPropertyDefinition.set === noop) {
    sharedPropertyDefinition.set = function () {
      warn(
        `Computed property "${key}" was assigned to but it has no setter.`,
        this
      )
    }
  }
  Object.defineProperty(target, key, sharedPropertyDefinition)
}

1. 由于 computed 属性是函数，可以直接使用 Object.defineProperty 代理到 vm 上，这样就可以通过 this.xxx 获取到 vm.computed.xxx的内容了

层层代理

function stateMixin(Vue) {
  // flow somehow has problems with directly declared definition object
  // when using Object.defineProperty, so we have to procedurally build up
  // the object here.
  var dataDef = {};
  dataDef.get = function () { return this._data };
  var propsDef = {};
  propsDef.get = function () { return this._props };
  {
    dataDef.set = function () {
      warn(
        'Avoid replacing instance root $data. ' +
        'Use nested data properties instead.',
        this
      );
    };
    propsDef.set = function () {
      warn("$props is readonly.", this);
    };
  }
  Object.defineProperty(Vue.prototype, '$data', dataDef);
  Object.defineProperty(Vue.prototype, '$props', propsDef);

  Vue.prototype.$set = set;
  Vue.prototype.$delete = del;

  Vue.prototype.$watch = function (
    expOrFn,
    cb,
    options
  ) {
    var vm = this;
    if (isPlainObject(cb)) {
      return createWatcher(vm, expOrFn, cb, options)
    }
    options = options || {};
    options.user = true;
    var watcher = new Watcher(vm, expOrFn, cb, options);
    if (options.immediate) {
      var info = "callback for immediate watcher "" + (watcher.expression) + """;
      pushTarget();
      invokeWithErrorHandling(cb, vm, [watcher.value], vm, info);
      popTarget();
    }
    return function unwatchFn() {
      watcher.teardown();
    }
  };
}

这里主要是处理 Vue.prototype 的，可以理解为基类的公共属性和方法，其中 $data 和 $props 分别代理指向了 this._data 和 this._props ，主要是用来处理已经声明了的情况

总结

this 指向问题应该是前端必备技能了，而 vue这里其实还有一个重要的思想就是元编程，通过 Object.defineProperty api代理来实现改变数据原本的行为达到操作其程序的行为；记得之前有道题

var a = ?
if (a == 1 && a == 2 && a == 3) {
    console.log('success')
}

这个用元编程就很容易实现。