vue响应式如何做到的

function defineReactive(obj, key, val) {  var dep = new Dep();  Object.defineProperty(obj, key, {    enumerable: true,    configurable: true,    get: function() {      dep.depend(); // 将当前watcher实例添加到Dep实例中      return val;    },    set: function(newVal) {      if (val === newVal) {        return;      }      val = newVal;      dep.notify(); // 通知所有watcher实例进行更新    }  });}function observe(obj) {  if (!obj || typeof obj !== "object") {    return;  }  Object.keys(obj).forEach(function(key) {    defineReactive(obj, key, obj[key]);  });}// 示例代码var data = { name: "Bob", age: 18 };observe(data);// 在组件中使用var vm = new Vue({  data: data,  created: function() {    // 访问数据的属性会触发get方法，实现依赖收集    console.log("My name is " + this.name + ", I'm " + this.age + " years old.");  }});

function Watcher(vm, exp, cb) {  this.vm = vm;  this.exp = exp;  this.cb = cb;  this.value = this.get(); // 保存初始状态的值}Watcher.prototype = {  constructor: Watcher,  get: function() {    Dep.target = this; // 将当前watcher实例设置到Dep类的静态属性中    var value = this.vm[exp]; // 访问数据的属性，实现依赖收集    Dep.target = null; // 重置Dep类的静态属性    return value;  },  update: function() {    var value = this.get();    if (value !== this.value) { // 值发生变化时执行回调函数      this.cb(value);      this.value = value;    }  }};// Dep类function Dep() {  this.subs = [];}Dep.prototype = {  constructor: Dep,  addSub: function(sub) {    this.subs.push(sub);  },  removeSub: function(sub) {    var index = this.subs.indexOf(sub);    if (index !== -1) {      this.subs.splice(index, 1);    }  },  depend: function() {    if (Dep.target) {      this.addSub(Dep.target); // 将当前watcher实例添加到依赖列表中    }  },  notify: function() {    this.subs.forEach(function(sub) {      sub.update(); // 通知所有watcher实例进行更新操作    });  }};Dep.target = null; // 静态属性，用于保存当前watcher实例// 示例代码var vm = new Vue({  data: { name: "Bob", age: 18 },  created: function() {    // 创建一个watcher实例，用于监听数据变化并执行回调函数    new Watcher(this, "name", function(value) {      console.log("My name is " + value);    });    new Watcher(this, "age", function(value) {      console.log("I'm " + value + " years old.");    });  }});// 改变数据的值，会触发回调函数的执行vm.name = "Alice";vm.age = 20;