var a = {};

Object.defineProperty(a, "b", {
 
 set: function (newValue) {
 
 console.log("我被赋值了！" + newValue);
 
 },
 
 get: function () {
 
 console.log("我被取值了！");
 
 return 2 
 }
})

a.b = 3; //我被赋值了！

console.log(a.b); //我被取值了! //打印 2

var data = {name: 'beidan'};

observe(data);

data.name = 'test'; // 监听到值变化了 beidan 变成 test

function observe(data) {
 
 if (!data || typeof data !== 'object') {
 
 return;
 
 }
 
 // 取出所有属性遍历
 
 Object.keys(data).forEach(function(key) {
 
  defineReactive(data, key, data[key]);

 });
}

function defineReactive(data, key, val) {


 Object.defineProperty(data, key, {

 enumerable: true, // 可枚举
 
  configurable: false, // 不能再define
  get: function() {
  
   return val;
 
  },
 
  set: function(newVal) {
  
   console.log('监听到值变化了 ', val, ' 变成 ', newVal);

   val = newVal;
 
  }
 
 });
}

function Dep() {
 
 this.subs = [];
}

Dep.prototype = {
 
 addSub: function (sub) {
 
 this.subs.push(sub);
 
 },
 
 notify: function (val) {
 
  this.subs.forEach(function (sub) {
  
  sub.update(val)
 
 });
 
}
};
function defineReactive(data, key, val) {
 Object.defineProperty(data, key, {
 ……
 set: function(newVal) {
  if (val === newVal) return;
  console.log('监听到值变化了 ', val, ' 变成 ', newVal);
  val = newVal;
  dep.notify(val); // 通知所有订阅者 
 } 
 });
}

bindText: function () {
 
 var textDOMs = this.el.querySelectorAll('[v-text]'),
bindText,_context = this;

 
 for (var i = 0; i < textDOMs.length; i++) {
  
 bindText = textDOMs[i].getAttribute('v-text');
 textDOMs[i].innerHTML = this.data[bindText];

 var val = textDOMs[i]

 
 var up = function (text) {
  
  val.innerText = text
 
 }

 _context.dep.addSub({
  
  value: textDOMs[i],
  
  update: up
 
 });
 
 }
},