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                <div id="app">
  <button @click="add">修改对象的count属性触发watch</button>
  <button @click="addPrototype">修改对象的a属性触发watch</button>




                const vm = new Vue({
  el: "#app",
  data: {
    obj: {
      count: 0,
      a: "默认",
        a: 1
    step: 1
  created:()=>{ // 箭头函数问题 this指向undefined
  watch: {
      handler(val, oldVal){
        console.log("啦啦啦", this.obj, val, oldVal);
      deep: true,
      immediate: true
    step: {
      handler(val, oldVal) {
        console.log("默认触发一次", val, oldVal);
      immediate: true
  methods: {
    add() {
      this.obj.count = this.obj.count + this.step;
      this.step += 1;
    addPrototype() {
      this.obj.c.a++; // 支持多层属性监听
      // this.obj.a = this.obj.count; 
      // this.obj.c= this.obj.count;
      //  如果把上面第一行注释,第二行取消注释
      // 那么将不会触发深度watch因为自己添加的属性里面没有get set方法 c属性不会被监听