Vue 侦听器 watch

侦听器 

只要数据发生变化，侦听器就会触发

监听器的应用场景

数据变化执行异步或开销较大的操作

侦听器的用法

watch: {
        firstName: function(val) {
          // val表示变化之后的值
          this.fullName = val + ' ' + this.lastName;
        },
        lastName: function(val) {
          this.fullName = this.firstName + ' ' + val;
        }
      }

简单的运用：

var vm = new Vue({
      el: '#app',
      data: {
        firstName: 'Jim',
        lastName: 'Green',
        fullName: 'Jim Green'
      },watch: {
        firstName: function(val) {
          // val表示变化之后的值
          this.fullName = val + ' ' + this.lastName;
        },
        lastName: function(val) {
          this.fullName = this.firstName + ' ' + val;
        }
      }
    });

<div id="app">
    <div>
      <span>名：</span>
      <span>
        <input type="text" v-model='firstName'>
      </span>
    </div>
    <div>
      <span>姓：</span>
      <span>
        <input type="text" v-model='lastName'>
      </span>
    </div>
    <div>{{fullName}}</div>
  </div>

运行结果：

 

 当修改输入框中的值时