// 键路径
vm.$watch('a.b.c', function (newVal, oldVal) {
 // 做点什么
})
// 函数
vm.$watch(
 function () {
  return this.a + this.b
 },
 function (newVal, oldVal) {
  // 做点什么
 }
)

var unwatch = vm.$watch('a', cb)
// 之后取消观察
unwatch()

<div id="watch">
    firstName:<input type="text" name="li" v-model="firstName">
    <br>
    lastName:<input type="text" name="fei" v-model="lastName">
    <p>fullName: {{fullName}}</p>
</div>

var vm = new Vue({
      el: '#watch',
      data: {
        firstName: 'a',
        lastName: 'fei',
        fullName: 'a fei'
      },
      watch: {
        firstName: function (val) {
          this.fullName = val + ' ' + this.lastName
        },
        lastName: function (val) {
          this.fullName = this.firstName + ' ' + val
        }
      }
})