1.何时用计算属性 何时用监听属性

https://cn.vuejs.org/v2/guide/computed.html

watch: {
    firstName: function (val) {
      this.fullName = val + ' ' + this.lastName
    },
    lastName: function (val) {
      this.fullName = this.firstName + ' ' + val
    }
  }

  computed: {
    fullName: function () {
      return this.firstName + ' ' + this.lastName
    }
  }