Vue3中的计算函数有两种写法：简写和完整写法。下面一一介绍下。

计算函数的简写

• main.js

import { createApp } from 'vue'
import App from './App.vue'

createApp(App).mount('#app')

• App.vue

<template>
  <Demo/>
</template>

<script>
import Demo from './components/Demo.vue'

export default {
  name: 'App',
  components: {
    Demo
  }
}
</script>

• Demo.vue

<template>
  姓：<input type="text" v-model="person.firstName">
  <br>
  名：<input type="text" v-model="person.lastName">
  <div>全名：{{fullName}}</div>
</template>

<script>
import {reactive,computed} from "vue";
export default {
  name: 'Demo',
  setup(){
    let person = reactive({
      firstName:"张",
      lastName:"三"
    })

    let fullName = computed(() => {
      return person.firstName + "-" + person.lastName;
    })

    return {
      person,
      fullName
    }
  }
}
</script>

• 启动应用，测试效果
  

其实，fullName也是应该是person的属性，所以修改Demo.vue，稍稍改进下。

<template>
  姓：<input type="text" v-model="person.firstName">
  <br>
  名：<input type="text" v-model="person.lastName">
  <div>全名：{{person.fullName}}</div>
</template>

<script>
import {reactive,computed} from "vue";
export default {
  name: 'Demo',
  setup(){
    let person = reactive({
      firstName:"张",
      lastName:"三"
    })

    person.fullName = computed(() => {
      return person.firstName + "-" + person.lastName;
    })

    return {
      person
    }
  }
}
</script>

计算函数的完整写法

• main.js

import { createApp } from 'vue'
import App from './App.vue'

createApp(App).mount('#app')

• App.vue

<template>
  <Demo/>
</template>

<script>
import Demo from './components/Demo.vue'

export default {
  name: 'App',
  components: {
    Demo
  }
}
</script>

• Demo.vue

<template>
  姓：<input type="text" v-model="person.firstName">
  <br>
  名：<input type="text" v-model="person.lastName">
  <br>
  姓名：<input type="text" v-model="person.fullName">
</template>

<script>
import {reactive,computed} from "vue";
export default {
  name: 'Demo',
  setup(){
    let person = reactive({
      firstName:"张",
      lastName:"三"
    })

    person.fullName = computed({
      get(){
        return person.firstName + "-" + person.lastName;
      },
      set(value){
        const tempArr = value.split("-");
        person.firstName = tempArr[0];
        person.lastName = tempArr[1];
      }
    })

    return {
      person
    }
  }
}
</script>

• 启动应用，测试效果