vue3 - computed，当数据发生变化的时候，触发computed计算属性

computed有两种书写方式：

1. 基础用法

例子1：

<template>
  <div>
    user: <input type="text" v-model="user">
    <br />
    pwd: <input type="text" v-model="pwd">
    <br />
    {{info}}
  </div>
</template>
<script setup lang='ts'>
  import { ref,computed } from 'vue'
  const user = ref<string>("")
  const pwd = ref<string>("")
  const info = computed(() => {
    return user.value + "--" + pwd.value
  })
</script>
<style scoped>
</style>

例子2：

<template>
  <div>
    <div v-if="show">显示</div>
    <div v-else>隐藏</div>
  </div>
</template>
<script setup lang='ts'>
  import { ref,computed, onMounted } from 'vue'
  const aaa = ref<boolean>()
  const bbb = ref<boolean>()
  const ccc = ref<boolean>()
  onMounted(() => {
    aaa.value = true
    bbb.value = false
    ccc.value = true
  })
  const show = computed(() => {
    return aaa.value && (!bbb.value || ccc.value)
  })
</script>
<style scoped>
</style>

例子3：

<template>
  <div>
    <input type="text" v-model="search" :placeholder="searchVal" />
  </div>
</template>
<script setup lang='ts'>
  import { ref,computed } from 'vue'
  const search = ref<string>()
  const searchVal = computed(() => {
    return "请输入搜索的商品"
  })
</script>
<style scoped>
</style>

2. watch - get / set

<template>
  <div>
    <input type="text" v-model="search1" />
    <input type="text" v-model="search2" />
  </div>
</template>
<script setup lang='ts'>
  import { ref,computed } from 'vue'
  const search1 = ref<string>("")
  const search2 = computed({
    get(){
      return search1.value.split('').reverse().join('')
    },
    set(newVal){
      search1.value = newVal
    }
  })
</script>
<style scoped>
</style>

computed的set可以写if条件判断，最后通过get返回结果，除了需要做复杂的计算属性之外，一般的都会直接用computed基础用法