一文详解vue中侦听器的使用

<script setup>
import { ref, watch } from 'vue'
 
const question = ref('')
const answer = ref('Questions usually contain a question mark. ;-)')
const loading = ref(false)
 
// 可以直接侦听一个 ref
watch(question, async (newQuestion, oldQuestion) => {
  if (newQuestion.includes('?')) {
    loading.value = true
    answer.value = 'Thinking...'
    try {
      const res = await fetch('https://yesno.wtf/api')
      answer.value = (await res.json()).answer
    } catch (error) {
      answer.value = 'Error! Could not reach the API. ' + error
    } finally {
      loading.value = false
    }
  }
})
</script>
 
<template>
  <p>
    Ask a yes/no question:
    <input v-model="question" :disabled="loading" />
  </p>
  <p>{{ answer }}</p>
</template>

const x = ref(0)
const y = ref(0)
 
// 单个 ref
watch(x, (newX) => {
  console.log(`x is ${newX}`)
})
 
// getter 函数
watch(
  () => x.value + y.value,
  (sum) => {
    console.log(`sum of x + y is: ${sum}`)
  }
)
 
// 多个来源组成的数组
watch([x, () => y.value], ([newX, newY]) => {
  console.log(`x is ${newX} and y is ${newY}`)
})

const obj = reactive({ count: 0 })
 
// 错误，因为 watch() 得到的参数是一个 number
watch(obj.count, (count) => {
  console.log(`count is: ${count}`)
})

// 提供一个 getter 函数
watch(
  () => obj.count,
  (count) => {
    console.log(`count is: ${count}`)
  }
)

const obj = reactive({ count: 0 })
 
watch(obj, (newValue, oldValue) => {
  // 在嵌套的属性变更时触发
  // 注意：`newValue` 此处和 `oldValue` 是相等的
  // 因为它们是同一个对象！
})
 
obj.count++

watch(
  () => state.someObject,
  () => {
    // 仅当 state.someObject 被替换时触发
  }
)

watch(
  () => state.someObject,
  (newValue, oldValue) => {
    // 注意：`newValue` 此处和 `oldValue` 是相等的
    // *除非* state.someObject 被整个替换了
  },
  { deep: true }
)]