深入探讨Vue 3中的组合式函数编程方式

<script setup>
import { ref, onMounted, onUnmounted } from 'vue'
const x = ref(0)
const y = ref(0)
function update(event) {
  x.value = event.pageX
  y.value = event.pageY
}
onMounted(() => window.addEventListener('mousemove', update))
onUnmounted(() => window.removeEventListener('mousemove', update))
</script>
<template>Mouse position is at: {{ x }}, {{ y }}</template>

// mouse.js
import { ref, onMounted, onUnmounted } from 'vue'
// 按照惯例，组合式函数名以“use”开头
export function useMouse() {
  // 被组合式函数封装和管理的状态
  const x = ref(0)
  const y = ref(0)
  // 组合式函数可以随时更改其状态。
  function update(event) {
    x.value = event.pageX
    y.value = event.pageY
  }
  // 一个组合式函数也可以挂靠在所属组件的生命周期上
  // 来启动和卸载副作用
  onMounted(() => window.addEventListener('mousemove', update))
  onUnmounted(() => window.removeEventListener('mousemove', update))
  // 通过返回值暴露所管理的状态
  return { x, y }
}

<script setup>
import { useMouse } from './mouse.js'
const { x, y } = useMouse()
</script>
<template>Mouse position is at: {{ x }}, {{ y }}</template>

// event.js
import { onMounted, onUnmounted } from 'vue'
export function useEventListener(target, event, callback) {
  // 如果你想的话，
  // 也可以用字符串形式的 CSS 选择器来寻找目标 DOM 元素
  onMounted(() => target.addEventListener(event, callback))
  onUnmounted(() => target.removeEventListener(event, callback))
}

// mouse.js
import { ref } from 'vue'
import { useEventListener } from './event'
export function useMouse() {
  const x = ref(0)
  const y = ref(0)
  useEventListener(window, 'mousemove', (event) => {
    x.value = event.pageX
    y.value = event.pageY
  })
  return { x, y }
}

<script setup>
import { ref } from 'vue'
const data = ref(null)
const error = ref(null)
fetch('...')
  .then((res) => res.JSON())
  .then((json) => (data.value = json))
  .catch((err) => (error.value = err))
</script>
<template>
  <div v-if="error">Oops! Error encountered: {{ error.message }}</div>
  <div v-else-if="data">
    Data loaded:
    <pre>{{ data }}</pre>
  </div>
  <div v-else>Loading...</div>
</template>

// fetch.js
import { ref } from 'vue'
export function useFetch(url) {
  const data = ref(null)
  const error = ref(null)
  fetch(url)
    .then((res) => res.json())
    .then((json) => (data.value = json))
    .catch((err) => (error.value = err))
  return { data, error }
}

<script setup>
import { useFetch } from './fetch.js'
const { data, error } = useFetch('...')
</script>

// fetch.js
import { ref, isRef, unref, watchEffect } from 'vue'
export function useFetch(url) {
  const data = ref(null)
  const error = ref(null)
  function doFetch() {
    // 在请求之前重设状态...
    data.value = null
    error.value = null
    // unref() 解包可能为 ref 的值
    fetch(unref(url))
      .then((res) => res.json())
      .then((json) => (data.value = json))
      .catch((err) => (error.value = err))
  }
  if (isRef(url)) {
    // 若输入的 URL 是一个 ref，那么启动一个响应式的请求
    watchEffect(doFetch)
  } else {
    // 否则只请求一次
    // 避免监听器的额外开销
    doFetch()
  }
  return { data, error }
}

import { unref } from 'vue'
function useFeature(maybeRef) {
  // 若 maybeRef 确实是一个 ref，它的 .value 会被返回
  // 否则，maybeRef 会被原样返回
  const value = unref(maybeRef)
}

// x 和 y 是两个 ref
const { x, y } = useMouse()

const mouse = reactive(useMouse())
// mouse.x 链接到了原来的 x ref
console.log(mouse.x)

Mouse position is at: {<!--{C}%3C!%2D%2D%20%2D%2D%3E-->{ mouse.x }}, {<!--{C}%3C!%2D%2D%20%2D%2D%3E-->{ mouse.y }}

<script setup>
import { useFeatureA } from './featureA.js'
import { useFeatureB } from './featureB.js'
import { useFeatureC } from './featureC.js'
const { foo, bar } = useFeatureA()
const { baz } = useFeatureB(foo)
const { qux } = useFeatureC(baz)
</script>

import { useMouse } from './mouse.js'
import { useFetch } from './fetch.js'
export default {
  setup() {
    const { x, y } = useMouse()
    const { data, error } = useFetch('...')
    return { x, y, data, error }
  },
  mounted() {
    // setup() 暴露的属性可以在通过 `this` 访问到
    console.log(this.x)
  }
  // ...其他选项
}