Vue3中watch監聽物件的屬性值(監聽源必須是一個getter函數)

<script lang="ts" setup>
	// 接受父元件傳遞的資料
    const props = defineProps({
        test: {
            type: String,
            default: ''
        }
    })
    
    // 使用 watch 偵聽 props 中的 test 屬性
    watch(
        // 這種寫法不會偵聽到 props 中 test 的變化
    	props.test,
        () => {
            console.log("偵聽成功")
        }
    )
    
    watch(
    	// 這種寫法會偵聽到 props 中 test 的變化
        () => props.test,
        () => {
            console.log("偵聽成功")
        }
    )
</script>

const obj = reactive({ count: 0 })

// 錯誤，因為 watch() 中的偵聽源是一個 number，最終 source 返回的 getter 函數是一個空，所以就得不到偵聽的資料
watch(obj.count, (count) => {
  console.log(`count is: ${count}`)
})

// 正確，主要思想是，將偵聽源轉化為以上4種型別（轉化為getter函數是最簡單方便的）
watch(
  () => obj.count,
  (count) => {
    console.log(`count is: ${count}`)
  }
)

export function watch<T = any, Immediate extends Readonly<boolean> = false>(
  source: T | WatchSource<T>,
  cb: any,
  options?: WatchOptions<Immediate>
): WatchStopHandle {
  if (__DEV__ && !isFunction(cb)) {
    warn(
      ``watch(fn, options?)` signature has been moved to a separate API. ` +
      `Use `watchEffect(fn, options?)` instead. `watch` now only ` +
      `supports `watch(source, cb, options?) signature.`
    )
  }
  return doWatch(source as any, cb, options)
}

function doWatch(
  source: WatchSource | WatchSource[] | WatchEffect | object,
  cb: WatchCallback | null,
  { immediate, deep, flush, onTrack, onTrigger }: WatchOptions = EMPTY_OBJ
): WatchStopHandle {
  // ...
// 當前元件範例
const instance = currentInstance
// 副作用函數，在初始化effect時使用
let getter: () => any
// 強制觸發偵聽
let forceTrigger = false
// 是否為多資料來源。
let isMultiSource = false
}

if (isRef(source)) {
  getter = () => source.value
  forceTrigger = isShallow(source)
}

if (isReactive(source)) {
  getter = () => source
  deep = true
}

<template>
  <div class="container">
    <h2>obj---{{ obj }}</h2>
    <button @click="changeName">修改名字</button>
    <button @click="changeAge">修改年齡</button>
  </div>
</template>

<script lang="ts" setup>
import { reactive, watch } from "vue";
const obj = reactive({
  name: "張三",
  age: 18,
});
const changeName = () => {
  obj.name += "++";
};
const changeAge = () => {
  obj.age += 1;
};
// obj 中的任一屬性變化了，都會被監聽到
watch(obj, () => {
  console.log("變化了");
});
</script>

if (isArray(source)) {
  isMultiSource = true
  forceTrigger = source.some(isReactive)
  getter = () =>
    source.map(s => {
      if (isRef(s)) {
        return s.value
      } else if (isReactive(s)) {
        return traverse(s)
      } else if (isFunction(s)) {
        return callWithErrorHandling(s, instance, ErrorCodes.WATCH_GETTER)
      } else {
        __DEV__ && warnInvalidSource(s)
      }
    })
}

if (isFunction(source)) {
  if (cb) {
    getter = () =>
      callWithErrorHandling(source, instance, ErrorCodes.WATCH_GETTER)
  } else {
    // watchEffect
    getter = () => {
      // 如果元件範例已經解除安裝，直接return
      if (instance && instance.isUnmounted) {
        return
      }
      // 如果清理函數，則執行清理函數
      if (cleanup) {
        cleanup()
      }
      // 執行source，傳入onCleanup，用來註冊清理函數
      return callWithAsyncErrorHandling(
        source,
        instance,
        ErrorCodes.WATCH_CALLBACK,
        [onCleanup]
      )
    }
  }
}

getter = NOOP
__DEV__ && warnInvalidSource(source)