我认为onSnapshot()返回unsubscribe，所以它应该像这样工作

<script>

    let unsubscribe

    onDestroy(() => {
        if(unsubscribe) unsubscribe();
    });

    $: if ($userStore) {
        unsubscribe =  onSnapshot(doc(db, 'userInfo', $userStore.uid), (doc) => {
            if (doc.data()) {
                console.log(doc.data());
                userInfoStore.set(doc.data() as UserInfo);
            }
        });
    }

</script>

用户注销时组件是否被销毁？因为如果用户注销，应该调用取消订阅？我认为在一个组件中可能不是处理逻辑的最佳位置。这将是一种通过定制商店

userInfoStore.js

export const userInfoStore = (() => {
        const initialValue = {}
        const {subscribe, set} = writable(initialValue)

        let unsubSnapshot

        return {
            subscribe,

            startListener(uid) {
                unsubSnapshot = onSnapshot(doc(db, 'userInfo', uid), (doc) => {
                    if (doc.data()) {
                        console.log(doc.data());
                        set(doc.data() as UserInfo);
                    }
                });
            },

            stopListener() {
                if(unsubSnapshot) unsubSnapshot()
            }
        }
    })();

auth.js

onAuthStateChanged(auth, user => {
        if(user) {
            userStore.set(user)
            userInfoStore.startListener(user.uid)
        }else {
            userInfoStore.stopListener()
        }
    })

App.svelte（主组件）

不知道这有多重要，或者当页面关闭时侦听器是否停止

<script>
import {onDestroy} from 'svelte'
import {userInfoStore} './userInfoStore'

 onDestroy(() => {
        userInfoStore.stopListener()
    });
</script>

Svelte宣布了Runes for v5，以下是如何实现它：

export function createUser() {
  let user = $state(null)

  function setUser(value) {
    user = value
  }

  return {
    // We need to use a getter here or the value will always be
    // whatever it is at the time function was called.
    get user() {
      return user
    },
    setUser
  }
}

export function createFirestore() {
  const data = createUser()
  let unsubscribe

  $effect(() => {
    if (data.user) {
        unsubscribe = onSnapshot(doc(db, 'userInfo', data.user.uid), (doc) => {
          data.setUser(doc.data())
        });
    }

    return () => {
        unsubscribe?.()
    }
  })

  return {
    get user() {
      return data.user
    },
  }
}

然后将这些函数导入到您的svelte组件中：

<script>
import { createUserFirestore } from './yourFunctions'

const data = createUserFirestore()
</script>

<User user={data.user} />

当然，您可以在组件内部执行所有这些逻辑。
请注意，Svelte v5尚未发布，将有变化，但只是想把它留在这里供将来参考。