我有一個帶有身份驗證和 firebase 后端的 React 應用程序。如何檢查用戶是否已登錄？我用 onAuthStateChanged 試過了。但它只返回：這是我的代碼：import * as FirestoreService from '../../firebase/firebase'; useEffect(() => {    let user = FirestoreService.isLoggedIn()                console.log( user)    }, []);現在在firebase文件中： export const isLoggedIn = () => {    return firebase.auth().onAuthStateChanged(function (user) {    if (user) {      console.log("User signed in");          }    else {      console.log("User signed out");          }  });  }我的檢查登錄是否成功：function handleLogin() {     FirestoreService.LoggIn(email, password).then(function (result) {        // result.user.tenantId should be ‘TENANT_PROJECT_ID’.        setloggedIn(true)            }).catch(function (error) {                setloggedIn(false);    });}火力基地文件：    export const LoggIn = (email, password) => {  return firebase.auth().signInWithEmailAndPassword(email, password)    .then(function (result) {      // result.user.tenantId should be ‘TENANT_PROJECT_ID’.      return result;    })    .catch(function (error) {      return error    });}