如何使用Angular Firebase实现isLoggedIn方法？

我正在尝试在Angular 6应用上使用Firebase实现isLoggedIn方法，并且由于firebase用户是“ subcription”对象而遇到了问题，并且我试图像常规方法isLoggedIn（）一样使用它。

这是我到目前为止的内容：

export class AngularFireAuthService {
    private firebaseUser$: Observable<firebase.User>;

    constructor(private angularFireAuth: AngularFireAuth) {
        this.firebaseUser$ = angularFireAuth.authState;
    }

    getFirebaseUser() {
        return this.firebaseUser$;
    }

    register(user: any) {
        return this.angularFireAuth.auth.createUserWithEmailAndPassword(user.email, user.password)
    }

    login(user: any) {
        return this.angularFireAuth.auth.signInWithEmailAndPassword(user.email, user.password)
    }

    logout() {
        return this.angularFireAuth.auth.signOut();
    }

    resetPassword(email: string) {
        return this.angularFireAuth.auth.sendPasswordResetEmail(email);
    }

    isLoggedIn() {
        //not sure how to do this.

        //I was trying to do something like this:
        // this.firebaseUser$.subscribe((user) => {
        //     if (user) {
        //         return true;
        //     } else {
        //         return false;
        //     }
        // });
    }
}

Is there a way for me to implement an isLoggedIn method, so that I could just simply call it in any of my components? I tried googling this and it seems like the way most people are doing it is to subscribe to the firebaseUser$ object, and if the result is not null, then assume the user is logged in. But I was wondering if I could get around having to subscribe in each of my components and instead just call a isLoggedIn method that would simply return true or false, like this:

if (this.angularFireAugthService.isLoggedIn()) {
   //do something
}