public register(credentials) {
    if (credentials.email === null || credentials.password === null) {
      return Observable.throw("Bitte geben Sie ihre vollständigen Daten ein");
    } else {
      //firebase user creation
      return Observable.create(observer => {
        this.users = this.af.database.list('/userData/', {preserveSnapshot: true});
        this.users.subscribe(snapshots => {
          snapshots.forEach(snapshot => {
            if (snapshot.val().username === credentials.username) {
              observer.error("Benutzername existiert bereits!");
              observer.next(false);
              observer.complete();
            }
          })
        })
        if (this.checkCredentials(credentials)) {
          this.af.auth.createUser({
            email: credentials.email,
            password: credentials.password
          }).then((success) => {
            observer.next(true);
            observer.complete();
            //adds the table userData with the username according to the user ids
            this.af.database.object(`/userData/${success.uid}`).update({
                username: credentials.username
            });
          }).catch((err) => {
            observer.next(false);
            observer.complete();
          });
        } else {
          observer.error(this.errorMessage);
          this.errorMessage = "";
          observer.complete();
        }
      })
    }
  }