LoadUser.swift

    func loadUser() {
        db.collection(Constant.FireStore.users).getDocuments { (querySnapshot, error) in
            if let error = error {
                print("There was an issue retrieving data to Firestore. \(error)")
            } else {
                if let snapshotDocuments = querySnapshot?.documents {
                    for doc in snapshotDocuments {
                        let data = doc.data()
                        if let userName = data[Constant.FireStore.userName] as? String {
                            // label顯示速度有差，再確認
                            DispatchQueue.main.async {
                                self.welcomeLabel.text = "Welcome, \(userName)"
                            }
                        }
                    }
                }
            }
        }
    }