今天是實作登入功能：
先建立一個變數用來儲存當前帳號是否登入：

var isLogin: Bool = false

程式碼：

        if (Auth.auth().currentUser == nil || !isLogin) {
                if (accountTextField.text == "" || passwordTextField.text == "") {
                    CustomFunc.customAlert(title: "請重新輸入帳號密碼！", message: "", vc: self, actionHandler: nil)
                } else {
                    Auth.auth().signIn(withEmail:accountTextField.text!, password: passwordTextField.text!) { (user, error) in
                        guard error == nil else {
                            CustomFunc.customAlert(title: "", message: "\(String(describing: error!.localizedDescription))", vc: self, actionHandler: nil)
                            return
                        }
                        CustomFunc.customAlert(title: "登入成功！", message: "", vc: self, actionHandler: self.getFirebaseUserInfo)
                        self.accountTextField.text = ""
                        self.passwordTextField.text = ""
                        self.isLogin = true
                    }
                }
            }

下面這可做可不做：
getFirebaseUserInfo：

    func getFirebaseUserInfo() {
        let currentUser = Auth.auth().currentUser
        guard let user = currentUser else {
            CustomFunc.customAlert(title: "使用者資訊", message: "無法取得使用者資料", vc: self, actionHandler: nil)
            return
        }
        let uid = user.uid
        let email = user.email
        CustomFunc.customAlert(title: "使用者資訊", message: "UID：\(uid)\nEmail：\(email!)", vc: self, actionHandler: nil)
    }