可为空和不可为空的值

如何解决可为空和不可为空的值

我使用 flutter 和 firebase 进行用户身份验证。

我遇到的问题是以下代码段：

import 'package:flutter/material.dart';

class AuthClass {

  final FirebaseAuth auth = FirebaseAuth.instance;

//Signing in a user
  Future<String> signIn({String email,String password}) async {
    try {
      await auth.signInWithEmailAndPassword(
          email: email,password: password
      );
      return "Welcome!";
    } on FirebaseAuthException catch (e) {
      if (e.code == 'user-not-found') {
        return 'No user found for that email.';
      } else if (e.code == 'wrong-password') {
        return 'Wrong password provided for that user.';
      }
    }
  }
}

我调用 signIn 函数的上下文就在这里：

import 'package:flutter_client_app/Provider/auth_provider.dart';
import 'package:flutter_client_app/Screens/home.dart';

class LoginPage extends StatefulWidget {
  @override
  _LoginPageState createState() => _LoginPageState();
}

class _LoginPageState extends State<LoginPage> {

  TextEditingController _email = TextEditingController();
  TextEditingController _password = TextEditingController();

  bool isLoading = false;
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(title: Text("Login"),),body: isLoading == false ? Padding(
          padding: const EdgeInsets.all(8.0),child: Column(
            children: [
              TextFormField(
                  controller: _email,decoration: InputDecoration(
                      hintText: "Email"
                  )
              ),const SizedBox(height: 30,TextFormField(
                  controller: _password,decoration: InputDecoration(
                      hintText: "Password"
                  )
              ),FlatButton(     <=============================================================
                  color: Colors.blue,onPressed: () {
                    print (_email.text);
                    print (_password.text);
                    setState(() {
                      isLoading = true;
                    });
                    AuthClass()
                        .signIn(
                          email: _email.text.trim(),password: _password.text.trim())
                        .then((value) {
                      if (value == "Welcome!") {
                        setState(() {
                          isLoading = false;
                        });
                        Navigator.pushAndRemoveUntil(
                            context,MaterialPageRoute(builder: (context) => HomePage()),(route) => false);
                      } else {
                        setState(() {
                          isLoading = false;
                        });
                        ScaffoldMessenger.of(context)
                            .showSnackBar(SnackBar(content: Text(value)));
                      }
                    }); <==============================================================
                  },child: Text("Login to account")),SizedBox(
                height: 20,// GestureDetector(
              //   onTap: () {
              //     Navigator.pushAndRemoveUntil(
              //         context,//         MaterialPageRoute(
              //             builder: (context) => RegisterPage()),(route) => false);
              //   },//   child: Text("Don't have an account? Register"),// ),//
              // const SizedBox(
              //   height: 10,MaterialPageRoute(builder: (context) => ResetPage()),//   child: Text("Forgot Password? reset"),//),],) : Center(
            child: CircularProgressIndicator(),)
    );
  }
}

我得到的错误如下：

29:33：错误：参数“email”的值不能为“null”，因为其类型为“String”，但隐式默认值为“null”。

29:47：错误：参数“password”的值不能为“null”，因为其类型为“String”，但隐式默认值为“null”。

29:18：错误：必须返回非空值，因为返回类型“字符串”不允许为空。

我已成功打印出 TextFormField 中记录的值，但在调用 signIn 异步函数时出现了问题。

解决方法

您正在使用 null-safety 功能。您可以通过将 pubspec.yaml 中的 dart sdk 更改为 environment: sdk: ">=2.7.0 <3.0.0" 而不是 environment: sdk: ">=2.12.0 <3.0.0" 来禁用它，或者将您的课程设置为 null-safety。此外，您应该在 else 部分返回 if of catch 的值。

import 'package:flutter/material.dart';

class AuthClass {

  final FirebaseAuth auth = FirebaseAuth.instance;

  //Signing in a user with nullable parameters
  Future<String> signIn({String? email,String? password}) async {
    try {
      await auth.signInWithEmailAndPassword(
          email: email as String,password: password as String,);
      return "Welcome!";
    } on FirebaseAuthException catch (e) {
      if (e.code == 'user-not-found') {
        return 'No user found for that email.';
      } else if (e.code == 'wrong-password') {
        return 'Wrong password provided for that user.';
      }
      else return "No Clear Error"
    }
  }
  //Signing in a user with default value parameters
  Future<String> signIn({String email = "",String password = ""}) async {
    try {
      await auth.signInWithEmailAndPassword(
          email: email,password: password,);
      return "Welcome!";
    } on FirebaseAuthException catch (e) {
      if (e.code == 'user-not-found') {
        return 'No user found for that email.';
      } else if (e.code == 'wrong-password') {
        return 'Wrong password provided for that user.';
      }
      else return "No Clear Error"
    }
  }
}