参数类型“字符串？”不能分配给参数类型

Question

参数类型“字符串？”不能分配给参数类型'String'.dart (argument_type_not_assignable.)

try{
  var key = 'email';
  await Provider.of<Authentication>(context, listen: false).signUp(
      _authData[key],
      _authData['password']
  );
  Navigator.of(context).pushReplacementNamed(HomeScreen.routeName);

} catch(error)
{
  var errorMessage = 'Authentication Failed. Please try again later.';
  _showErrorDialog(errorMessage);
}

}

try{
  await Provider.of<Authentication>(context, listen: false).logIn(
      _authData['email'],
      _authData['password']
  );
  Navigator.of(context).pushReplacementNamed(HomeScreen.routeName);

} catch (error)
{
  var errorMessage = 'Authentication Failed. Please try again later.';
  _showErrorDialog(errorMessage);
}

}

Screenshot

Screenshot

Answer 1

这是null safety的问题。基本上，您的 signUp 方法接受一个字符串作为参数，但您传递的是一个字符串？ （问号表示该值可以为空）。为了解决这种情况，您必须检查_authData[key] 和_authData['password']（logIn 也一样）是否不为空。像这样的：

try{
  var key = 'email';
  if(_authData[key] != null && _authData['password'] != null) {
   await Provider.of<Authentication>(context, listen: false).signUp(
          _authData[key]!,
          _authData['password']!,
   );
   Navigator.of(context).pushReplacementNamed(HomeScreen.routeName);
  } else {
    throw Exception('Error');
  }
} catch(error) {
  var errorMessage = 'Authentication Failed. Please try again later.';
  _showErrorDialog(errorMessage);
}

Answer 2

缺少“State.build”的具体实现。 尝试实现缺少的方法，或将类设为 abstract.dart(non_abstract_class_inherits_abstract_member)

signup_screen.dart

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import '../models/authentication.dart';

import 'home_screen.dart';
import 'login_screen.dart';

class SignupScreen extends StatefulWidget {
  static const routeName = '/signup';
  @override
  _SignupScreenState createState() => _SignupScreenState();
}

class _SignupScreenState extends State<SignupScreen> {

  final GlobalKey<FormState> _formKey = GlobalKey();
  TextEditingController _passwordController = new TextEditingController();

  Map<String, String> _authData = {
    'email' : '',
    'password' : ''
  };

  void _showErrorDialog(String msg)
  {
    showDialog(
        context: context,
        builder: (ctx) => AlertDialog(
          title: Text('An Error Occured'),
          content: Text(msg),
          actions: <Widget>[
            FlatButton(
              child: Text('Okay'),
              onPressed: (){
                Navigator.of(ctx).pop();
              },
            )
          ],
        )
    );
  }

  Future<void> _submit() async
  {
    if(!_formKey.currentState!.validate())
      {
        return;
      }
    _formKey.currentState!.save();

    try{
  var key = 'email';
  if(_authData[key] != null && _authData['password'] != null) {
   await Provider.of<Authentication>(context, listen: false).signUp(
          _authData[key]!,
          _authData['password']!,
   );
   Navigator.of(context).pushReplacementNamed(HomeScreen.routeName);
  } else {
    throw Exception('Error');
  }
} catch(error) {
  var errorMessage = 'Authentication Failed. Please try again later.';
  _showErrorDialog(errorMessage);
}
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Signup'),

        actions: <Widget>[
          FlatButton(
            child: Row(
              children: <Widget>[
                Text('Login'),
                Icon(Icons.person)
              ],
            ),
            textColor: Colors.white,
            onPressed: (){
              Navigator.of(context).pushReplacementNamed(LoginScreen.routeName);
            },
          )
        ],
      ),
      body: Stack(
        children: <Widget>[
          Container(
            decoration: BoxDecoration(
                gradient: LinearGradient(
                    colors: [
                      Colors.limeAccent,
                      Colors.redAccent,
                    ]
                )
            ),
          ),
          Center(
            child: Card(
              shape: RoundedRectangleBorder(
                borderRadius: BorderRadius.circular(10.0),
              ),
              child: Container(
                height: 300,
                width: 300,
                padding: EdgeInsets.all(16),
                child: Form(
                  key: _formKey,
                  child: SingleChildScrollView(
                    child: Column(
                      children: <Widget>[
                        //email
                        TextFormField(
                          decoration: InputDecoration(labelText: 'Email'),
                          keyboardType: TextInputType.emailAddress,
                          validator: (value)
                          {
                            if(value!.isEmpty || !value.contains('@'))
                            {
                              return 'invalid email';
                            }
                            return null;
                          },
                          onSaved: (value)
                          {
                            _authData['email'] = value!;
                          },
                        ),

                        //password
                        TextFormField(
                          decoration: InputDecoration(labelText: 'Password'),
                          obscureText: true,
                          controller: _passwordController,
                          validator: (value)
                          {
                            if(value!.isEmpty || value.length<=5)
                            {
                              return 'invalid password';
                            }
                            return null;
                          },
                          onSaved: (value)
                          {
                            _authData['password'] = value!;
                          },
                        ),

                        //Confirm Password
                        TextFormField(
                          decoration: InputDecoration(labelText: 'Confirm Password'),
                          obscureText: true,
                          validator: (value)
                          {
                            if(value!.isEmpty || value != _passwordController.text)
                            {
                              return 'invalid password';
                            }
                            return null;
                          },
                          onSaved: (value)
                          {

                          },
                        ),
                        SizedBox(
                          height: 30,
                        ),
                        RaisedButton(
                          child: Text(
                              'Submit'
                          ),
                          onPressed: ()
                          {
                            _submit();
                          },
                          shape: RoundedRectangleBorder(
                            borderRadius: BorderRadius.circular(30),
                          ),
                          color: Colors.blue,
                          textColor: Colors.white,
                        )
                      ],
                    ),
                  ),
                ),
              ),
            ),
          )
        ],
      ),
    );
  }
}
}

login_screen.dart

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';

import 'signup_screen.dart';
import 'home_screen.dart';
import '../models/authentication.dart';

class LoginScreen extends StatefulWidget {
  static const routeName = '/login';
  @override
  _LoginScreenState createState() => _LoginScreenState();
}

class _LoginScreenState extends State<LoginScreen> {

  final GlobalKey<FormState> _formKey = GlobalKey();

  Map<String, String> _authData = {
    'email' : '',
    'password': ''
  };

  var email;

  void _showErrorDialog(String msg)
  {
    showDialog(
        context: context,
      builder: (ctx) => AlertDialog(
        title: Text('An Error Occured'),
        content: Text(msg),
        actions: <Widget>[
          FlatButton(
            child: Text('Okay'),
            onPressed: (){
              Navigator.of(ctx).pop();
            },
          )
        ],
      )
    );
  }

  Future<void> _submit() async
  {
    if(!_formKey.currentState!.validate())
      {
        return;
      }
    _formKey.currentState!.save();

   

try{
  
  if(_authData[email] != null && _authData['password'] != null) {
   await Provider.of<Authentication>(context, listen: false).logIn(
          _authData['email']!,
          _authData['password']!,
   );
   Navigator.of(context).pushReplacementNamed(HomeScreen.routeName);
  } else {
    throw Exception('Error');
  }
} catch(error) {
  var errorMessage = 'Authentication Failed. Please try again later.';
  _showErrorDialog(errorMessage);
}}
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Login'),
        actions: <Widget>[
          FlatButton(
            child: Row(
              children: <Widget>[
                Text('Signup'),
                Icon(Icons.person_add)
              ],
            ),
            textColor: Colors.white,
            onPressed: (){
              Navigator.of(context).pushReplacementNamed(SignupScreen.routeName);
            },
          )
        ],
      ),
      body: Stack(
        children: <Widget>[
          Container(
            decoration: BoxDecoration(
              gradient: LinearGradient(
                colors: [
                  Colors.lightGreenAccent,
                  Colors.blue,
                ]
              )
            ),
          ),
          Center(
            child: Card(
              shape: RoundedRectangleBorder(
                borderRadius: BorderRadius.circular(10.0),
              ),
              child: Container(
                height: 260,
                width: 300,
                padding: EdgeInsets.all(16),
                child: Form(
                  key: _formKey,
                  child: SingleChildScrollView(
                    child: Column(
                      children: <Widget>[
                        //email
                        TextFormField(
                          decoration: InputDecoration(labelText: 'Email'),
                          keyboardType: TextInputType.emailAddress,
                          validator: (value)
                          {
                            if(value!.isEmpty || !value.contains('@'))
                              {
                                return 'invalid email';
                              }
                            return null;
                          },
                          onSaved: (value)
                          {
                            _authData['email'] = value!;
                          },
                        ),

                        //password
                        TextFormField(
                          decoration: InputDecoration(labelText: 'Password'),
                          obscureText: true,
                          validator: (value)
                          {
                            if(value!.isEmpty || value.length<=5)
                              {
                                return 'invalid password';
                              }
                            return null;
                          },
                          onSaved: (value)
                          {
                            _authData['password'] = value!;
                          },
                        ),
                        SizedBox(
                          height: 30,
                        ),
                        RaisedButton(
                          child: Text(
                            'Submit'
                          ),
                          onPressed: ()
                          {
                            _submit();
                          },
                          shape: RoundedRectangleBorder(
                            borderRadius: BorderRadius.circular(30),
                          ),
                          color: Colors.blue,
                          textColor: Colors.white,
                        )
                      ],
                    ),
                  ),
                ),
              ),
            ),
          )
        ],
      ),
    );
  }
}

Photo