Flutter – Simple login page navigation issue

Thanks for posting the full class! I took some time to read through all of it; Codefarmer’s answer is 100% on point, but I have a few small pieces of advice I wanted to share:

Single-line functions

If a function only has one expression, you can use => instead of {} to make it look a little nicer. These two function calls have the same behavior:

buildTextFormField(Icons.lock, "Password", true, (value) {
  Password = value!;
}),

buildTextFormField(Icons.lock, "Password", true, (value) => Password = value!),

Function vs. Class

Here’s some advice that will make your app run faster: if you want to clean up a build method, the best practice is to create another class instead of making a function. Functions are evaluated any time the widget is rebuilt, but widget classes are only rebuilt when needed.

Don’t do this:

TextFormField buildTextFormField(
  IconData icon, String hint, bool isPassword, String val) {
  return TextFormField(...);
}

Do this instead:

class _TextFormField extends StatelessWidget {
  const _TextFormField(this.icon, this.hint, this.isPassword, this.onSaved);
  final IconData icon;
  final String hint;
  final bool isPassword;
  final ValueChanged<String?> onSaved;

  @override
  Widget build(BuildContext context) {
    return TextFormField(...);
  }
}

It’s more code to write, but it runs better 🙂

There were a couple of other small things; I’ll paste my version of the class below so you can take a look:

class LoginRegister extends StatefulWidget {
  const LoginRegister({super.key});
  @override
  State<LoginRegister> createState() => _LoginRegisterState();
}

class _LoginRegisterState extends State<LoginRegister> {
  /// you can initialize [emailController] and [passwordController] here;
  /// no need for [initState] or [dispose] unless you add other stuff
  late final emailController = TextEditingController();
  late final passwordController = TextEditingController();

  /// In general, Dart classes are capitalized and variables aren't.
  /// (no need to initialize to empty string since they're initialized to 'a' later)
  late String username, password;
  final _formKey = GlobalKey<FormState>();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Center(child: Text('Login-Register'))),
      body: SafeArea(
        child: Padding(
          padding: const EdgeInsets.all(40),
          child: Form(
            key: _formKey,
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: [
                _TextFormField(
                  Icons.person,
                  'Username',
                  onSaved: (value) => username = value!,
                ),
                _TextFormField(
                  Icons.lock,
                  'Password',
                  isPassword: true,
                  onSaved: (value) => password = value!,
                ),
                SizedBox(
                  width: double.infinity,
                  child: ElevatedButton(
                    // ElevatedButton.styleFrom() is really convenient,
                    // since you don't have to use MaterialStatePropertyAll
                    style: ElevatedButton.styleFrom(backgroundColor: Colors.green),
                    child: const Text('Sign-In'),
                    onPressed: () {
                      if (_formKey.currentState!.validate()) {
                        _formKey.currentState!.save();

                        print('Username: $username');
                        print('Password: $password');
                        if (username == 'a' && password == 'a') print('it worked!');
                      }
                    },
                  ),
                ),
                Row(
                  mainAxisAlignment: MainAxisAlignment.spaceAround,
                  children: [
                    TextButton(onPressed: () {}, child: const Text('Forgot Password')),
                    TextButton(onPressed: () {}, child: const Text('Register')),
                  ],
                )
              ],
            ),
          ),
        ),
      ),
    );
  }
}

class _TextFormField extends StatelessWidget {
  const _TextFormField(
    this.icon,
    this.hint, {
    this.isPassword = false,
    required this.onSaved,
  });
  final IconData icon;
  final String hint;
  final bool isPassword;
  final ValueChanged<String?> onSaved;

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.only(bottom: 15),
      child: TextFormField(
        initialValue: 'a',
        onSaved: onSaved,
        obscureText: isPassword,
        decoration: InputDecoration(
          prefixIcon: Icon(icon),
          border: const OutlineInputBorder(
            borderRadius: BorderRadius.all(Radius.circular(40)),
          ),
          hintText: hint,
        ),
      ),
    );
  }
}