main.dart

  void main() => runApp(DemoApp());

  class DemoApp extends StatelessWidget {
    const DemoApp();

    @override
    Widget build(BuildContext context) {
      return MaterialApp(
        home: Scaffold(
          resizeToAvoidBottomInset: false,
          appBar: AppBar(
            title: const Text("Login form"),
          ),
          body: LayoutBuilder(
            builder: (context, dimensions) {
              final width = dimensions.maxWidth / 1.5;
              final height = dimensions.maxHeight / 2;

              return Center(
                child: SingleChildScrollView(
                  child: SizedBox(
                    width: width,
                    height: height,
                    child: LoginForm(width),
                  ),
                ),
              );
            },
          ),
        ),
      );
    }
  }
  

login_form.dart

  class LoginForm extends StatelessWidget {
    final _key = GlobalKey<FormState>();

    String? _validateEmail(String value) {
      if (value.isEmpty) {
        return "Field cannot be empty";
      } else {
        return null;
      }
    }

    String? _validatePassword(String value) {
      if (value.length < 8) {
        return "At least 8 chars!";
      } else {
        return null;
      }
    }

    void _login() {
      if (_key.currentState?.validate() ?? false) {
        debugPrint("Yay! :)");
      } else {
        debugPrint("Error :(");
      }
    }

    @override
    Widget build(BuildContext context) {
      return Form(
        key: _key,
        child: Column(
          mainAxisAlignment: MainAxisAlignment.spaceAround,
          children: <Widget>[
            // Email
            TextFormField(
              decoration: const InputDecoration(
                  icon: Icon(Icons.mail),
                  hintText: "Email"
              ),
              validator: _validateEmail,
            ),

            // Password
            TextFormField(
              decoration: const InputDecoration(
                  icon: Icon(Icons.vpn_key),
                  hintText: "Password"
              ),
              obscureText: true,
              validator: _validatePassword,
            ),

            // Confirm Button
            RaisedButton(
              child: const Text("Login"),
              onPressed: _login,
            ),
          ],
        ),
      );
    }

  }