Cannot toggle between the checkboxes – Flutter

You just have to declare the Boolean values outside the build context. Build context method is called whenever the state is changed. Since you declare and assign values inside the build context function, variables are getting the default value. Also, you don’t need stateful builder.

Full Code : –

import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: const Text('AlertDialog Sample')),
        body: const Center(
          child: DialogExample(),
        ),
      ),
    );
  }
}

class DialogExample extends StatefulWidget {
  const DialogExample({super.key});

  @override
  State<DialogExample> createState() => _DialogExampleState();
}

class _DialogExampleState extends State<DialogExample> {
  bool mIsChecked = false;
  bool fIsChecked = false;
  @override
  Widget build(BuildContext context) {
    Size size = MediaQuery.of(context).size;

    return Scaffold(
      appBar: AppBar(
        title: const Text(
          'BMI Calculator',
          style: TextStyle(fontSize: 15),
        ),
        centerTitle: true,
        elevation: 0,
      ),
      body: Column(
        children: [
          const Text(
            'I am a ',
          ),
          const SizedBox(height: 50),
          Row(
            mainAxisAlignment: MainAxisAlignment.spaceEvenly,
            children: [
              Column(
                children: [
                  const Text(
                    'male',
                  ),
                  Transform.scale(
                    scale: 1.5,
                    child: Checkbox(
                      value: mIsChecked,
                      onChanged: (bool? value) {
                        setState(() {
                          if (fIsChecked == true) {
                            fIsChecked = false;
                          }

                          mIsChecked = value!;
                        });
                        // print('isChecked: $isChecked');
                      },
                      // activeColor: Colors.green,
                      fillColor: MaterialStateProperty.resolveWith<Color>(
                          (Set<MaterialState> states) {
                        if (states.contains(MaterialState.disabled)) {
                          return const Color(0xFF00BB6E).withOpacity(.32);
                        }
                        return const Color(0xFF00BB6E);
                      }),
                      materialTapTargetSize: MaterialTapTargetSize.padded,
                    ),
                  )
                ],
              ),
              Column(
                children: [
                  const Text(
                    'female',
                  ),
                  Transform.scale(
                    scale: 1.5,
                    child: Checkbox(
                      value: fIsChecked,
                      onChanged: (bool? value) {
                        setState(() {
                          print(mIsChecked);
                          if (mIsChecked == true) {
                            mIsChecked = false;
                          }
                          fIsChecked = value!;
                        });
                        setState(
                          () {},
                        );
                        // print('isChecked: $isChecked');
                      },
                      // activeColor: Colors.green,
                      fillColor: MaterialStateProperty.resolveWith<Color>(
                          (Set<MaterialState> states) {
                        if (states.contains(MaterialState.disabled)) {
                          return const Color(0xFF00BB6E).withOpacity(.32);
                        }
                        return const Color(0xFF00BB6E);
                      }),
                      materialTapTargetSize: MaterialTapTargetSize.padded,
                    ),
                  )
                ],
              ),
            ],
          )
        ],
      ),
    );
  }
}

Output : –