How to setState inside showModalBottomSheet

I also faced the same problem. Its a small trick, you need to insert StatefulBuilder in the showModalBottomSheet. I will use a different code to make someone out there understand easily using checkbox now that the answer comes way late.

showModalBottomSheet(
    context: context,
    isScrollControlled: true,
    builder: (BuildContext context) {
    mpesachecked =false;
    return StatefulBuilder(builder: (BuildContext context, StateSetter mystate) {
          return Padding(
          padding: const EdgeInsets.fromLTRB(10.0, 70.0, 20.0, 20.0),
          child: Column(
            children: <Widget>[
                 Checkbox(
                      value: mpesachecked,
                      activeColor: Colors.green,
                      onChanged: (value) {
                        mystate(() {
                          mpesachecked = value;
                        });
                      }),
                 ])
                 ));
               });
    )

NOTE: the new state within the showModalBottomSheet is mystate

The problem here is that the BottomSheet you are creating is not part of your StatefulWidget. If you only made your widget stateful for the purpose of using setState inside of showModalBottomSheet, you can revert that change now.

What you really want to do is set the state inside of your BottomSheet. You do that by either passing a StatefulWidget to the builder or by using a StatefulBuilder, which I will do for this example for the sake of simplicity:

showModalBottomSheet(
  context: context,
  builder: (BuildContext context) {
    return BottomSheet(
      onClosing: () {},
      builder: (BuildContext context) {
        bool b = false;
        return StatefulBuilder(
          builder: (BuildContext context, setState) => Switch(
            onChanged: (bool v) {
              setState(() => b = v);
            },
            value: b,
          ),
        );
      },
    );
  },
);

I also moved the b value inside the builder function of the BottomSheet.

If you want to use the value of b inside of your original StatefulWidget as well, you would move it out again and you probably want to call this.setState as well to also update the other widget (only if you need it to update).