Flutter onClosing callback for showModalBottomSheet

Perhaps it's not the best solution, but showModalBottomSheet return a "Future" so you can used it.

For example:

void _showModal() {
    Future<void> future = showModalBottomSheet<void>(
      context: context,
      builder: (BuildContext context) {
        return Container(height: 260.0, child: Text('I am text'));
      },
    );
    future.then((void value) => _closeModal(value));
}
void _closeModal(void value) {
    print('modal closed');
}

You can also achieve it by use of whenComplete function of showModalBottomSheet.

Let's see below code

showModalBottomSheet<void>(
        context: context,
        builder: (BuildContext context) {
          return Container(
            height: 260.0,
            child: Text('I am text')
        );
      },
).whenComplete(() {
  print('Hey there, I\'m calling after hide bottomSheet');
});