compound_widget.dart

import 'package:flutter/material.dart';

class CompoundWidget extends StatefulWidget {
  final Function onChanged;

  const CompoundWidget({Key key, this.onChanged}) : super(key: key);

  @override
  _CompoundWidgetState createState() => _CompoundWidgetState();
}

class _CompoundWidgetState extends State<CompoundWidget> {
  int _counter = 0;
  Function _onChanged = (value) {};

  @override
  Widget build(BuildContext context) {
    return Row(
      mainAxisSize: MainAxisSize.min,
      mainAxisAlignment: MainAxisAlignment.spaceBetween,
      children: [
        FloatingActionButton(
          onPressed: () {
            setState(() {
              _counter--;
            });
            widget.onChanged(_counter);
          },
          child: Text(
            '-',
            style: TextStyle(color: Colors.white, fontSize: 24),
          ),
        ),
        SizedBox(
          width: 10,
        ),
        Text('$_counter', style: TextStyle(fontSize: 24)),
        SizedBox(
          width: 10,
        ),
        FloatingActionButton(
          onPressed: () {
            setState(() {
              _counter++;
            });
            widget.onChanged(_counter);
          },
          child: Text(
            '+',
            style: TextStyle(color: Colors.white, fontSize: 24),
          ),
        ),
      ],
    );
  }
}