export_functions.dart

import 'package:flutter/material.dart';

final Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
 final DataItem dataItem = DataItem();
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        floatingActionButton: FloatingActionButton.extended(
          label: Text('Append'),
          icon: Icon(Icons.add),
          onPressed: () {
            dataItem.appendValue();
          },
        ),
        body: Center(
          child: dataItem,
        ),
      ),
    );
  }
}



class DataItem extends StatefulWidget {
  final _DataItemState dataItemState = new _DataItemState();
  void appendValue() => dataItemState.appendValue();
  @override
  _DataItemState createState() => dataItemState;
}

class _DataItemState extends State<DataItem> {
  int value = 0;
  
  void appendValue() {
    setState(() {
      value ++;
    });
  }
  
  @override  
  build(BuildContext context) {
    return new Container(
      child: Text(
          value.toString()
    ));
  }
}