A simple and powerful flutter calculator widget

dependencies:
  flutter_calculator:
    git:
      url: git://github.com/flytreeleft/flutter_calculator.git

import 'package:flutter/material.dart';

import 'package:flutter_calculator/flutter_calculator.dart';

void main() => runApp(FlutterCalculatorExample());

class FlutterCalculatorExample extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      // ...
      home: const HomePage(title: 'Flutter Calculator Example'),
    );
  }
}

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  final FocusNode _focusNode = FocusNode();
  final TextEditingController _textController = TextEditingController(text: '0.00');

  @override
  void initState() {
    this._focusNode.addListener(this._showCalculatorDialog);
    super.initState();
  }

  @override
  void dispose() {
    this._focusNode.removeListener(this._showCalculatorDialog);
    this._textController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      // ...
      body: TextField(
        showCursor: false,
        readOnly: true,
        focusNode: this._focusNode,
        controller: this._textController,
      ),
    );
  }

  void _showCalculatorDialog() async {
    if (!this._focusNode.hasFocus) {
      return;
    }

    final result = await showCalculator(context: this.context);

    this._textController.value = this._textController.value.copyWith(
          text: result?.toString() ?? '0.00',
        );
  }
}