calculator on flutter

main.dart

import 'package:flutter/material.dart';
import 'package:flutter_layout_grid/flutter_layout_grid.dart';
import 'package:math_expressions/math_expressions.dart';

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

class MyApp extends StatefulWidget {
  const MyApp({Key? key}) : super(key: key);

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

class _MyAppState extends State<MyApp> {
  void refresh() {
    setState(() {});
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        title: 'Flutter Demo',
        home: Scaffold(
          appBar: AppBar(title: const Text('Calculator')),
          body:
              Column(crossAxisAlignment: CrossAxisAlignment.stretch, children: [
            Expanded(
              child: Text(CalcOperations.operationStr,
                  style: const TextStyle(
                      fontSize: 150,
                      fontWeight: FontWeight.w100,
                      color: Colors.blueAccent)),
            ),
            Expanded(
              flex: 2,
              child: LayoutGrid(areas: ''' 
                      buttonSeven buttonEight buttonNine buttonMinus
                      buttonFour  buttonFive  buttonSix  buttonPlus
                      buttonOne   buttonTwo   buttonThree buttonDivide
                      buttonClear   buttonZero  buttonEqual buttonMultiply
                    ''', rowSizes: const [
                FlexibleTrackSize(100),
                FlexibleTrackSize(100),
                FlexibleTrackSize(100),
                FlexibleTrackSize(100),
              ], columnSizes: const [
                FlexibleTrackSize(100),
                FlexibleTrackSize(100),
                FlexibleTrackSize(100),
                FlexibleTrackSize(100),
              ], children: [
                CalcButton(value: 7, notifyParent: refresh)
                    .inGridArea('buttonSeven'),
                CalcButton(value: 8, notifyParent: refresh)
                    .inGridArea('buttonEight'),
                CalcButton(value: 9, notifyParent: refresh)
                    .inGridArea('buttonNine'),
                CalcButton(value: '-', notifyParent: refresh)
                    .inGridArea('buttonMinus'),
                CalcButton(value: 4, notifyParent: refresh)
                    .inGridArea('buttonFour'),
                CalcButton(value: 5, notifyParent: refresh)
                    .inGridArea('buttonFive'),
                CalcButton(value: 6, notifyParent: refresh)
                    .inGridArea('buttonSix'),
                CalcButton(value: '+', notifyParent: refresh)
                    .inGridArea('buttonPlus'),
                CalcButton(value: 1, notifyParent: refresh)
                    .inGridArea('buttonOne'),
                CalcButton(value: 2, notifyParent: refresh)
                    .inGridArea('buttonTwo'),
                CalcButton(value: 3, notifyParent: refresh)
                    .inGridArea('buttonThree'),
                CalcButton(value: '/', notifyParent: refresh)
                    .inGridArea('buttonDivide'),
                CalcButton(value: 'C', notifyParent: refresh)
                    .inGridArea('buttonClear'),
                CalcButton(value: 0, notifyParent: refresh)
                    .inGridArea('buttonZero'),
                CalcButton(value: '=', notifyParent: refresh)
                    .inGridArea('buttonEqual'),
                CalcButton(value: '*', notifyParent: refresh)
                    .inGridArea('buttonMultiply'),
              ]),
            ),
          ]),
        ));
  }
}

class CalcButton extends StatelessWidget {
  const CalcButton({Key? key, required this.value, required this.notifyParent})
      : super(key: key);

  final Object value;
  final Function() notifyParent;

  @override
  Widget build(BuildContext context) {
    return OutlinedButton(
      style: ButtonStyle(minimumSize: MaterialStateProperty.all(Size.infinite)),
      child: Text(value.toString()),
      onPressed: () {
        if (value == '=' && CalcOperations.operationStr != '') {
          CalcOperations.operationStr = Parser()
              .parse(CalcOperations.operationStr)
              .evaluate(EvaluationType.REAL, ContextModel())
              .round()
              .toString();
          notifyParent();
          return;
        }
        CalcOperations.operationStr += value.toString();
        notifyParent();
      },
    );
  }
}

class CalcOperations {
  static String operationStr = '';
}