Untitled

import 'package:flutter/material.dart';

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

class CalculatorApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Simple Calculator',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: CalculatorScreen(),
    );
  }
}

class CalculatorScreen extends StatefulWidget {
  @override
  _CalculatorScreenState createState() => _CalculatorScreenState();
}

class _CalculatorScreenState extends State<CalculatorScreen> {
  String _display = '';
  double _result = 0.0;

  void _onButtonPressed(String value) {
    setState(() {
      if (value == '=') {
        try {
          _result = evalExpression(_display);
        } catch (e) {
          _result = 0.0;
        }
        _display = _result.toString();
      } else if (value == 'C') {
        _display = '';
        _result = 0.0;
      } else {
        _display += value;
      }
    });
  }

  double evalExpression(String expression) {
    return double.parse(expression);
  }

  Widget _buildButton(String label) {
    return Expanded(
      child: InkWell(
        onTap: () => _onButtonPressed(label),
        child: Container(
          alignment: Alignment.center,
          decoration: BoxDecoration(
            border: Border.all(),
          ),
          child: Text(
            label,
            style: TextStyle(fontSize: 24.0),
          ),
        ),
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Simple Calculator'),
      ),
      body: Column(
        children: <Widget>[
          Expanded(
            child: Container(
              alignment: Alignment.bottomRight,
              padding: EdgeInsets.all(16.0),
              child: Text(
                _display,
                style: TextStyle(fontSize: 48.0),
              ),
            ),
          ),
          Divider(),
          Row(
            children: <Widget>[
              _buildButton('7'),
              _buildButton('8'),
              _buildButton('9'),
              _buildButton('/'),
            ],
          ),
          Row(
            children: <Widget>[
              _buildButton('4'),
              _buildButton('5'),
              _buildButton('6'),
              _buildButton('*'),
            ],
          ),
          Row(
            children: <Widget>[
              _buildButton('1'),
              _buildButton('2'),
              _buildButton('3'),
              _buildButton('-'),
            ],
          ),
          Row(
            children: <Widget>[
              _buildButton('C'),
              _buildButton('0'),
              _buildButton('='),
              _buildButton('+'),
            ],
          ),
        ],
      ),
    );
  }
}