https://stackoverflow.com/questions/60093354

mian.dart

import 'package:flutter/material.dart';

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

class CalculatorApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(title: 'Bending Calculator', home: Calculator());
  }
}

class Calculator extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => Calculatore();
}

class Calculatore extends State<Calculator> {
  final a = TextEditingController();
  final b = TextEditingController();
  final c = TextEditingController();
  // controller mentioned
  String total = "";

  void calculate() {
    print('hello');
    int numberA = int.tryParse(a.text);
    int numberB = int.tryParse(b?.text);
    int numberC = int.tryParse(c?.text);
    int result;
    
    // if numberA have value then answer will be a+c

    // what condition to do here for between choosing between  textfields a or b.
    // i tried numberB ==null   that does not work
    // very much confused, no idea what to do please help
    if (numberA != null && numberC != null) {
      result = numberA + numberC;
    } else if (numberB != null && numberC != null){
      result = numberB + numberC;
    } 

    setState(() {
      total = "$result";
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(title: Text("Calculator")),
        body: SafeArea(
            child: ListView(
          children: <Widget>[
            Row(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
// first textfield

                Container(
                  width: MediaQuery.of(context).size.width * 0.45,
                  height: 50,
                  child: TextField(
                      controller: a,
                      decoration: InputDecoration(hintText: " Enter a value"),
                      keyboardType: TextInputType.number),
                ),
                Container(
                  width: MediaQuery.of(context).size.width * 0.04,
                  height: 50,
                ),
                // second textfield
                Container(
                  width: MediaQuery.of(context).size.width * 0.45,
                  height: 50,
                  child: TextField(
                      controller: b,
                      decoration: InputDecoration(hintText: " Enter  b value"),
                      keyboardType: TextInputType.number),
                ),
              ],
            ),
            Row(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
// third textfield

                Container(
                  width: MediaQuery.of(context).size.width * 0.9,
                  height: 50,
                  child: TextField(
                      controller: c,
                      decoration: InputDecoration(hintText: "   Enter c value"),
                      keyboardType: TextInputType.number),
                ),
              ],
            ),
            Row(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
//button

                RaisedButton(
                  onPressed: calculate,
                  child: Text('Calculate'),
                ),
              ],
            ),
            Text(
              "   Total : $total",
              style: TextStyle(fontSize: 20.0),
            ),
          ],
        )));
  }
}