Call a function whenever there is a change in Flutter - flutter

Question

how can I call a function whenever there is a change in TextFormField .
I have many of them and it's more like an invoice.
So whenever the user changes a value, I need to make some mathematical operations.
The problem is that sometimes some TextFormField could be NULL (User did'tn insert a value yet) and that causes an error.
import 'package:flutter/material.dart';
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
#override
Widget build(BuildContext context) {
late TextEditingController age = TextEditingController();
late TextEditingController height = TextEditingController();
late TextEditingController weight = TextEditingController();
late TextEditingController result = TextEditingController();
return Scaffold(
appBar: AppBar(
title: const Text('BMI !'),
),
body: Center(
child: DecoratedBox(
decoration: const BoxDecoration(
color: Colors.white10,
),
child: Padding(
padding: const EdgeInsets.all(48.0),
child: Column(
mainAxisSize: MainAxisSize.min,
crossAxisAlignment: CrossAxisAlignment.start,
children: [
TextFormField(
controller:age,
decoration: const InputDecoration(
border: UnderlineInputBorder(),
labelText: 'AGE',
),
),
TextFormField(
controller:height,
decoration: const InputDecoration(
border: UnderlineInputBorder(),
labelText: 'HEIGHT',
),
),
TextFormField(
controller:weight,
decoration: const InputDecoration(
border: UnderlineInputBorder(),
labelText: 'WEIGHT',
),
),
TextFormField(
enabled:false,
controller:result,
decoration: const InputDecoration(
border: UnderlineInputBorder(),
labelText: 'Result',
),
),
],
),
),
),
),
);
}
}
void main() {
runApp(
MaterialApp(
home: MyApp(),
),
);
}

Answer 1

To call a function every time the TextFormField has a change, there's a property for exactly that!
the onChanged property, which fires every time something is typed:
return TextFormField(
onChanged: (value) {
// do something with `value`
},);
Since this is only called once there's a change, you don't have to worry about null
Edit: you can check if the controler.isNotEmpty and do something based on that:
void onChanged(String value){
if (_controller.text.isNotEmpty && _secondController.text.isNotEmpty) {
// do something
} else {
// do something
}
}
return TextFormField(
controller: _controller,
onChanged: (value) => onChanged(value),);