Passing a function in onChanged Flutter - flutter

Question

I'm having these 4 inputs (TextFormFields) , and I want to handle a change that whenver the user changes what is written inside , a function is triggered and updates the sum of another TextFormField.
I thought using the onChanged is the best way to handle these changes , but I'm unable to make it.
I have a TextEditingController that I want to update whenever there is a change in the inputs :
TextEditingController totalDocument;
void _CalculTotal() {
setState(() {
total = total + double.tryParse(widget.controllers.last.text);
totalDocument.text = total.toString();
});
}
These are my inputs inside a class :
class InputRefNomProduit extends StatefulWidget {
final String label, label2, label3, label4;
final String content, content2, content3, content4;
var fieldController = TextEditingController();
var fieldController2 = TextEditingController();
var fieldController3 = TextEditingController();
var fieldController4 = TextEditingController();
final FunctionStringCallback Prix;
FormFieldValidator<String> fieldValidator = (_) {};
InputRefNomProduit(
{this.label,
this.content,
this.label2,
this.content2,
this.label3,
this.content3,
this.label4,
this.content4,
this.fieldController,
this.fieldController2,
this.fieldValidator,
this.fieldController3,
this.fieldController4,
**this.Prix**});
. . .
Expanded(
flex: 3,
child: Container(
width: MediaQuery.of(context).size.width / 3.7,
color: Color.fromARGB(255, 255, 255, 255),
child: TextFormField(
**onChanged: widget.Prix,**
enabled: true,
controller: widget.fieldController4,
validator: widget.fieldValidator,
style: TextStyle(
fontSize: 15.0,
color: Colors.black,
),
decoration: InputDecoration(
contentPadding: EdgeInsets.all(10.0),
hintText: "${widget.content4}",
hintStyle: TextStyle(
color: Color.fromARGB(255, 190, 190, 190),
fontSize: 14),
fillColor: Color.fromARGB(255, 0, 0, 0),
),
),
),
),
The variable Prix is used to get the function _CalculTotal().
This is my sum TextFormField that will hold the total :
SizedBox(
width: 200,
child: Padding(
padding: const EdgeInsets.only(top: 15),
child: TextFormField(
**controller: totalDocument,**
keyboardType: TextInputType.text,
decoration: InputDecoration(
enabled: false,
hintText: '$total',
hintStyle: TextStyle(
fontFamily: 'Montserrat',
fontWeight: FontWeight.w400,
color: Colors.grey.shade400,
),
prefixIcon: Icon(
Icons.attach_money_outlined,
color: Colors.grey.shade400),
enabledBorder: OutlineInputBorder(
borderSide: BorderSide(
width: 1,
color: Colors.grey.shade200),
borderRadius:
BorderRadius.circular(5),
),
disabledBorder: OutlineInputBorder(
borderSide: BorderSide(
width: 1,
color: Colors.grey.shade200),
borderRadius:
BorderRadius.circular(5),
),
focusedBorder: OutlineInputBorder(
borderRadius:
BorderRadius.circular(5),
borderSide: BorderSide(
color: Colors.grey.shade400,
width: 1,
),
),
focusedErrorBorder: OutlineInputBorder(
borderRadius:
BorderRadius.circular(5),
borderSide: BorderSide(
color: Colors.grey.shade400,
width: 1,
),
),
errorBorder: OutlineInputBorder(
borderRadius:
BorderRadius.circular(5),
borderSide: BorderSide(
color: Colors.grey.shade400,
width: 1,
),
),
contentPadding:
EdgeInsets.fromLTRB(15, 0, 15, 0),
),
),
),
),

Answer 1

Use the addListener() method of your TextEditingController to listen to changes in the TextFormFields where you assigned your TextEditingController as a controller.
Checkout this flutter cookbook article for more details. It clearly explains the difference between setting the onChanged attribute of a TextFormField and using the addListener() of a controller to handle changes in your TextFields.
https://docs.flutter.dev/cookbook/forms/text-field-changes