Connection between child and parent - flutter

Question

I created my own TextField widget and i want to use onChange() function of textField, so i created a callbackFunction as you can see below, im trying to print given param of function but receiving null, how can i handle these callbacks;
CallBack Function
void childCallBack(dynamic value, dynamic property) {
setState(() {
property = value;
});
}
How i call my TextField in main state
TextFieldForProduct(
property: widget.product.name,
func: childCallBack,
)
And my CustomTextFieldWidget
class TextFieldForProduct extends StatelessWidget {
TextFieldForProduct({#required this.property, #required this.func});
var property;
Function func;
#override
Widget build(BuildContext context) {
return Container(
child: Padding(
padding: const EdgeInsets.all(8.0),
child: TextField(
onChanged: (value) {
func(value, property);
},
),
);
}
}

Answer 1

What is the point of passing widget.product.name to TextFieldForProduct, you can directly set the widget.product.name = value; in the childCallBack since both are available in the same class. Please see the code below:
void childCallBack(String value) {
setState(() {
widget.product.name = value;
});
}
How to call TextField in main state :
TextFieldForProduct(
func: childCallBack,
),
CustomTextFieldWidget
class TextFieldForProduct extends StatelessWidget {
const TextFieldForProduct({#required this.func});
final Function func;
#override
Widget build(BuildContext context) {
return Container(
child: Padding(
padding: const EdgeInsets.all(8.0),
child: TextField(
onChanged: (String value) => func(value),
),
),
);
}
}