Flutter/Dart Reload widget and send new data - flutter

Question

I would like to have a text field in a widget where you can search for something. If the string changes, the widget should be reloaded and the new data sent.
I've read many things about a callback function, unfortunately I don't quite understand it at flutter.
Below you will find the sample code:
Thanks for your help
class _HomepageState extends State<Homepage> {
String data;
String day;
...
TextField(
onChanged: (str){
setState(() {
data = str;
});
},
),
...
Nextpage(data: data, day: day,)
...
}
class Nextpage extends StatefulWidget {
final String data;
final String day;
Nextpage({this.data, this.day});
...
print(widget.data);
...
}

Answer 1

Below is an example where you have a TextField, and as its input changes, the text is shown on a different Text widget (you can copy it and paste it to DartPad to see how it works in action).
import 'package:flutter/material.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatefulWidget {
#override
_MyAppState createState() => _MyAppState();
}
class _MyAppState extends State<MyApp> {
String myText = '';
TextField buildTextField() {
return TextField(
decoration: InputDecoration(
labelText: 'Enter Text',
),
onChanged: (changedText) => updateText(changedText),
onEditingComplete: () => print("complete"),
);
}
void updateText(newText) {
setState(() {
myText = newText;
});
}
#override
Widget build(BuildContext context) {
return MaterialApp(
title: "Example",
theme: ThemeData(primarySwatch: Colors.deepPurple),
home: Scaffold(
body: Container(
child: Column(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: [
buildTextField(),
Text(myText),
]),
),
));
}
}
The callback is this part of the code:
onChanged: (changedText) => updateText(changedText)
and the callback function is updateText.
This is another example of a callback where the print function (the callback function) is called after the editing is complete.
onEditingComplete: () => print("complete")
I hope this helps.