How to enable buttons - flutter

Question

In one of my Flutter app pages, I want to disable the next button at first. If the user enters any new value in the textfield, I want to enable next button. Here is my code, but the next button stays disabled no matter what. How can I enable the next button when a user enters a value?
#override
Widget build(BuildContext context) {
bool btnenabled = false;
return Scaffold(
appBar: AppBar(
backgroundColor: Colors.red,
title: Text('Address'),
),
body: Column(
children: <Widget>[
.....
TextFormField(
onChanged: (newValue) {
_currentaddress = '$addressJSON' + newValue;
if (newValue.length > 0) {
setState(() {
btnenabled = true;
});
}
}),
TextButton(
style: TextButton.styleFrom(
backgroundColor: Colors.blue[700], primary: Colors.white),
child: Text('next',
),
onPressed: btnenabled == true
? () => Navigator.push(context,
MaterialPageRoute(builder: (context) => Summary()))
: null)
],
),
);
}
}

Answer 1

You can copy paste run full code below
You can move bool btnenabled = false; out of Widget build
change from
Widget build(BuildContext context) {
bool btnenabled = false;
to
bool btnenabled = false;
...
Widget build(BuildContext context) {
working demo
full code
import 'package:flutter/material.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
#override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
bool btnenabled = false;
String _currentaddress = "";
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
backgroundColor: Colors.red,
title: Text('Address'),
),
body: Column(
children: <Widget>[
TextFormField(onChanged: (newValue) {
_currentaddress = 'addressJSON' + newValue;
if (newValue.length > 0) {
setState(() {
btnenabled = true;
});
}
}),
TextButton(
style: TextButton.styleFrom(
backgroundColor: Colors.blue[700], primary: Colors.white),
child: Text(
'next',
),
onPressed: btnenabled == true
? () => Navigator.push(context,
MaterialPageRoute(builder: (context) => Summary()))
: null)
],
),
);
}
}
class Summary extends StatelessWidget {
#override
Widget build(BuildContext context) {
return Text("Summary");
}
}