pageview index not updating

page.dart

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatefulWidget {
  MyApp({Key? key}) : super(key: key);

  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        title: 'Flutter Demo',
        theme: ThemeData(
          primarySwatch: Colors.blue,
        ),
        home: HomePage());
  }
}

class HomePage extends StatefulWidget {
  const HomePage({Key? key}) : super(key: key);

  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  PageController _controller = PageController(initialPage: 0);

  int length = 25;

  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    if (_controller.hasClients) {
      _controller.addListener(increment);
    }
  }

  void increment() {
    index++;
    print(index);
  }

  int index = 0;
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text(
            '${index}',
          ),
        ),
        body: PageView.builder(
          itemCount: length,
          scrollDirection: Axis.horizontal,
          controller: _controller,
          onPageChanged: (x) {
            print(x);
          },
          itemBuilder: (BuildContext context, int x) {
            return Center(
              child: Text('item $x'),
            );
          },
        ));
  }
}