Flutter-BottomNavigationBar-KeepAlive

main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key, required this.title}) : super(key: key);

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final indexes = List.generate(10, (index) => index);

  PageController _pageController = PageController();

  @override
  void initState() {
    super.initState();
    _pageController = PageController();
  }
  
  @override
  void dispose() {
    super.dispose();
    _pageController.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      bottomNavigationBar: StatefulNaviBar(
        items: [
          for (var i in indexes)
            BottomNavigationBarItem(
              icon: const Icon(Icons.abc),
              label: 'l$i',
              backgroundColor: Colors.grey,
            ),
        ],
        onTap: (index) => _pageController.jumpToPage(index),
      ),
      body: PageView(
        controller: _pageController,
        physics: const NeverScrollableScrollPhysics(),
        children: [
          ...indexes.map((i) {
            return i % 2 == 0 ? PlainWidget(i: i) : AliveWidget(i: i);
          })
        ],
      ),
    );
  }
}

class StatefulNaviBar extends StatefulWidget {
  const StatefulNaviBar({
    Key? key,
    required this.items,
    this.onTap,
    this.initialIndex = 0,
  }) : super(key: key);

  final List<BottomNavigationBarItem> items;

  final ValueChanged<int>? onTap;

  final int initialIndex;

  @override
  State<StatefulWidget> createState() => _StatefulNaviBarState();
}

class _StatefulNaviBarState extends State<StatefulNaviBar> {
  int _selectedIndex = 0;

  @override
  void initState() {
    super.initState();
    _selectedIndex = widget.initialIndex;
  }

  @override
  Widget build(BuildContext context) {
    return BottomNavigationBar(
      currentIndex: _selectedIndex,
      items: widget.items,
      onTap: (index) {
        setState(() => _selectedIndex = index);
        widget.onTap?.call(index);
      },
    );
  }
}

class PlainWidget extends StatelessWidget {
  const PlainWidget({
    Key? key,
    required this.i,
  }) : super(key: key);

  final int i;

  @override
  Widget build(BuildContext context) {
    debugPrint('PlainWidget build $i');
    return Center(
      child: Text('PlainWidget$i'),
    );
  }
}

class AliveWidget extends StatefulWidget {
  const AliveWidget({
    Key? key,
    required this.i,
  }) : super(key: key);

  final int i;

  @override
  State<StatefulWidget> createState() => AliveWidgetState();
}

class AliveWidgetState extends State<AliveWidget> with AutomaticKeepAliveClientMixin {
  @override
  bool get wantKeepAlive => true;

  @override
  Widget build(BuildContext context) {
    super.build(context);

    debugPrint('AliveWidget build ${widget.i}');
    return Center(
      child: Text('AliveWidget${widget.i}'),
    );
  }
}