Flutter - ListView inside on a TabBarView loses its scroll position

Issue

I have a very simple Flutter app with a TabBarView with two views (Tab 1 and Tab 2), one of them (Tab 1) has a ListView with many simple Text Widgets, the problem with this is that after I scroll down the ListView elements of Tab 1, if I swipe from Tab 1 to Tab 2 and finally I swipe from Tab 2 to Tab 1, the previous scroll position in the ListView of Tab 1 get lost.

Here is the 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: 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
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage>
    with SingleTickerProviderStateMixin {
  late TabController controller;

  @override
  void initState() {
    super.initState();
    controller = TabController(
      length: 2,
      vsync: this,
    );
  }

  @override
  void dispose() {
    controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    var tabs = const <Tab>[
      Tab(icon: Icon(Icons.home), text: 'Tab 1'),
      Tab(icon: Icon(Icons.account_box), text: 'Tab 2')
    ];

    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: TabBarView(controller: controller, children: <Widget>[
        ListView(children: <Widget>[
          Column(children: <Widget>[
            for (int i = 0; i < 48; i++) Text('Data $i'),
          ])
        ]),
        const Center(child: Text('Tab 2'))
      ]),
      bottomNavigationBar: Material(
        color: Colors.deepOrange,
        child: TabBar(controller: controller, tabs: tabs),
      ),
    );
  }
}

I have even separated the TabBarView childrens (Tab 1 and Tab 2) in another classes and I have noticed that the

@override
  Widget build(BuildContext context) {
  ...
}

method of each child (Tab 1 and Tab 2) is executed every time I swipe to it's container tab.

My questions are:

1.- How can I keep the scroll of the ListView even if I move from tab to tab?

2.- Is there a way to execute the

@override
Widget build(BuildContext context) {

}

method only once if I separate the TabBarView childrens (Tab 1 and Tab 2) to another classes? I mean, if I have to retrieve data when the Tab 1 and Tab 2 are created I don't want to do this every time it's Tab is swiped in. That would be expensive.

3.- In general, Is there a way to prevent that a tab view (including it's variables, data, etc.) be rebuilt every time I swipe to that tab?

Thank you in advance.

Solution

👇👇👇👇👇👇👇👇👇👇👇👇👇👇👇

Jordan Nelson's answer is the correct one. Don't use mine.

👆👆👆👆👆👆👆👆👆👆👆👆👆👆👆

1.- How can I keep the scroll of the ListView even if I move from tab to tab?

Ok, it wasn't so easy as I thought but I think I managed to do it.

My idea is to keep listview's offset in HomePageState, and when we scroll listview we just get offset from notifier and set it via setter (please make it cleaner and share!).

Then when we rebuild listview we just ask our main widget to give us saved offset and by ScrollController we initialize list with that offset.

I also changed your listview since it had one column element with 50 texts to use 50 elements with one text each. Hope you don't mind :)

The code:

import 'package:flutter/material.dart';

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

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

typedef double GetOffsetMethod();
typedef void SetOffsetMethod(double offset);

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

  final String title;

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage>
    with SingleTickerProviderStateMixin {
  TabController controller;
  double listViewOffset=0.0;

  @override
  void initState() {
    super.initState();
    controller = new TabController(
      length: 2,
      vsync: this,
    );
  }

  @override
  void dispose() {
    controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    var tabs = <Tab>[
      new Tab(icon: new Icon(Icons.home), text: 'Tab 1'),
      new Tab(icon: new Icon(Icons.account_box), text: 'Tab 2')
    ];

    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      body: new TabBarView(
      controller: controller,
      children: <Widget>[
        new StatefulListView(
          getOffsetMethod: () => listViewOffset,
          setOffsetMethod: (offset) => this.listViewOffset = offset,
        ),
        new Center(child: new Text('Tab 2'))
      ]),
      bottomNavigationBar: new Material(
        color: Colors.deepOrange,
        child: new TabBar(controller: controller, tabs: tabs),
      ),
    );
  }
}

class StatefulListView extends StatefulWidget {
  StatefulListView({Key key, this.getOffsetMethod, this.setOffsetMethod}) : super(key: key);

  final GetOffsetMethod getOffsetMethod;
  final SetOffsetMethod setOffsetMethod;

  @override
  _StatefulListViewState createState() => new _StatefulListViewState();
}

class _StatefulListViewState extends State<StatefulListView> {

  ScrollController scrollController;

  @override
  void initState() {
    super.initState();
    scrollController = new ScrollController(
      initialScrollOffset: widget.getOffsetMethod()
    );
  }

  @override
  Widget build(BuildContext context) {
    return new NotificationListener(
      child: new ListView.builder(
        controller: scrollController,
        itemCount: 50,
        itemBuilder: (BuildContext context, int index) {
          return new Text("Data "+index.toString());
        },
      ),
      onNotification: (notification) {
        if (notification is ScrollNotification) {
          widget.setOffsetMethod(notification.metrics.pixels);
        }
      },
    );
  }
}

Answered By - Marcin Szałek