Flutter Setting initialScrollOffset on a large list becomes laggy and very slow

Question

I am trying to save the list position so when the user closes the app and come back to it, they can pick up where they left off. for a small list, this works fine, but on a larger list, setting the initialscrolloffset hangs the UI (very slow) for a few seconds.

class MyApp extends StatelessWidget {
  final List<String> items;

  MyApp({Key key, @required this.items}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final title = 'Long List';

    ScrollController _controller = ScrollController(initialScrollOffset: 29900);

    return MaterialApp(
      title: title,
      home: Scaffold(
        appBar: AppBar(
          title: Text(title),
        ),
        body: ListView.builder(
          itemCount: items.length,
          controller: _controller,
          itemBuilder: (context, index) {
            return ListTile(
              title: Text('${items[index]}'),
            );
          },
        ),

Answer 1

You can copy paste run full code below
From https://github.com/flutter/flutter/issues/12319#issuecomment-569296423
You can use scrollable_positioned_list
It provide jumpTo
code snippet

import 'package:flutter_widgets/flutter_widgets.dart';
...
itemScrollController.jumpTo(index: 150);
...
Widget list(Orientation orientation) => ScrollablePositionedList.builder(
            itemCount: numberOfItems,
            itemBuilder: (context, index) => item(index, orientation),
            itemScrollController: itemScrollController,
            itemPositionsListener: itemPositionsListener,
            reverse: reversed,
            scrollDirection: orientation == Orientation.portrait
                ? Axis.vertical
                : Axis.horizontal,
          );

working demo

full code

import 'package:flutter/material.dart';
import 'dart:math';
import 'package:flutter_widgets/flutter_widgets.dart';

const numberOfItems = 5001;
const minItemHeight = 20.0;
const maxItemHeight = 150.0;
const scrollDuration = Duration(seconds: 2);

/// Example widget that uses [ScrollablePositionedList].
///
/// Shows a [ScrollablePositionedList] along with the following controls:
///   - Buttons to jump or scroll to certain items in the list.
///   - Slider to control the alignment of the items being scrolled or jumped
///   to.
///   - A checkbox to reverse the list.
///
/// If the device this example is being used on is in portrait mode, the list
/// will be vertically scrollable, and if the device is in landscape mode, the
/// list will be horizontally scrollable.
class ScrollablePositionedListPage extends StatefulWidget {
  const ScrollablePositionedListPage({Key key}) : super(key: key);

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

class _ScrollablePositionedListPageState
    extends State<ScrollablePositionedListPage> {
  /// Controller to scroll or jump to a particular item.
  final ItemScrollController itemScrollController = ItemScrollController();

  /// Listener that reports the position of items when the list is scrolled.
  final ItemPositionsListener itemPositionsListener =
      ItemPositionsListener.create();
  List<double> itemHeights;
  List<Color> itemColors;
  bool reversed = false;

  /// The alignment to be used next time the user scrolls or jumps to an item.
  double alignment = 0;

  @override
  void initState() {
    super.initState();
    final heightGenerator = Random(328902348);
    final colorGenerator = Random(42490823);
    itemHeights = List<double>.generate(
        numberOfItems,
        (int _) =>
            heightGenerator.nextDouble() * (maxItemHeight - minItemHeight) +
            minItemHeight);
    itemColors = List<Color>.generate(
        numberOfItems,
        (int _) =>
            Color(colorGenerator.nextInt(pow(2, 32) - 1)).withOpacity(1));
  }

  @override
  Widget build(BuildContext context) => Material(
        child: OrientationBuilder(
          builder: (context, orientation) => Column(
            children: <Widget>[
              Expanded(
                child: list(orientation),
              ),
              positionsView,
              Row(
                children: <Widget>[
                  Column(
                    children: <Widget>[
                      scrollControlButtons,
                      jumpControlButtons,
                      alignmentControl,
                    ],
                  ),
                ],
              )
            ],
          ),
        ),
      );

  Widget get alignmentControl => Row(
        mainAxisSize: MainAxisSize.max,
        children: <Widget>[
          const Text('Alignment: '),
          SizedBox(
            width: 200,
            child: Slider(
              value: alignment,
              onChanged: (double value) => setState(() => alignment = value),
            ),
          ),
        ],
      );

  Widget list(Orientation orientation) => ScrollablePositionedList.builder(
        itemCount: numberOfItems,
        itemBuilder: (context, index) => item(index, orientation),
        itemScrollController: itemScrollController,
        itemPositionsListener: itemPositionsListener,
        reverse: reversed,
        scrollDirection: orientation == Orientation.portrait
            ? Axis.vertical
            : Axis.horizontal,
      );

  Widget get positionsView => ValueListenableBuilder<Iterable<ItemPosition>>(
        valueListenable: itemPositionsListener.itemPositions,
        builder: (context, positions, child) {
          int min;
          int max;
          if (positions.isNotEmpty) {
            // Determine the first visible item by finding the item with the
            // smallest trailing edge that is greater than 0.  i.e. the first
            // item whose trailing edge in visible in the viewport.
            min = positions
                .where((ItemPosition position) => position.itemTrailingEdge > 0)
                .reduce((ItemPosition min, ItemPosition position) =>
                    position.itemTrailingEdge < min.itemTrailingEdge
                        ? position
                        : min)
                .index;
            // Determine the last visible item by finding the item with the
            // greatest leading edge that is less than 1.  i.e. the last
            // item whose leading edge in visible in the viewport.
            max = positions
                .where((ItemPosition position) => position.itemLeadingEdge < 1)
                .reduce((ItemPosition max, ItemPosition position) =>
                    position.itemLeadingEdge > max.itemLeadingEdge
                        ? position
                        : max)
                .index;
          }
          return Row(
            children: <Widget>[
              Expanded(child: Text('First Item: ${min ?? ''}')),
              Expanded(child: Text('Last Item: ${max ?? ''}')),
              const Text('Reversed: '),
              Checkbox(
                  value: reversed,
                  onChanged: (bool value) => setState(() {
                        reversed = value;
                      }))
            ],
          );
        },
      );

  Widget get scrollControlButtons => Row(
        children: <Widget>[
          const Text('scroll to'),
          scrollButton(0),
          scrollButton(5),
          scrollButton(10),
          scrollButton(100),
          scrollButton(1000),
          scrollButton(5000),
        ],
      );

  Widget get jumpControlButtons => Row(
        children: <Widget>[
          const Text('jump to'),
          jumpButton(0),
          jumpButton(5),
          jumpButton(10),
          jumpButton(100),
          jumpButton(1000),
          jumpButton(5000),
        ],
      );

  Widget scrollButton(int value) => GestureDetector(
        key: ValueKey<String>('Scroll$value'),
        onTap: () => scrollTo(value),
        child: Padding(
            padding: const EdgeInsets.symmetric(horizontal: 20),
            child: Text('$value')),
      );

  Widget jumpButton(int value) => GestureDetector(
        key: ValueKey<String>('Jump$value'),
        onTap: () => jumpTo(value),
        child: Padding(
            padding: const EdgeInsets.symmetric(horizontal: 20),
            child: Text('$value')),
      );

  void scrollTo(int index) => itemScrollController.scrollTo(
      index: index,
      duration: scrollDuration,
      curve: Curves.easeInOutCubic,
      alignment: alignment);

  void jumpTo(int index) =>
      itemScrollController.jumpTo(index: index, alignment: alignment);

  /// Generate item number [i].
  Widget item(int i, Orientation orientation) {
    return SizedBox(
      height: orientation == Orientation.portrait ? itemHeights[i] : null,
      width: orientation == Orientation.landscape ? itemHeights[i] : null,
      child: Container(
        color: itemColors[i],
        child: Center(
          child: Text('Item $i'),
        ),
      ),
    );
  }
}

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: ScrollablePositionedListPage(),
    );
  }
}