keep_alive.dart

import 'package:flutter/material.dart';

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

class KeepAliveExampleApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: Scaffold(
        body: KeepAlivePage(),
      ),
    );
  }
}

class KeepAlivePage extends StatefulWidget {
  @override
  _KeepAlivePageState createState() => _KeepAlivePageState();
}

class _KeepAlivePageState extends State<KeepAlivePage> {
  var _offset = Offset.zero;

  void _updateOffset(Offset offset) => setState(() => _offset = offset);

  @override
  Widget build(BuildContext context) {
    return CustomScrollView(
      slivers: [
        SliverAppBar(
          title: Text('Drag Position: $_offset'),
          pinned: true,
        ),
        SliverPadding(
          padding: const EdgeInsets.only(top: 50),
        ),
        SliverFixedExtentList(
          itemExtent: 50,
          delegate: SliverChildBuilderDelegate(
            (itemContext, index) {
              var keepAlive = index == 2;
              return KeepAlive(
                key: ValueKey<int>(index),
                keepAlive: keepAlive,
                child: KeepAliveListItem(index, _updateOffset),
              );
            },
            findChildIndexCallback: (Key key) {
              final valKey = key as ValueKey<int>;
              return valKey.value;
            },
            addRepaintBoundaries: false,
            addSemanticIndexes: false,
            addAutomaticKeepAlives: false,
          ),
        ),
      ],
      cacheExtent: 0,
    );
  }
}

class KeepAliveListItem extends StatelessWidget {
  const KeepAliveListItem(this.index, this.updateOffset);
  final int index;
  final Function(Offset) updateOffset;

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      behavior: HitTestBehavior.opaque,
      child: Container(
        height: 50,
        width: 100,
        color: Colors.lightBlue[100 * (index % 9)],
        child: Center(
          child: Text('Item $index ${index == 2 ? " (KEEPALIVE)" : ""}'),
        ),
      ),
      onVerticalDragStart: (details) {},
      onVerticalDragUpdate: (details) {
        RenderBox getBox = context.findRenderObject() as RenderBox;
        final local = getBox.globalToLocal(details.globalPosition);
        updateOffset(local);
      },
      onVerticalDragEnd: (details) {},
    );
  }
}