Copy this, wrap your widget widget KeepAlivePage and you will get the AutomaticKeepAliveClientMixin on that widget

keep_alive.dart

import 'package:flutter/material.dart';

class KeepAlivePage extends StatefulWidget {
  const KeepAlivePage({
    Key key,
    @required this.child,
  }) : super(key: key);

  final Widget child;

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

class _KeepAlivePageState extends State<KeepAlivePage>
    with AutomaticKeepAliveClientMixin {
  @override
  void dispose() {
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    super.build(context);
    return widget.child;
  }

  @override
  bool get wantKeepAlive => true;
}