RefreshIndicator with NestedScrollview

Question

I want 2 tab pages with a ListView each to share a single RefreshIndicator. However, a RefreshIndicator must have Scrollable as a child (which a TabBarView isn't) so instead I tried making 2 RefreshIndicators per tab as shown in the code below.

But this brings a different problem, I also wanted a floating AppBar which meant I had to use a NestedScrollView. So as a result I end up triggering both RefreshIndicators' onRefresh method whenever I scroll down. Whereas I only need one to refresh.

import 'package:flutter/material.dart';

main() {
    runApp(
        MaterialApp(
            home: DefaultTabController(
                length: 2,
                child: Scaffold(
                    body: NestedScrollView(
                        headerSliverBuilder: (context, innerBoxIsScrolled) {
                            return [
                                SliverAppBar(
                                    floating: true,
                                    snap: true,
                                    bottom: TabBar(
                                        tabs: [
                                            Tab(text: 'Page1'),
                                            Tab(text: 'Page2'),
                                        ],
                                    ),
                                ),
                            ];
                        },
                        body: TabBarView(
                            children: [
                                Page(1),
                                Page(2),
                            ],
                        ),
                    ),
                ),
            ),
        ),
    );
}

class Page extends StatefulWidget {
    final pageNumber;
    Page(this.pageNumber);
    createState() => PageState();
}

class PageState extends State<Page> with AutomaticKeepAliveClientMixin {
    get wantKeepAlive => true;

    build(context){
        super.build(context);
        return RefreshIndicator(
            onRefresh: () => Future(() async {
                print('Refreshing page no. ${widget.pageNumber}');  // This prints twice once both tabs have been opened
                await Future.delayed(Duration(seconds: 5));
            }),
            child: ListView.builder(
                itemBuilder: ((context, index){
                    return ListTile(
                        title: Text('Item $index')
                    );
                }),
            )
        );
    }
}

The AutomaticKeepAliveClientMixin is there to prevent the pages rebuilding every time I switch tabs as this would be an expensive process in my actual app.

A solution that uses a single RefreshIndicator for both tabs would be most ideal, but any help is appreciated.

Answer 1

This code is working for me, it's mostly built on top of @Nuts answer, but it needed some tweaking and my edit was rejected

DefaultTabController(
      length: tabs.length,
      child: RefreshIndicator(
        notificationPredicate: (notification) {
          // with NestedScrollView local(depth == 2) OverscrollNotification are not sent
          return notification.depth == 2;
        },
        onRefresh: () => Future.value(null),
        child: NestedScrollView(
          headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
            return [
              SliverAppBar(...)
            ];
          },
          body: TabBarView(
            children: tabs,
          ),
        ),
      ),
    )