android - Flutter TabBar and SliverAppBar that hides when you scroll down

Question

I am trying to create an app with a top application bar and a tab bar below. When you scroll down, the bar should hide by moving off the screen (but tabs should stay), and when you scroll back up, the application bar should show again. This behaviour can be seen in WhatsApp. Please see this video for a demonstration. (Taken from Material.io). This is a similar behaviour, although the app bar and tab bar are hidden on scroll, so it is not exactly the behaviour I am looking for.

I have been able to achieve the autohiding, however, there are a few issues:

1. I have to set the snap of the SliverAppBar to true. Without this, the application bar will not show when I scroll back up.

   Although this is works, it is not the behaviour I am looking for. I want the application bar to show smoothly (similar to WhatsApp) rather than coming into view even if you scroll very little.

   To clarify, when I scroll all the way down, even if I scroll up very little, the app bar should come into view. I do not want to have to scroll all the way up to see the app bar.

2. When I scroll down and change tabs, a little bit of the content is cut out of view.

   Below is a GIF showing the behaviour:

   

   (See the part when I scroll down on the listView (tab1), then move back to tab2)

Here is the code for the DefaultTabController:

DefaultTabController(
  length: 2,
  child: new Scaffold(
    body: new NestedScrollView(
      headerSliverBuilder:
          (BuildContext context, bool innerBoxIsScrolled) {
        return <Widget>[
          new SliverAppBar(
            title: Text("Application"),
            floating: true,
            pinned: true,
            snap: true,    // <--- this is required if I want the application bar to show when I scroll up
            bottom: new TabBar(
              tabs: [ ... ],    // <-- total of 2 tabs
            ),
          ),
        ];
      },
      body: new TabBarView(
        children: [ ... ]    // <--- the array item is a ListView
      ),
    ),
  ),
),

In case it is needed, the full code is in this GitHub repository. main.dart is here.

I also found this related question: Hide Appbar on Scroll Flutter?. However, it did not provide the solution. The same problems persist, and when you scroll up, the SliverAppBar will not show. (So snap: true is required)

I also found this issue on Flutter's GitHub. (Edit: someone commented that they are waiting for the Flutter team to fix this. Is there a possibility that there is no solution?)

This is the output of flutter doctor -v: Pastebin. Certain issues are found, but from what I have learned, they should not have an impact.

Edit: There are two issues for this:

• https://github.com/flutter/flutter/issues/29561 (closed)
• https://github.com/flutter/flutter/issues/17518

Answer 1

You need to use SliverOverlapAbsorber/SliverOverlapInjector, the following code works for me (Full Code):

@override
  Widget build(BuildContext context) {
    return Material(
      child: Scaffold(
        body: DefaultTabController(
          length: _tabs.length, // This is the number of tabs.
          child: NestedScrollView(
            headerSliverBuilder:
                (BuildContext context, bool innerBoxIsScrolled) {
              // These are the slivers that show up in the "outer" scroll view.
              return <Widget>[
                SliverOverlapAbsorber(
                  // This widget takes the overlapping behavior of the SliverAppBar,
                  // and redirects it to the SliverOverlapInjector below. If it is
                  // missing, then it is possible for the nested "inner" scroll view
                  // below to end up under the SliverAppBar even when the inner
                  // scroll view thinks it has not been scrolled.
                  // This is not necessary if the "headerSliverBuilder" only builds
                  // widgets that do not overlap the next sliver.
                  handle:
                      NestedScrollView.sliverOverlapAbsorberHandleFor(context),
                  child: SliverSafeArea(
                    top: false,
                    sliver: SliverAppBar(
                      title: const Text('Books'),
                      floating: true,
                      pinned: true,
                      snap: false,
                      primary: true,
                      forceElevated: innerBoxIsScrolled,
                      bottom: TabBar(
                        // These are the widgets to put in each tab in the tab bar.
                        tabs: _tabs.map((String name) => Tab(text: name)).toList(),
                      ),
                    ),
                  ),
                ),
              ];
            },
            body: TabBarView(
              // These are the contents of the tab views, below the tabs.
              children: _tabs.map((String name) {
                return SafeArea(
                  top: false,
                  bottom: false,
                  child: Builder(
                    // This Builder is needed to provide a BuildContext that is "inside"
                    // the NestedScrollView, so that sliverOverlapAbsorberHandleFor() can
                    // find the NestedScrollView.
                    builder: (BuildContext context) {
                      return CustomScrollView(
                        // The "controller" and "primary" members should be left
                        // unset, so that the NestedScrollView can control this
                        // inner scroll view.
                        // If the "controller" property is set, then this scroll
                        // view will not be associated with the NestedScrollView.
                        // The PageStorageKey should be unique to this ScrollView;
                        // it allows the list to remember its scroll position when
                        // the tab view is not on the screen.
                        key: PageStorageKey<String>(name),
                        slivers: <Widget>[
                          SliverOverlapInjector(
                            // This is the flip side of the SliverOverlapAbsorber above.
                            handle:
                                NestedScrollView.sliverOverlapAbsorberHandleFor(
                                    context),
                          ),
                          SliverPadding(
                            padding: const EdgeInsets.all(8.0),
                            // In this example, the inner scroll view has
                            // fixed-height list items, hence the use of
                            // SliverFixedExtentList. However, one could use any
                            // sliver widget here, e.g. SliverList or SliverGrid.
                            sliver: SliverFixedExtentList(
                              // The items in this example are fixed to 48 pixels
                              // high. This matches the Material Design spec for
                              // ListTile widgets.
                              itemExtent: 60.0,
                              delegate: SliverChildBuilderDelegate(
                                (BuildContext context, int index) {
                                  // This builder is called for each child.
                                  // In this example, we just number each list item.
                                  return Container(
                                      color: Color((math.Random().nextDouble() *
                                                      0xFFFFFF)
                                                  .toInt() <<
                                              0)
                                          .withOpacity(1.0));
                                },
                                // The childCount of the SliverChildBuilderDelegate
                                // specifies how many children this inner list
                                // has. In this example, each tab has a list of
                                // exactly 30 items, but this is arbitrary.
                                childCount: 30,
                              ),
                            ),
                          ),
                        ],
                      );
                    },
                  ),
                );
              }).toList(),
            ),
          ),
        ),
      ),
    );
  }