Place a floating app bar above a list

To make it easier for users to view a list of items, you might want to hide the app bar as the user scrolls down the list. This is especially true if your app displays a “tall” app bar that occupies a lot of vertical space.

Typically, you create an app bar by providing an appBar property to the Scaffold widget. This creates a fixed app bar that always remains above the body of the Scaffold.

Moving the app bar from a Scaffold widget into a CustomScrollView allows you to create an app bar that scrolls offscreen as you scroll through a list of items contained inside the CustomScrollView.

This recipe demonstrates how to use a CustomScrollView to display a list of items with an app bar on top that scrolls offscreen as the user scrolls down the list using the following steps:

1. Create a CustomScrollView.
2. Use SliverAppBar to add a floating app bar.
3. Add a list of items using a SliverList.

1. Create a CustomScrollView

To create a floating app bar, place the app bar inside a CustomScrollView that also contains the list of items. This synchronizes the scroll position of the app bar and the list of items. You might think of the CustomScrollView widget as a ListView that allows you to mix and match different types of scrollable lists and widgets together.

The scrollable lists and widgets provided to the CustomScrollView are known as slivers. There are several types of slivers, such as SliverList, SliverGridList, and SliverAppBar. In fact, the ListView and GridView widgets use the SliverList and SliverGrid widgets to implement scrolling.

For this example, create a CustomScrollView that contains a SliverAppBar and a SliverList. In addition, remove any app bars that you provide to the Scaffold widget.

Scaffold(
  // No appBar property provided, only the body.
  body: CustomScrollView(
    // Add the app bar and list of items as slivers in the next steps.
    slivers: <Widget>[]
  ),
);

2. Use SliverAppBar to add a floating app bar

Next, add an app bar to the CustomScrollView. Flutter provides the SliverAppBar widget which, much like the normal AppBar widget, uses the SliverAppBar to display a title, tabs, images and more.

However, the SliverAppBar also gives you the ability to create a “floating” app bar that scrolls offscreen as the user scrolls down the list. Furthermore, you can configure the SliverAppBar to shrink and expand as the user scrolls.

To create this effect:

1. Start with an app bar that displays only a title.
2. Set the floating property to true. This allows users to quickly reveal the app bar when they scroll up the list.
3. Add a flexibleSpace widget that fills the available expandedHeight.

CustomScrollView(
  slivers: [
    // Add the app bar to the CustomScrollView.
    const SliverAppBar(
      // Provide a standard title.
      title: Text(title),
      // Allows the user to reveal the app bar if they begin scrolling
      // back up the list of items.
      floating: true,
      // Display a placeholder widget to visualize the shrinking size.
      flexibleSpace: Placeholder(),
      // Make the initial height of the SliverAppBar larger than normal.
      expandedHeight: 200,
    ),
  ],
)

3. Add a list of items using a SliverList

Now that you have the app bar in place, add a list of items to the CustomScrollView. You have two options: a SliverList or a SliverGrid. If you need to display a list of items one after the other, use the SliverList widget. If you need to display a grid list, use the SliverGrid widget.

The SliverList and SliverGrid widgets take one required parameter: a SliverChildDelegate, which provides a list of widgets to SliverList or SliverGrid. For example, the SliverChildBuilderDelegate allows you to create a list of items that are built lazily as you scroll, just like the ListView.builder widget.

// Next, create a SliverList
SliverList(
  // Use a delegate to build items as they're scrolled on screen.
  delegate: SliverChildBuilderDelegate(
    // The builder function returns a ListTile with a title that
    // displays the index of the current item.
    (context, index) => ListTile(title: Text('Item #$index')),
    // Builds 1000 ListTiles
    childCount: 1000,
  ),
)

Interactive example

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    const title = 'Floating App Bar';

    return MaterialApp(
      title: title,
      home: Scaffold(
        // No appbar provided to the Scaffold, only a body with a
        // CustomScrollView.
        body: CustomScrollView(
          slivers: [
            // Add the app bar to the CustomScrollView.
            const SliverAppBar(
              // Provide a standard title.
              title: Text(title),
              // Allows the user to reveal the app bar if they begin scrolling
              // back up the list of items.
              floating: true,
              // Display a placeholder widget to visualize the shrinking size.
              flexibleSpace: Placeholder(),
              // Make the initial height of the SliverAppBar larger than normal.
              expandedHeight: 200,
            ),
            // Next, create a SliverList
            SliverList(
              // Use a delegate to build items as they're scrolled on screen.
              delegate: SliverChildBuilderDelegate(
                // The builder function returns a ListTile with a title that
                // displays the index of the current item.
                (context, index) => ListTile(title: Text('Item #$index')),
                // Builds 1000 ListTiles
                childCount: 1000,
              ),
            ),
          ],
        ),
      ),
    );
  }
}