How to overlap SliverList on a SliverAppBar

Question

I'm trying to overlap a SliverList a few pixels over the SliverAppBar. Similar to this post. I'd like the image in the FlexibleSpaceBar to go under the radius of my SliverList. I'm attempting to achieve the below.

I can only get the radius like so. Without the ability to overlap the SliverList onto he SliverAppBar.

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: CustomScrollView(
        slivers: <Widget>[
          SliverAppBar(
            floating: false,
            expandedHeight: MediaQuery.of(context).size.height * 0.50,
            flexibleSpace: FlexibleSpaceBar(
              background: Image.network(pet.photos.first)
            ),
          ),
          SliverList(
            delegate: SliverChildListDelegate([
              Container(
                height: 40,
                decoration: BoxDecoration(
                  color: Colors.red,
                  borderRadius: BorderRadius.only(
                    topLeft: Radius.circular(30),
                    topRight: Radius.circular(30),
                  ),
                ),
              ),
            ]),
          )
        ],
      ),
    );
  }

Any direction would be appreciated, thanks!

Answer 1

You can use the following widget which uses Stack and scroll listeners to achieve something like the screenshots. It's based on https://pub.dartlang.org/packages/sliver_fab.

The widget:

import 'package:flutter/material.dart';

class DetailScaffold extends StatefulWidget {
  final ScrollController controller;
  final ScrollPhysics physics;
  final List<Widget> slivers;

  final double expandedHeight;

  /// Changes edge behavior to account for [SliverAppBar.pinned].
  ///
  /// Hides the edge when the [ScrollController.offset] reaches the collapsed
  /// height of the [SliverAppBar] to prevent it from overlapping the app bar.
  final bool hasPinnedAppBar;

  DetailScaffold({
    @required this.expandedHeight,
    this.controller,
    this.physics,
    this.slivers,
    this.hasPinnedAppBar = false,
  }) {
    assert(expandedHeight != null);
    assert(hasPinnedAppBar != null);
  }

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

class _DetailScaffoldState extends State<DetailScaffold> {
  ScrollController ctrl;

  @override
  void initState() {
    super.initState();

    ctrl = widget.controller ?? ScrollController();
    ctrl.addListener(() => setState(() {}));
  }

  @override
  void dispose() {
    if (widget.controller == null) {
      ctrl.dispose();
    }

    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Stack(
      children: <Widget>[
        CustomScrollView(
          controller: ctrl,
          physics: widget.physics,
          slivers: widget.slivers,
        ),
        _buildEdge(),
      ],
    );
  }

  _buildEdge() {
    var edgeHeight = 12.0;
    var paddingTop = MediaQuery.of(context).padding.top;

    var defaultOffset =
        (paddingTop + widget.expandedHeight) - edgeHeight;

    var top = defaultOffset;
    var edgeSize = edgeHeight;

    if (ctrl.hasClients) {
      double offset = ctrl.offset;
      top -= offset > 0 ? offset : 0;

      if (widget.hasPinnedAppBar) {
        // Hide edge to prevent overlapping the toolbar during scroll.
        var breakpoint =
            widget.expandedHeight - kToolbarHeight - edgeHeight;

        if (offset >= breakpoint) {
          edgeSize = edgeHeight - (offset - breakpoint);
          if (edgeSize < 0) {
            edgeSize = 0;
          }

          top += (edgeHeight - edgeSize);
        }
      }
    }

    return Positioned(
      top: top,
      left: 0,
      right: 0,
      child: Container(
        height: edgeSize,
        decoration: BoxDecoration(
          color: Theme.of(context).canvasColor,
          borderRadius: BorderRadius.vertical(
            top: Radius.circular(12),
          ),
        ),
      ),
    );
  }
}

Using it:

class Home extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    var expandedHeight = 128.0;

    return DetailScaffold(
      expandedHeight: expandedHeight,
      slivers: <Widget>[
        SliverAppBar(
          expandedHeight: expandedHeight,
          flexibleSpace: FlexibleSpaceBar(
            background: Container(color: Colors.purple),
          ),
        ),
        SliverList(
          delegate: SliverChildBuilderDelegate((_, i) {
            return ListTile(title: Text('Item $i'));
          }, childCount: 50),
        ),
      ],
    );
  }
}

Answer 2

I found an easier way to achieve that:

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'How to overlap SliverList on a SliverAppBar',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Home(),
    );
  }
}

class Home extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: CustomScrollView(
        slivers: <Widget>[
          SliverAppBar(
            bottom: PreferredSize(
              child: Container(),
              preferredSize: Size(0, 20),
            ),
            pinned: false,
            expandedHeight: MediaQuery.of(context).size.height * 0.4,
            flexibleSpace: Stack(
              children: [
                Positioned(
                    child: Image(
                      fit: BoxFit.cover,
                      image: NetworkImage(
                        "https://images.pexels.com/photos/62389/pexels-photo-62389.jpeg?auto=compress&cs=tinysrgb&dpr=2&h=750&w=1260",
                      ),
                    ),
                    top: 0,
                    left: 0,
                    right: 0,
                    bottom: 0),
                Positioned(
                  child: Container(
                    height: 30,
                    decoration: BoxDecoration(
                      color: Colors.lightBlue[100],
                      borderRadius: BorderRadius.vertical(
                        top: Radius.circular(50),
                      ),
                    ),
                  ),
                  bottom: -1,
                  left: 0,
                  right: 0,
                ),
              ],
            ),
          ),
          SliverFixedExtentList(
            itemExtent: 50.0,
            delegate: SliverChildBuilderDelegate(
              (BuildContext context, int index) {
                return Container(
                  alignment: Alignment.center,
                  color: Colors.lightBlue[100 * (index + 1 % 9)],
                  child: Text('List Item $index'),
                );
              },
            ),
          ),
        ],
      ),
    );
  }
}

You can see it in DartPad: https://dartpad.dev/e2976ad6ede98813b75ee44b1217cc96

The important part is to use a stack in the flexibleSpace that contains the content you wanna show as a positioned widget and another position widget with the border decoration.

You also need to use the bottom section of the SliverAppBar setting the preferred size as the same as the widget with border decoration.

The DartPad does not render the widget properly (it shows a margin at the bottom, but this does not happen in the iOS/Android).