Flutter: Display SliverList in "shaped" out area of SliverPersistentHeader

Question

I have a CustomScrollView which holds

• a SliverPersistentHeader as well as
• a SliverList.

I apply some ShapeBorder to the Widget returned within the SliverPersistentHeaderDelegate.

I would now like my "underneath" placed SliverList to use some of the cut out area of the SliverPersistentHeader, like shown in this screenshot:

How is this possible?

This is my code:

import 'package:flutter/material.dart';

void main() => runApp(
    MediaQuery(data: MediaQueryData(), child: MaterialApp(home: MyApp())));

class MyApp extends StatefulWidget {
  // This widget is the root of your application.
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        body: Container(
            child: CustomScrollView(
      slivers: <Widget>[
        SliverPersistentHeader(
          pinned: true,
          delegate: CustomSliverPersistentHeader(),
        ),
        SliverList(delegate: SliverChildBuilderDelegate(
          (BuildContext context, int index) {
            return Card(
                child: Padding(
              padding: EdgeInsets.all(10),
              child: Text('text $index'),
            ));
          },
        ))
      ],
    )));
  }
}

class CustomSliverPersistentHeader extends SliverPersistentHeaderDelegate {
  @override
  Widget build(
      BuildContext context, double shrinkOffset, bool overlapsContent) {
    return LayoutBuilder(builder: (context, constraints) {
      return Container(
          decoration: ShapeDecoration(
              color: Colors.amber, shape: CustomShape(shrinkOffset)),
          height: constraints.maxHeight,
          child: Container());
    });
  }

  @override
  bool shouldRebuild(SliverPersistentHeaderDelegate _) => true;

  @override
  double get maxExtent => 400.0;

  @override
  double get minExtent => 100.0;
}

class CustomShape extends ShapeBorder {
  final double shrinkOffset;

  CustomShape(this.shrinkOffset);

  @override
  Path getOuterPath(Rect rect, {TextDirection textDirection}) {
    double scrollOffset = 0.0;
    if (shrinkOffset <= 100) {
      scrollOffset = 100.00 - shrinkOffset;
    }

    Offset controllPoint1 = Offset(0, rect.size.height - scrollOffset);
    Offset endPoint1 = Offset(scrollOffset, rect.size.height - scrollOffset);
    Offset controllPoint2 =
        Offset(rect.size.width, rect.size.height - scrollOffset);
    Offset endPoint2 =
        Offset(rect.size.width, rect.size.height - scrollOffset * 2);

    return Path()
      ..lineTo(0, rect.size.height)
      ..quadraticBezierTo(
          controllPoint1.dx, controllPoint1.dy, endPoint1.dx, endPoint1.dy)
      ..lineTo(rect.size.width - scrollOffset, rect.size.height - scrollOffset)
      ..quadraticBezierTo(
          controllPoint2.dx, controllPoint2.dy, endPoint2.dx, endPoint2.dy)
      ..lineTo(rect.size.width, 0);
  }

  @override
  EdgeInsetsGeometry get dimensions => EdgeInsets.only(bottom: 0);

  @override
  Path getInnerPath(Rect rect, {TextDirection textDirection}) => null;

  @override
  void paint(Canvas canvas, Rect rect, {TextDirection textDirection}) {}

  @override
  ShapeBorder scale(double t) => this;
}

Answer 1

Thanks to @pskink's great help in this regard, I learned that I had to use an OverflowBox (which lets its child overflow itself):

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

void main() => runApp(
    MediaQuery(data: MediaQueryData(), child: MaterialApp(home: MyApp())));

class MyApp extends StatefulWidget {
  // This widget is the root of your application.
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        body: Container(
            child: CustomScrollView(
      slivers: <Widget>[
        SliverPersistentHeader(
          pinned: true,
          delegate: CustomSliverPersistentHeader(),
        ),
        SliverList(delegate: SliverChildBuilderDelegate(
          (BuildContext context, int index) {
            return Card(
                child: Padding(
              padding: EdgeInsets.all(10),
              child: Text('text $index'),
            ));
          },
        ))
      ],
    )));
  }
}

class CustomSliverPersistentHeader extends SliverPersistentHeaderDelegate {
  @override
  Widget build(
      BuildContext context, double shrinkOffset, bool overlapsContent) {
    return LayoutBuilder(builder: (context, constraints) {
      return OverflowBox(
        maxHeight: constraints.biggest.height + 100,
        alignment: Alignment.topCenter,
        child: SizedBox.fromSize(
          size: constraints.biggest + Offset(0, max(0, 100 - shrinkOffset)),
          // The following Container can be replaced by a ClipPath with a
          // ShapeBorderClipper, albeit at the expense of not being able to add
          // shadows and other fancy "Container" stuff ;-)
          child: Container(
            clipBehavior: Clip.antiAlias,
            decoration: ShapeDecoration(shape: CustomShape(shrinkOffset)),
            child: Container(
              color: Colors.amber,
            ),
          ),
        ),
      );
    });
  }

  @override
  bool shouldRebuild(SliverPersistentHeaderDelegate _) => true;

  @override
  double get maxExtent => 400.0;

  @override
  double get minExtent => 100.0;
}

class CustomShape extends ShapeBorder {
  final double shrinkOffset;

  CustomShape(this.shrinkOffset);

  @override
  Path getOuterPath(Rect rect, {TextDirection textDirection}) {
    double scrollOffset = 0.0;
    if (shrinkOffset <= 100) {
      scrollOffset = 100.00 - shrinkOffset;
    }

    Offset controllPoint1 = Offset(0, rect.size.height - scrollOffset);
    Offset endPoint1 = Offset(scrollOffset, rect.size.height - scrollOffset);
    Offset controllPoint2 =
        Offset(rect.size.width, rect.size.height - scrollOffset);
    Offset endPoint2 =
        Offset(rect.size.width, rect.size.height - scrollOffset * 2);

    return Path()
      ..lineTo(0, rect.size.height)
      ..quadraticBezierTo(
          controllPoint1.dx, controllPoint1.dy, endPoint1.dx, endPoint1.dy)
      ..lineTo(rect.size.width - scrollOffset, rect.size.height - scrollOffset)
      ..quadraticBezierTo(
          controllPoint2.dx, controllPoint2.dy, endPoint2.dx, endPoint2.dy)
      ..lineTo(rect.size.width, 0);
  }

  @override
  EdgeInsetsGeometry get dimensions => EdgeInsets.only(bottom: 0);

  @override
  Path getInnerPath(Rect rect, {TextDirection textDirection}) => null;

  @override
  void paint(Canvas canvas, Rect rect, {TextDirection textDirection}) {}

  @override
  ShapeBorder scale(double t) => this;
}

Should you want to add some spacing in between the SliverPersistentHeader and the SliverList you can wrap the latter in a SliverPadding.