app_bar_parts_floating_header_v0.dart

import 'dart:math' as math;

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

class Item extends StatelessWidget {
  const Item({ super.key, required this.title, required this.color });

  final String title;
  final Color color;

  @override
  Widget build(BuildContext context) {
    return Card(
      color: color,
      child: ListTile(
        textColor: Colors.white,
        title: Text(title),
      ),
    );
  }
}

// A placeholder SliverList of 50 items.
class ItemList extends StatelessWidget {
  const ItemList({
    super.key,
    required this.startColor,
    required this.endColor,
    this.itemCount = 50,
  });

  final Color startColor;
  final Color endColor;
  final int itemCount;

  @override
  Widget build(BuildContext context) {
    return SliverList(
      delegate: SliverChildBuilderDelegate(
        (BuildContext context, int index) {
          return Item(
            title: 'Item $index',
            color: Color.lerp(startColor, endColor, index / itemCount)!
          );
        },
        childCount: itemCount,
      ),
    );
  }
}

class FloatingHeaderSliver extends StatefulWidget {
  const FloatingHeaderSliver({ super.key, required this.child });

  final Widget child;

  @override
  State<FloatingHeaderSliver> createState() => _FloatingHeaderSliverState();
}

class _FloatingHeaderSliverState extends State<FloatingHeaderSliver> with SingleTickerProviderStateMixin {
  @override
  Widget build(BuildContext context) {
    return _FloatingHeaderSliver(
      child: widget.child,
    );
  }
}

class _FloatingHeaderSliver extends SingleChildRenderObjectWidget {
  const _FloatingHeaderSliver({
    super.child,
  });

  @override
  _RenderFloatingHeaderSliver createRenderObject(BuildContext context) {
    return _RenderFloatingHeaderSliver();
  }

  @override
  void updateRenderObject(BuildContext context, _RenderFloatingHeaderSliver renderObject) {
  }
}

class _RenderFloatingHeaderSliver extends RenderSliverSingleBoxAdapter {
  _RenderFloatingHeaderSliver();

  ScrollDirection scrollDirection = ScrollDirection.idle;
  double? effectiveScrollOffset;
  double? scrollAnchor;

  double get childExtent {
    if (child == null) {
      return 0.0;
    }
    assert(child!.hasSize);
    return switch (constraints.axis) {
      Axis.vertical => child!.size.height,
      Axis.horizontal => child!.size.width,
    };
  }

  @override
  double childMainAxisPosition(covariant RenderObject child) => 0;

  @override
  void performLayout() {
    final SliverConstraints constraints = this.constraints;
    child?.layout(constraints.asBoxConstraints(), parentUsesSize: true);

    late final double scrollOffset;
    late final double paintExtent;
    print(constraints.userScrollDirection);
    if (constraints.userScrollDirection == ScrollDirection.forward && (scrollAnchor != null || constraints.scrollOffset >= childExtent)) {
      scrollAnchor ??= constraints.scrollOffset;
      final double scrollAnchorDelta = scrollAnchor! - constraints.scrollOffset;
      scrollOffset = clampDouble(childExtent - scrollAnchorDelta, 0, constraints.remainingPaintExtent);
      paintExtent = childExtent - scrollOffset;
    } else {
      scrollAnchor = null;
      paintExtent = clampDouble(childExtent - constraints.scrollOffset, 0, constraints.remainingPaintExtent);
      scrollOffset = clampDouble(constraints.scrollOffset, 0, childExtent);
    }

    geometry = SliverGeometry(
      scrollExtent: childExtent,
      paintOrigin: math.max(-scrollOffset, -childExtent),
      paintExtent: paintExtent,
      layoutExtent: paintExtent,
      maxPaintExtent: childExtent,
      maxScrollObstructionExtent: childExtent,
      cacheExtent: calculateCacheOffset(constraints, from: 0.0, to: childExtent),
      hasVisualOverflow: true, // Conservatively say we do have overflow to avoid complexity.
    );
  }
}

class AppBarParts extends StatefulWidget {
  const AppBarParts({ super.key });

  @override
  State<AppBarParts> createState() => _AppBarPartsState();
}

class _AppBarPartsState extends State<AppBarParts> {
  late final ScrollController scrollController;

  @override
  void initState() {
    super.initState();
    scrollController = ScrollController();
  }

  @override
  void dispose() {
    scrollController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    const EdgeInsets horizontalPadding = EdgeInsets.symmetric(horizontal: 8);

    return Scaffold(
      body: SafeArea(
        child: Padding(
          padding: const EdgeInsets.symmetric(horizontal: 4),
          child: CustomScrollView(
            controller: scrollController,
            slivers: const <Widget>[
              SliverPadding(
                padding: horizontalPadding,
                sliver: ItemList(
                  startColor: Colors.blue,
                  endColor: Colors.red,
                  itemCount: 3,
                ),
              ),
              SliverPadding(
                padding: horizontalPadding,
                sliver: FloatingHeaderSliver(
                  child: SizedBox(
                    height: 128,
                    child: Item(
                      title: 'FloatingHeaderSliver',
                      color: Colors.orange,
                    ),
                  ),
                ),
              ),
              SliverPadding(
                padding: horizontalPadding,
                sliver: ItemList(
                  startColor: Colors.blue,
                  endColor: Colors.red,
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}

class AppBarPartsApp extends StatelessWidget {
  const AppBarPartsApp({ super.key });

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(useMaterial3: true),
      home: const AppBarParts(),
    );
  }
}

void main() {
  runApp(const AppBarPartsApp());
}