app_bar_parts_pinned_header_0.dart

// PinnedHeaderSliver demo
// https://github.com/flutter/flutter/pull/143196

import 'dart:math' as math;

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

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

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

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

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

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

class RenderPinnedHeaderSliver extends RenderSliverSingleBoxAdapter {
  RenderPinnedHeaderSliver({ super.child });

  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);

    final double layoutExtent = clampDouble(childExtent - constraints.scrollOffset, 0, constraints.remainingPaintExtent);
    final double paintExtent = math.min(childExtent, constraints.remainingPaintExtent - constraints.overlap);
    geometry = SliverGeometry(
      scrollExtent: childExtent,
      paintOrigin: constraints.overlap,
      paintExtent: paintExtent,
      layoutExtent: layoutExtent,
      maxPaintExtent: childExtent,
      maxScrollObstructionExtent: childExtent,
      cacheExtent: calculateCacheOffset(constraints, from: 0.0, to: childExtent),
      hasVisualOverflow: true, // Conservatively say we do have overflow to avoid complexity.
    );
  }
}

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

  @override
  State<PinnedHeaderSliverExample> createState() => _PinnedHeaderSliverExampleState();
}

class _PinnedHeaderSliverExampleState extends State<PinnedHeaderSliverExample> {
  int count = 0;
  late final ScrollController scrollController;

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

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

  @override
  Widget build(BuildContext context) {
    final ThemeData theme = Theme.of(context);
    final ColorScheme colorScheme = theme.colorScheme;

    final Widget header = Container(
      color: colorScheme.background,
      padding: const EdgeInsets.all(4),
      child: Material(
        color: colorScheme.primaryContainer,
        shape: RoundedRectangleBorder(
          borderRadius: BorderRadius.circular(8),
          side: BorderSide(
            width: 7,
            color: colorScheme.outline,
          ),
        ),
        child: Container(
          alignment: Alignment.center,
          padding: const EdgeInsets.symmetric(vertical: 48),
          child: Text(
            count.isOdd ? 'Alternative Title\nWith Two Lines' : 'PinnedHeaderSliver',
            style: theme.textTheme.headlineMedium!.copyWith(
              color: colorScheme.onPrimaryContainer,
            ),
          ),
        ),
      ),
    );

    return Scaffold(
      body: SafeArea(
        child: Padding(
          padding: const EdgeInsets.symmetric(horizontal: 4),
          child: Scrollbar(
            controller: scrollController,
            child: CustomScrollView(
              controller: scrollController,
              slivers: <Widget>[
                PinnedHeaderSliver(child: header),
                const ItemList(),
              ],
            ),
          ),
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          setState(() {
            count += 1;
          });
        },
        child: const Icon(Icons.add),
      ),
    );
  }
}

// A placeholder SliverList of 25 items.
class ItemList extends StatelessWidget {
  const ItemList({
    super.key,
    this.itemCount = 25,
  });

  final int itemCount;

  @override
  Widget build(BuildContext context) {
    final ColorScheme colorScheme = Theme.of(context).colorScheme;
    return SliverList(
      delegate: SliverChildBuilderDelegate(
        (BuildContext context, int index) {
          return Card(
            color: colorScheme.onSecondary,
            child: ListTile(
              textColor: colorScheme.secondary,
              title: Text('Item $index'),
            ),
          );
        },
        childCount: itemCount,
      ),
    );
  }
}