Flutter Sliver Header

sliver_header.dart

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

class SliverHeader extends SliverPersistentHeaderDelegate {
  SliverHeader({
    this.minExtent = 130,
    this.leading,
    this.middle,
    this.trailing,
    this.bottom,
    this.automaticallyImplyLeading = true,
  });

  final Widget? leading;
  
  final Widget? middle;

  final Widget? trailing;

  final PreferredSizeWidget? bottom;

  final bool automaticallyImplyLeading;

  @override
  final double minExtent;

  @override
  double get maxExtent => minExtent + (bottom?.preferredSize.height ?? 0);

  @override
  Widget build(
    BuildContext context,
    double shrinkOffset,
    bool overlapsContent,
  ) {
    final theme = Theme.of(context);
    final topPadding = MediaQuery.of(context).padding.top;

    final fixedSpace = minExtent + topPadding;
    final currentExtent = math.max(fixedSpace, maxExtent - shrinkOffset);

    final parentRoute = ModalRoute.of(context);
    final canPop = parentRoute?.canPop ?? false;

    Widget? leading = this.leading;
    if (leading == null && automaticallyImplyLeading) {
      if (canPop || (parentRoute?.impliesAppBarDismissal ?? false)) {
        leading = const BackButton();
      }
    }
    leading = ConstrainedBox(
      constraints: BoxConstraints.tightFor(width: minExtent - topPadding),
      child: Center(child: leading),
    );

    final trailing = ConstrainedBox(
      constraints: BoxConstraints.tightFor(width: minExtent - topPadding),
      child: Center(child: this.trailing),
    );
    
    final middle = ConstrainedBox(
      constraints: const BoxConstraints.tightFor(height: 40),
      child: Center(child: this.middle),
    );
    
    final toolbar = IconTheme.merge(
      data: theme.primaryIconTheme,
      child: NavigationToolbar(
        leading: leading,
        middle: middle,
        trailing: trailing,
      ),
    );

    return MediaQuery.removePadding(
      context: context,
      removeBottom: true,
      child: AnnotatedRegion<SystemUiOverlayStyle>(
        value: theme.appBarTheme.systemOverlayStyle!,
        child: Material(
          type: MaterialType.transparency,
          child: Container(
            alignment: Alignment.topCenter,
            constraints: BoxConstraints.tightFor(
              height: math.max(minExtent, currentExtent - 50),
            ),
            decoration: const BoxDecoration(
              borderRadius: BorderRadius.vertical(
                bottom: Radius.circular(32),
              ),
              gradient: LinearGradient(
                begin: Alignment.topLeft,
                end: Alignment.bottomRight,
                colors: [Color(0xFFC80C0F), Color(0XFFF20A0E)],
                stops: [.21, 1],
              ),
            ),
            child: SafeArea(
              child: toolbar,
            ),
          ),
        ),
      ),
    );
  }

  @override
  bool shouldRebuild(covariant SliverPersistentHeaderDelegate oldDelegate) {
    return bottom != (oldDelegate as SliverHeader).bottom;
  }
}