import 'package:flutter/material.dart';

import 'common.dart';

/// Builds header content for a given collapse progress: 0 fully open, 1 fully
/// collapsed.
typedef CollapsingHeaderBuilder =
    Widget Function(BuildContext context, double t);

/// The pinned brand header used by the main tabs.
///
/// It stays on screen while the content scrolls beneath it, and shrinks from
/// [expandedHeight] to [collapsedHeight] on the way. Pinning a tall header at
/// full size would cost a quarter of a phone screen for the whole session;
/// collapsing keeps the context and gives the space back.
class KrowSliverHeader extends StatelessWidget {
  const KrowSliverHeader({
    super.key,
    required this.expandedHeight,
    required this.collapsedHeight,
    required this.builder,
  });

  /// Heights exclude the status bar, which is added for you.
  final double expandedHeight;
  final double collapsedHeight;
  final CollapsingHeaderBuilder builder;

  @override
  Widget build(BuildContext context) {
    final top = MediaQuery.paddingOf(context).top;
    return SliverPersistentHeader(
      pinned: true,
      delegate: _HeaderDelegate(
        minHeight: collapsedHeight + top,
        maxHeight: expandedHeight + top,
        topInset: top,
        builder: builder,
      ),
    );
  }
}

class _HeaderDelegate extends SliverPersistentHeaderDelegate {
  _HeaderDelegate({
    required this.minHeight,
    required this.maxHeight,
    required this.topInset,
    required this.builder,
  });

  final double minHeight;
  final double maxHeight;
  final double topInset;
  final CollapsingHeaderBuilder builder;

  @override
  double get minExtent => minHeight;

  @override
  double get maxExtent => maxHeight;

  @override
  Widget build(BuildContext context, double shrinkOffset, bool overlaps) {
    final range = maxHeight - minHeight;
    final t = range <= 0 ? 1.0 : (shrinkOffset / range).clamp(0.0, 1.0);
    return BrandSurface(
      child: Padding(
        padding: EdgeInsets.only(top: topInset),
        child: builder(context, t),
      ),
    );
  }

  @override
  bool shouldRebuild(_HeaderDelegate old) =>
      old.minHeight != minHeight ||
      old.maxHeight != maxHeight ||
      old.topInset != topInset ||
      old.builder != builder;
}

/// Cross-fades a header between its open and collapsed states.
///
/// The open content keeps its natural size and is clipped as the header
/// shrinks — letting it compress instead would squash the type on the way
/// down, and would overflow before it finished fading.
class HeaderCrossFade extends StatelessWidget {
  const HeaderCrossFade({
    super.key,
    required this.t,
    required this.expandedHeight,
    required this.expanded,
    required this.collapsed,
  });

  final double t;
  final double expandedHeight;
  final Widget expanded;
  final Widget collapsed;

  @override
  Widget build(BuildContext context) {
    // The open state clears out over the first two thirds; the collapsed bar
    // arrives over the last half. They overlap briefly, which reads as one
    // state becoming the other rather than a swap.
    final openOpacity = (1 - t * 1.5).clamp(0.0, 1.0);
    final shutOpacity = ((t - 0.5) * 2).clamp(0.0, 1.0);

    return Stack(
      fit: StackFit.expand,
      children: [
        if (openOpacity > 0)
          ClipRect(
            child: OverflowBox(
              alignment: Alignment.topCenter,
              minHeight: 0,
              // Unbounded, so content taller than the header lays out at its
              // natural size and is clipped. Capping it here instead would
              // throw a layout overflow — and one only visible in a debug
              // build, so it reaches a device before it reaches you.
              maxHeight: double.infinity,
              child: Opacity(
                opacity: openOpacity,
                child: IgnorePointer(ignoring: t > 0.5, child: expanded),
              ),
            ),
          ),
        if (shutOpacity > 0)
          Opacity(
            opacity: shutOpacity,
            child: IgnorePointer(ignoring: t < 0.5, child: collapsed),
          ),
      ],
    );
  }
}