import "package:flutter/widgets.dart";

/// Wraps a scrollable panel body so content approaching the top or bottom
/// edge fades to transparent over [fadeExtent] pixels instead of getting cut
/// off flat by the scroll view's own default hard clip. Purely a paint
/// effect - the scroll views clip still keeps everything contained, this
/// just masks the alpha near each edge on top of that.
///
/// Colour-blind on purpose: the gradient only carries alpha (dstIn blends
/// against whatever's already painted), so it works unchanged over any panel
/// background/scheme rather than needing its own colour token.
class ScrollEdgeFade extends StatelessWidget {
  const ScrollEdgeFade({super.key, required this.child, this.fadeExtent = 6});

  final Widget child;
  final double fadeExtent;

  @override
  Widget build(BuildContext context) {
    return ShaderMask(
      shaderCallback: (bounds) {
        // fraction of the box height the fade eats into from each end - a
        // fixed pixel extent thats clamped so a very short panel doesnt end
        // up with the two fades overlapping and cancelling out the middle.
        final fraction = bounds.height > 0
            ? (fadeExtent / bounds.height).clamp(0.0, 0.5)
            : 0.0;
        return LinearGradient(
          begin: Alignment.topCenter,
          end: Alignment.bottomCenter,
          stops: [0.0, fraction, 1 - fraction, 1.0],
          colors: const [
            Color(0x00ffffff),
            Color(0xffffffff),
            Color(0xffffffff),
            Color(0x00ffffff),
          ],
        ).createShader(bounds);
      },
      blendMode: BlendMode.dstIn,
      child: child,
    );
  }
}