DropExtraChildren.dart

import 'package:flutter/material.dart';

class DropExtraChildren extends StatelessWidget {
  final Axis direction;
  final double spacing;
  final List<Widget> children;

  const DropExtraChildren({
    super.key,
    this.direction = Axis.horizontal,
    required this.spacing,
    required this.children,
  });

  @override
  Widget build(BuildContext context) {
    return LayoutBuilder(
      builder: (context, constraints) {
        return _DropExtraChildrenDelegate(
          direction: direction,
          spacing: spacing,
          constraints: constraints,
          children: children,
        );
      },
    );
  }
}

class _DropExtraChildrenDelegate extends MultiChildLayoutDelegate {
  final Axis direction;
  final double spacing;
  final BoxConstraints constraints;
  final List<Widget> children;

  _DropExtraChildrenDelegate({
    required this.direction,
    required this.spacing,
    required this.constraints,
    required this.children,
  });

  @override
  void performLayout(Size size) {
    double mainAxisOffset = 0.0;
    double crossAxisMax = 0.0;
    int visibleCount = 0;

    // Measure children and determine visible subset
    for (int i = 0; i < children.length; i++) {
      if (hasChild(i)) {
        final childSize = layoutChild(
          i,
          BoxConstraints.loose(constraints.biggest),
        );

        final childMainExtent = (direction == Axis.horizontal)
            ? childSize.width
            : childSize.height;
        final childCrossExtent = (direction == Axis.horizontal)
            ? childSize.height
            : childSize.width;

        // Add spacing between items (except first)
        final totalSpacing = (mainAxisOffset > 0) ? spacing : 0;
        final newMainOffset = mainAxisOffset + totalSpacing + childMainExtent;

        // Check against main axis constraints
        final maxMainExtent = (direction == Axis.horizontal)
            ? constraints.maxWidth
            : constraints.maxHeight;

        if (newMainOffset <= maxMainExtent) {
          mainAxisOffset = newMainOffset;
          crossAxisMax = math.max(crossAxisMax, childCrossExtent);
          visibleCount++;
        } else {
          break; // Stop processing remaining children
        }
      }
    }

    // Position visible children
    double layoutOffset = 0.0;
    for (int i = 0; i < visibleCount; i++) {
      if (hasChild(i)) {
        final childSize = getChildSize(i)!;
        final childMainExtent = (direction == Axis.horizontal)
            ? childSize.width
            : childSize.height;

        // Apply spacing between items
        if (i > 0) layoutOffset += spacing;

        // Position child
        if (direction == Axis.horizontal) {
          positionChild(i, Offset(layoutOffset, 0));
        } else {
          positionChild(i, Offset(0, layoutOffset));
        }

        layoutOffset += childMainExtent;
      }
    }

    // Set final size (main axis: visible children width, cross axis: max child height)
    size = Size(
      (direction == Axis.horizontal) ? layoutOffset : crossAxisMax,
      (direction == Axis.horizontal) ? crossAxisMax : layoutOffset,
    );
  }

  @override
  bool shouldRelayout(_DropExtraChildrenDelegate oldDelegate) {
    return oldDelegate.spacing != spacing ||
        oldDelegate.direction != direction ||
        oldDelegate.constraints != constraints ||
        oldDelegate.children != children;
  }
}