FlowLayout.swift

import SwiftUI

@available(iOS 16, *)
public struct FlowLayout: Layout {

    private let spacing: CGFloat
    
    public init(spacing: CGFloat = 0) {
        self.spacing = spacing
    }

    public func makeCache(subviews: Subviews) -> FlowLayoutCache {
        FlowLayoutCache(
            sizes: subviews.map { $0.sizeThatFits(.unspecified) }
        )
    }

    public func placeSubviews(
        in bounds: CGRect,
        proposal: ProposedViewSize,
        subviews: Subviews,
        cache: inout FlowLayoutCache
    ) {
        let maxX = bounds.minX + (proposal.width ?? 0)
        var lineX = bounds.minX
        var lineY = bounds.minY
        var lineHeight: CGFloat = 0

        func moveToNextLine() {
            lineY += lineHeight + spacing
            lineHeight = 0
            lineX = bounds.minX
        }

        func placeSubview(_ subview: LayoutSubviews.Element, size: CGSize) {
            subview.place(
                at: CGPoint(x: lineX, y: lineY),
                proposal: ProposedViewSize(size)
            )
            lineHeight = max(lineHeight, size.height)
            lineX += size.width + spacing
        }

        for index in subviews.indices {
            let subview = subviews[index]
            let size = cache.sizes[index]
            if lineX + size.width > maxX {
                moveToNextLine()
            }
            placeSubview(subview, size: size)
        }
    }

    public func sizeThatFits(
        proposal: ProposedViewSize,
        subviews: Subviews,
        cache: inout FlowLayoutCache
    ) -> CGSize {
        var totalHeight: CGFloat = 0
        var totalWidth: CGFloat = 0

        var lineWidth: CGFloat = 0
        var lineHeight: CGFloat = 0

        for size in cache.sizes {
            if lineWidth + spacing + size.width > (proposal.width ?? 0) {
                totalHeight += lineHeight + spacing
                lineWidth = size.width
                lineHeight = size.height
            } else {
                lineWidth += size.width + spacing
                lineHeight = max(lineHeight, size.height)
            }

            totalWidth = max(totalWidth, lineWidth)
        }

        totalHeight += lineHeight
        return .init(width: totalWidth, height: totalHeight)
    }

    public func spacing(subviews: Subviews, cache: inout FlowLayoutCache) -> ViewSpacing {
        .zero
    }
}

public struct FlowLayoutCache {
    fileprivate let sizes: [CGSize]
}