flower-story/FlowerApp/Platforms/iOS/Controls/FlowLayout.cs

using CoreGraphics;
using Foundation;
using UIKit;

namespace Blahblah.FlowerApp.Platforms.iOS.Controls;

public class FlowLayout : UICollectionViewFlowLayout
{
    public CalculateCellHeightHandler? CalculateCellHeight { get; set; }

    private readonly List<UICollectionViewLayoutAttributes> layoutAttributes = new();

    private CGSize oldBounds;
    private bool dirty;
    private int cols;
    private nfloat[]? yArray;
    private nfloat maxHeight;

    public FlowLayout(int cols = 2)
    {
        SetCols(cols);
    }

    public void SetCols(int cols)
    {
        if (this.cols != cols)
        {
            dirty = true;
            this.cols = cols;
        }
    }

    public void Invalidate()
    {
        dirty = true;
    }

    private void Clean()
    {
        dirty = false;
        yArray = new nfloat[cols];
        maxHeight = 0f;
        layoutAttributes.Clear();
    }

    public override CGSize CollectionViewContentSize => new(CollectionView.Bounds.Width, maxHeight);

    public override bool ShouldInvalidateLayoutForBoundsChange(CGRect newBounds)
    {
        return newBounds.Width != CollectionView.Bounds.Width;
    }

    public override void PrepareLayout()
    {
        base.PrepareLayout();

        var bounds = CollectionView.Bounds.Size;
        if (dirty || oldBounds.Width != bounds.Width)
        {
            oldBounds = bounds;
            Clean();
        }

        var sectionLeft = SectionInset.Left;
        var sectionTop = SectionInset.Top;
        var minSpacing = MinimumInteritemSpacing;
        var itemWidth = (bounds.Width - sectionLeft - SectionInset.Right - minSpacing * (cols - 1)) / cols;
        var itemCount = CollectionView.NumberOfItemsInSection(0);
        for (nint i = layoutAttributes.Count; i < itemCount; i++)
        {
            var indexPath = NSIndexPath.FromItemSection(i, 0);
            var attr = UICollectionViewLayoutAttributes.CreateForCell(indexPath);
            var itemHeight = CalculateCellHeight?.Invoke(indexPath, itemWidth) ?? 20;
            nfloat value = nfloat.MaxValue;
            int minHeightIndex = 0;
            if (yArray?.Length >= cols)
            {
                for (var n = 0; n < cols; n++)
                {
                    if (yArray[n] < value)
                    {
                        value = yArray[n];
                        minHeightIndex = n;
                    }
                }
            }
            var itemY = value;
            if (itemY < sectionTop)
            {
                itemY = sectionTop;
            }
            if (i >= cols)
            {
                itemY += minSpacing;
            }

            var itemX = sectionLeft + (itemWidth + minSpacing) * minHeightIndex;
            attr.Frame = new CGRect(itemX, itemY, itemWidth, itemHeight);
            layoutAttributes.Add(attr);
            if (yArray != null)
            {
                yArray[minHeightIndex] = itemY + itemHeight;
            }
        }
        nfloat y = 0f;
        if (yArray != null)
        {
            for (var i = 0; i < yArray.Length; i++)
            {
                if (yArray[i] > y)
                {
                    y = yArray[i];
                }
            }
        }
        maxHeight = y + SectionInset.Bottom;
    }

    public override UICollectionViewLayoutAttributes[] LayoutAttributesForElementsInRect(CGRect rect)
    {
        return layoutAttributes.Where(a => a.Frame.IntersectsWith(rect)).ToArray();
    }
}

public delegate nfloat CalculateCellHeightHandler(NSIndexPath indexPath, nfloat itemWidth);