GridControl.cs

public class VerticalGridLayout : UICollectionViewLayout
    {
        public VerticalGridLayout(
            IGridCellMetricProvider gridMetricProvider,
            double cellHeight
        )
        {
            _gridMetricProvider = gridMetricProvider;
            _cellHeight = cellHeight;
        }
        private IGridCellMetricProvider _gridMetricProvider;
        private double _cellHeight;
        private Dictionary<NSIndexPath, UICollectionViewLayoutAttributes> attributesCache = new Dictionary<NSIndexPath, UICollectionViewLayoutAttributes>();
        private nfloat ContentWidth
        {
            get => CollectionView.Frame.Width - (CollectionView.ContentInset.Left + CollectionView.ContentInset.Right);
        }
        private nfloat _contentHeigth;
        //bounding rect for all items
        public override CoreGraphics.CGSize CollectionViewContentSize
        {
            get
            {
                var contentHeight = (nfloat)(_gridMetricProvider.GridSize.Height * _cellHeight);
                return new CoreGraphics.CGSize(ContentWidth,contentHeight);
                //return base.CollectionViewContentSize;
            }
        }
        //which items to show in scrollview
        public override UICollectionViewLayoutAttributes[] LayoutAttributesForElementsInRect(CoreGraphics.CGRect rect)
        {
            return attributesCache.Values.ToArray();            //return base.LayoutAttributesForElementsInRect(rect);
        }
        //retrieve layout attributes for specific item
        public override UICollectionViewLayoutAttributes LayoutAttributesForItem(Foundation.NSIndexPath indexPath)
        {
            double cellWidth = ContentWidth / _gridMetricProvider.GridSize.Width;
			var gridMetrics = _gridMetricProvider.GridCellInfoForIndexPath(indexPath);
            var itemAttributes = UICollectionViewLayoutAttributes.CreateForCell(indexPath);

			double x = cellWidth * gridMetrics.column;
			double w = cellWidth * gridMetrics.colspan;
			double y = _cellHeight * gridMetrics.row;
			double h = _cellHeight * gridMetrics.rowspan;

			itemAttributes.Frame = new CoreGraphics.CGRect(x, y, w, h);
            return itemAttributes;
        }
        //if collection view size changed or device is rotated
        public override bool ShouldInvalidateLayoutForBoundsChange(CoreGraphics.CGRect newBounds)
        {
            if (CollectionView == null)
                return false;
            return CollectionView.Bounds.Width != newBounds.Width;
        }
        //for generating cache
        public override void PrepareLayout()
        {
            if (attributesCache.Count != 0) return;

                for (int sectionIndex = 0; sectionIndex < CollectionView.NumberOfSections(); sectionIndex++)
                {
                    for (int itemIndex = 0; itemIndex < CollectionView.NumberOfItemsInSection(sectionIndex); itemIndex++)
                    {
                        NSIndexPath indexPath = NSIndexPath.FromItemSection(itemIndex, sectionIndex);
                        attributesCache.Add(indexPath, LayoutAttributesForItem(indexPath));
                    }
                }

        }
        //}
        //clear cache
        public override void InvalidateLayout()
        {
            attributesCache.Clear();
            base.InvalidateLayout();
        }
    }
    
    public interface IGridCellMetricProvider
    {
        GridCellInfo GridCellInfoForIndexPath(NSIndexPath path);
        Size GridSize { get; }
    }

    public struct GridCellInfo
    {
        public GridCellInfo(int col,int colspan,int row,int rowspan)
        {
            this.row = row;
            this.rowspan = rowspan;
            this.column = col;
            this.colspan = colspan;
        }
        public int column;
        public int colspan;
        public int row;
        public int rowspan;
    }
    
    ...
    CollectionView.SetCollectionViewLayout(new VerticalGridLayout(_dataSource, cellHeight: ApplicationSettings.DefaultCellHeight), false);
    ...
    class which implements IGridCellMetricProvider
    ...
            public GridCellInfo GridCellInfoForIndexPath(NSIndexPath path)
            {
                var formControl = GetControlAt(path.Row);//row will be index
                return new GridCellInfo(formControl.Left, formControl.Width, formControl.Top, formControl.Height);
            }

            public Size GridSize => new Size(_form.Width,_form.Height);
    ...