UICollection View Flow Layout Vertical Align

Question

By default, when you are using the flow layout in a collection view, cells are centered vertically. Is there a way to change this alignment ?

Answer 1

@DongXu: Your solution worked for me too. Here is the Xamarin.iOS version if it:

public class TopAlignedCollectionViewFlowLayout : UICollectionViewFlowLayout
{
    public override UICollectionViewLayoutAttributes[] LayoutAttributesForElementsInRect(CGRect rect)
    {
        if (base.LayoutAttributesForElementsInRect(rect) is UICollectionViewLayoutAttributes[] attrs)
        {
            // Find all the cells and group them together by the rows they appear on
            var cellsGroupedByRow = attrs
                .Where(attr => attr.RepresentedElementCategory == UICollectionElementCategory.Cell)
                // The default flow layout aligns cells in the middle of the row.
                // Thus, cells with the same Y center point are in the same row.
                // Convert to int, otherwise float values can be slighty different for cells on the same row and cause bugs.
                .GroupBy(attr => Convert.ToInt32(attr.Frame.GetMidY()));

            foreach (var cellRowGroup in cellsGroupedByRow)
            {
                TopAlignCellsOnSameLine(cellRowGroup.ToArray());
            }

            return attrs;
        }

        return null;
    }

    private static void TopAlignCellsOnSameLine(UICollectionViewLayoutAttributes[] cells)
    {
        // If only 1 cell in the row its already top aligned.
        if (cells.Length <= 1) return;

        // The tallest cell has the correct Y value for all the other cells in the row
        var tallestCell = cells.OrderByDescending(cell => cell.Frame.Height).First();

        var topOfRow = tallestCell.Frame.Y;

        foreach (var cell in cells)
        {
            if (cell.Frame.Y == topOfRow) continue;

            var frame = cell.Frame;

            frame.Y = topOfRow;

            cell.Frame = frame;
        }
    }
}