Collection view grid last row appears differently despite equal widths - swift iOS

Question

I've a fixed grid of collection view cells (UICollectionView) but the cells in the bottom row always appears with a slightly smaller width on screen. The frame size (or bounds) and calculated width used within collectionViewLayout: UICollectionViewLayout sizeForItemAt are the same for all rows.

        func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize
{
    let settings = currentContents[indexPath.item]
    let height = CGFloat(30)

    // https://stackoverflow.com/questions/54915227/uicollectionview-remove-space-between-cells-with-7-items-per-row
    var cellWidth = CGFloat()
    let availableWidth = collectionView.bounds.size.width
    print("available width", availableWidth)
    let minimumWidth = floor(availableWidth / collectionContents.cellsPerRow5)
    print("minmum width", minimumWidth)
    cellWidth = minimumWidth * settings.portion - 1
    print("cell width", cellWidth)

    return CGSize(width: cellWidth, height: height)
}

I'd like to get the bottom row to line up with the other rows, but can't imagine what is happening that is changing the widths after returning the value in the layout delegate method (or how to fix).

Answer 1

The best way I found was to capture the remaining space (minus 1 to avoid rounding up) after calculating the initial column width, and then use this remaining space for additional columns.

For a two column example:

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize
{
    if indexPath.item == 0 || indexPath.item == 1
    { rowHeight = 40 }
    else { rowHeight = 30 }
    var cellWidth = CGFloat()
    let minimumWidth = floor(availableWidth / numberOfColumns)
    var columnPortion = CGFloat()
    let columnNumber = indexPath.item % Int(numberOfColumns)
    switch columnNumber
    {
    case 0:
        columnPortion = columnPortion1
        cellWidth = minimumWidth * columnPortion - 10
        remainder = availableWidth - cellWidth - 1
    case 1:
        columnPortion = columnPortionFinal
        cellWidth = remainder
    default:
        break
    }
    return CGSize(width: cellWidth, height: rowHeight)
}