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

UIKit does not like values with more than 2 decimals, round them or it will do it for you.
Here, UICollectionViewFlowLayout rounds your cell size and starts to fill the lines with an "interitemspacing" at least equal to the minimumInteritemSpacing you specified. On the last line, it used exactly the minimumInteritemSpacing value and doesn't fill entirely the line.
Fix it using better rounded values, giving the illusion that all is perfectly aligned.
I usually use those extensions:
extension CGFloat {
func xx_rounded(_ rule: FloatingPointRoundingRule = .down, toDecimals decimals: Int = 2) -> CGFloat {
let multiplier = CGFloat(pow(10.0, CGFloat(decimals)))
return (self * multiplier).rounded(.down) / multiplier
}
}
extension CGSize {
func xx_rounded(rule: FloatingPointRoundingRule = .down, toDecimals: Int = 2) -> CGSize {
return CGSize(
width: width.xx_rounded(rule, toDecimals: toDecimals),
height: height.xx_rounded(rule, toDecimals: toDecimals)
)
}
}
Change:
return CGSize(width: cellWidth, height: height)
to
return CGSize(width: cellWidth, height: height).xx_rounded()

Answer 2

Just replace
cellWidth = (minimumWidth * settings.portion) - 0.5
WITH
cellWidth = (minimumWidth * settings.portion)
Specified minimumInteritemSpacingForSectionAt to 0.5 instead of 0
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumInteritemSpacingForSectionAt section: Int) -> CGFloat {
return 0.5
}
CVTVCell.swift
Just reload your UICollectionView in awakeFromNib method after your data set into currentContents. Please refer below sample code.
override func awakeFromNib(){
super.awakeFromNib()
currentContents = collectionContents.allEndings[0]
self.cView.reloadData()
}
Download code from here

Answer 3

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)
}