Determine UICollectionView cell height only by autolayout - ios

Question

I need a collection view which displays cells in a grid. So a standard flow layout is fine for me. However, I want to tell how many cells to show per row, while the cell height should be determined by the autolayout constraints that I put on the cell. Here is my cell layout:
It is quite simple - an image view and two labels below it. Now the image view has an aspect ratio constraint (1:1) which means whenever the width is known for the cell the height should automatically be known by the auto layout rules (there are vertical constraints going through: celltop-image-label1-label2-cellbottom).
Now, since I don't know any other good way to tell the collection view to show 2 items per row, I have overridden UICollectionViewDelegateFlowLayout methods:
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
let availableWidth = collectionView.frame.width - padding
let widthPerItem = availableWidth / itemsPerRow
return CGSize(width: widthPerItem, height: widthPerItem)
}
As you can see, since I don't know the item height I return the same thing as the width, hoping that the autolayout will fix it later. I also set the estimatedItemSize in order the whole mechanism to start working.
The results are quite strange - it seems like the collection view doesn't event take into account the width I return there, mostly depending on the label lengths:
I have seen some other answers where people recommend manually calculating the cell size for width, like telling "layout yourself, then measure yourself, then give me your size for this width", and even though it would still run the autolayout rules under the hood, I would like to know if there is a way of doing this without manually messing with the sizes.

Answer 1

You can easily find out the height of your collectionView cell in the storyboard's Size inspector, as shown below:
Now, just pick up this height from here, and pass it to the overridden UICollectionViewDelegateFlowLayout method:
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
let availableWidth = collectionView.frame.width - padding
let widthPerItem = availableWidth / itemsPerRow
return CGSize(width: widthPerItem, height: **114**)
}
And you will get the desired output.

Answer 2

I ended up implementing a trick to move everything to autolayout. I completely removed the delegate method func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize and added a width constraint for my cell content in the interface builder (set the initial value to something, that's not important). Then, I created an outlet for that constraint in the custom cell class:
class MyCell: UICollectionViewCell {
#IBOutlet weak var cellContentWidth: NSLayoutConstraint!
func updateCellWidth(width: CGFloat) {
cellContentWidth.constant = width
}
}
Later, when the cell is created, I update the width constraint to the precalculated value according to the number of cells that I want per row:
private var cellWidth: CGFloat {
let paddingSpace = itemSpacing * (itemsPerRow - 1) + sectionInsets.left + sectionInsets.right
let availableWidth = collectionView.frame.width - paddingSpace
return availableWidth / itemsPerRow
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "MyCell", for: indexPath) as! MyCell
...
cell.updateCellWidth(width: cellWidth)
return cell
}
And this, together with autolayout cell sizing enabled, will lay out the cells correctly.