UICollectionView edge to edge layout - ios

Question

Goal: edge-to-edge UICollectionView with 2 cells on all size iPhones.
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize {
let screenSize = collectionView.bounds
let screenWidth = screenSize.width
print("Zhenya: \(screenWidth)") // prints correct 375, which is half of iPhone 6
let cellEdgeLength: CGFloat = screenWidth / 2.0
return CGSize(width: cellEdgeLength, height: cellEdgeLength)
}
Also
override func viewDidLoad() {
super.viewDidLoad()
collectionView.delegate = self
collectionView.dataSource = self
let flow = UICollectionViewFlowLayout()
flow.scrollDirection = UICollectionViewScrollDirection.vertical
flow.minimumInteritemSpacing = 0
flow.minimumLineSpacing = 0
collectionView.collectionViewLayout = flow
}
However on iPhone 6:
Collection Cell attributes:
Collection View attributes:
Update:
func for gradient, that actually gets the right width:
(located at custom UICollectionViewCell class)
func addGradient () {
let gradient = CAGradientLayer()
gradient.frame = gradientView.bounds
let topColor = UIColor(red:0.07, green:0.07, blue:0.07, alpha:1)
let botomColor = UIColor.clear
gradient.colors = [topColor.cgColor, botomColor.cgColor]
if gradientWasRemoved == false {
gradientView.layer.insertSublayer(gradient, at: 0)
} else if gradientWasRemoved == true {
self.addSubview(gradientView)
}
Update 2:
Note: Testing on iPhone 7 Plus.
I found that UICollectionViewFlowLayout overrides cell size crated in sizeForItemAtIndexPath: (seems like it)
With this code:
let flow = UICollectionViewFlowLayout()
let screenSize = collectionView.bounds
let screenWidth = screenSize.width
let cellEdgeLength: CGFloat = screenWidth / 2.0
flow.itemSize = CGSize(width: cellEdgeLength, height: cellEdgeLength)
flow.scrollDirection = UICollectionViewScrollDirection.vertical
flow.minimumInteritemSpacing = 0
flow.minimumLineSpacing = 0
collectionView.collectionViewLayout = flow
I have this:
Then I decided manual specify cell edge length (half of iPhone7Plust width = 207):
let cellSide: CGFloat = 207
flow.itemSize = CGSize(width: cellSide, height: cellSide)
flow.scrollDirection = UICollectionViewScrollDirection.vertical
flow.minimumInteritemSpacing = 0
flow.minimumLineSpacing = 0
collectionView.collectionViewLayout = flow
I get this:

Answer 1

Found the solution.
Problem was in the inner imageView autolayout settings. I didn't specify them.
Why that little piece of BS was getting on the way of my flow.itemSize, I don't know.
What is more weird to me, is why when manually specified cellEdgeLength as 207 in the second update, all of the sudden, it did override the lack of imageView constraints.