CollectionView estimatedItemSize not working as expected - ios

Question

Im trying to use estimatedItemSize to dynamically resize collectionView cells to fit they're content tower I'm having a few problems with different device sizes and particularly the width not resizing, its just staying the same size as it is in interface builder.
I have done my Interface Builder stuff on an iPhone 7 so the screen width is 375. In my storyBoard i have the following.
outerCollectionView pinned to all 4 sides of the superView with 0 margin
outerCollectionView Cell of w: 339 h:550
Inside this cell i have:
innerCollectionView pinned to all 4 sides of the cell with 0 margin
innerCollectionView constant w: 339 h: 550 added in IB
Inside my DataSource and Delegate class i have defined my flow layout in the viewDidLoad function:
let width = UIScreen.main.bounds.size.width
let height = UIScreen.main.bounds.size.height
let flow = outerCollectionView.collectionViewLayout as! UICollectionViewFlowLayout
flow.estimatedItemSize = CGSize(width: width - ((width / 18.75)), height: height / 4)
flow.minimumLineSpacing = (width / 37.5)
flow.sectionInset = UIEdgeInsets(top: (width / 37.5), left: (width / 37.5), bottom: (width / 37.5), right: (width / 37.5))
And inside the custom cell class that holds the "inner" collection i set some of these constant constraints and define the flow layout to size the items:
// This is the correct screen size
// let width = innerCollectionView.frame.size.width
let width = UIScreen.main.bounds.size.width
let flow = innerCollectionView.collectionViewLayout as! UICollectionViewFlowLayout
flow.sectionInset = UIEdgeInsetsMake((width / 37.5), (width / 37.5), (width / 37.5), (width / 37.5))
flow.itemSize = CGSize(width: (width / 6 - (width / 37.5)), height: (width / 6 - (width / 37.5)))
flow.minimumInteritemSpacing = (width / 37.5)
flow.minimumLineSpacing = (width / 37.5)
innerCollectionHeight.constant = innerCollectionView.collectionViewLayout.collectionViewContentSize.height
innerCollectionWidth.constant = innerCollectionView.collectionViewLayout.collectionViewContentSize.width
Now all this works fine and the cells resize vertically to fit my content nicely on the iPhone 7 simulator, showing a single column cells correctly populated. But when i step up a device size does not adjust the width of the cell. And when i go down to a smaller device i get a whole bunch of Xcode errors about how the width of the cell is is to big because it hasn't been resized, but nothing displays. (some pics below)
2017-02-16 12:53:49.632 ParseStarterProject-Swift[26279:394906] The behavior of the UICollectionViewFlowLayout is not defined because:
2017-02-16 12:53:49.632 ParseStarterProject-Swift[26279:394906] the item width must be less than the width of the UICollectionView minus the section insets left and right values, minus the content insets left and right values.
2017-02-16 12:53:49.632 ParseStarterProject-Swift[26279:394906] Please check the values returned by the delegate.
2017-02-16 12:53:49.633 ParseStarterProject-Swift[26279:394906] The relevant UICollectionViewFlowLayout instance is <UICollectionViewFlowLayout: 0x7be38920>, and it is attached to <UICollectionView: 0x7db03600; frame = (0 0; 320 504); clipsToBounds = YES; autoresize = RM+BM; gestureRecognizers = <NSArray: 0x7d61a550>; layer = <CALayer: 0x7be95880>; contentOffset: {0, 0}; contentSize: {320, 20}> collection view layout: <UICollectionViewFlowLayout: 0x7be38920>.
So obviously i cant use
flow.itemSize =
to set the size of the outer cell because i need the estimated size to adjust the vertical length and hug the content. I just cant figure out why the width isnt rising to suit the screen size??
Large sized device - margin too large: CV width 339
Iphone 7 as i built it - margins normal: CV width 339
---- EDIT -----
tried overriding szeForItemAt but there is no change.
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
let width = self.view.frame.size.width
let height = self.view.frame.size.height
if collectionView == self.outerCollectionView {
return CGSize(width: width - 100, height: height / 4)
} else {
return CGSize(width: (width / 6 - (width / 75)), height: (width / 6 - (width / 75)))
}
}
---- EDIT ----
I can edit the preferred item seize of the cell. Using the compressed size option for height and setting my own size for width, (width / 18.7) is the width minus left and right inset previously defined
override func preferredLayoutAttributesFitting(_ layoutAttributes: UICollectionViewLayoutAttributes) -> UICollectionViewLayoutAttributes {
let attributes = layoutAttributes.copy() as! UICollectionViewLayoutAttributes
let width = UIScreen.main.bounds.size.width
let desiredWidth = width - (width / 18.75)
//let desiredWidth = systemLayoutSizeFitting(UILayoutFittingExpandedSize).width
let desiredHeight = systemLayoutSizeFitting(UILayoutFittingCompressedSize).height
attributes.frame.size.width = desiredWidth
attributes.frame.size.height = desiredHeight
return attributes
}
Bu this stuffs up the constraints as shown below in the image, which i think might just be a script i have running to fillet the top corners of UIViews. But its also stuffing up the cells as it cramming more per line.
Smaller screen, similar problem with the cells:

Answer 1

The solution is to implement UICollectionViewDelegateFlowLayout and call sizeForItemAt like this:
extension YourClass: UICollectionViewDelegateFlowLayout {
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout:UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
//return size needed
return CGSize(width: (width / 6 - (width / 37.5)), height: (width / 6 - (width / 37.5)))
}
At runtime the returned size will be the one giving the item Size.
Note: implementing sizeForItemAt without UICollectionViewDelegateFlowLayout will not succeed