the behavior of the UICollectionViewFlowLayout is not defined, because the cell width is greater than collectionView width

Question

2015-08-18 16:07:51.523 Example[16070:269647] the behavior of the UICollectionViewFlowLayout is not defined because: 2015-08-18 16:07:51.523
Example

Answer 1

I am using a subclass of UICollectionViewFlowLayout and using its itemSize property to specify the cell size (instead of the collectionView:sizeForItemAtIndexPath: delegate method). Every time I rotate the screen to a shorter width one (e.g. landscape -> portrait) I get this huge warning in question.

I was able to fix it by doing 2 steps.

Step 1: In UICollectionViewFlowLayout subclass's prepareLayout() function, move super.prepareLayout() to after where self.itemSize is set. I think this makes the super class to use the correct itemSize value.

import UIKit

extension UICollectionViewFlowLayout {

  var collectionViewWidthWithoutInsets: CGFloat {
    get {
      guard let collectionView = self.collectionView else { return 0 }
      let collectionViewSize = collectionView.bounds.size
      let widthWithoutInsets = collectionViewSize.width
        - self.sectionInset.left - self.sectionInset.right
        - collectionView.contentInset.left - collectionView.contentInset.right
      return widthWithoutInsets
    }
  }

}

class StickyHeaderCollectionViewLayout: UICollectionViewFlowLayout {

  // MARK: - Variables
  let cellAspectRatio: CGFloat = 3/1

  // MARK: - Layout
  override func prepareLayout() {
    self.scrollDirection = .Vertical
    self.minimumInteritemSpacing = 1
    self.minimumLineSpacing = 1
    self.sectionInset = UIEdgeInsets(top: 0, left: 0, bottom: self.minimumLineSpacing, right: 0)
    let collectionViewWidth = self.collectionView?.bounds.size.width ?? 0
    self.headerReferenceSize = CGSize(width: collectionViewWidth, height: 40)

    // cell size
    let itemWidth = collectionViewWidthWithoutInsets
    self.itemSize = CGSize(width: itemWidth, height: itemWidth/cellAspectRatio)

    // Note: call super last if we set itemSize
    super.prepareLayout()
  }

  // ...

}

Note that the above change will somehow make the layout size change when screen rotates stops working. This is where step 2 comes in.

Step 2: Put this in the view controller that holds the collection view.

  override func viewWillTransitionToSize(size: CGSize, withTransitionCoordinator coordinator: UIViewControllerTransitionCoordinator) {
    super.viewWillTransitionToSize(size, withTransitionCoordinator: coordinator)
    collectionView.collectionViewLayout.invalidateLayout()
  }

Now the warning is gone :)

---

Some Notes:

1. Make sure you are adding constraints to the collectionView and not using collectionView.autoresizingMask = [.flexibleWidth, .flexibleHeight]

2. Make sure you are not calling invalidateLayout in viewWillTransitionToSize() because the width of an edge-to-edge cell in landscape is larger than the collection view’s frame width in portrait. See below references.

References

• Making Adaptive Forms using UICollectionView
• Collection view with self-sizing cells