UICollectionViewFlowLayout not invalidating right

Question

i have a UICollectionView with a UICollectionViewFlowLayout. I also implement the UICollectionViewDelegateFlowLayout protocol.

In my datasource i have a bunch of UIViewControllers which respond to a custom protocol so i can ask for their size and some other stuff.

In in the FlowLayout delegate when it asks for the sizeForItemAtIndexPath:, i return the item size which i get from my protocol. The ViewControllers which implement my protocol return a different item size depending on the orientation.

Now if i change the device orientation from portrait to landscape theres no problem (Items are larger in landscape) but if i change it back, i get this warning:

    the item width must be less that the width of the UICollectionView minus the section insets left and right values.
    Please check the values return by the delegate.

It still works but i don't like it to get warnings so maybe you can tell me what i am doing wrong. Also there is another problem. If i don't tell my collectionViews collectionViewLayout to invalidate in willAnimateRotationToInterfaceOrientation: the sizeForItemAtIndexPath: is never called.

Hope you understand what i mean. If you need additional information let me know :)

Answer 1

A variation on @meelawsh's answer, but in Swift, and without a self.sizeForCollectionView property:

override func viewWillTransitionToSize(size: CGSize, withTransitionCoordinator coordinator: UIViewControllerTransitionCoordinator) {

    super.viewWillTransitionToSize(size, withTransitionCoordinator: coordinator)

    guard let collectionView = self.collectionView else {
        return
    }

    if size.width > collectionView.frame.width {
        // bigger

        let animation: (UIViewControllerTransitionCoordinatorContext)->(Void) = {
            context in
            collectionView.collectionViewLayout.invalidateLayout()
        }

        coordinator.animateAlongsideTransition(animation, completion: nil)
    } else {
        // smaller
        collectionView.collectionViewLayout.invalidateLayout()
    }
}