Nested UICollectionViews, AutoLayout and rotation in iOS 8

Question

I started to use AutoLayout for a large project and was positively surprised about it. However, now I have to adjust the project to accommodate for rotation and size classes, an

Answer 1

Just in case anyone is looking for a Swift 4 solution:

I had a very similar problem and I found the solution to be a combination of two answers above: use the coordinator animate function to call invalidateLayout() on the top collection view, then, in the completion hander, loop through each cell and call invalidateLayout() on each cell's collection view. I didn't have to maintain selections or expanded views so this worked without having to call reloadData().

override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
    super.viewWillTransition(to: size, with: coordinator)

    // get the device orientation so we can change the number of rows & columns
    // in landscape and portrait modes
    let orientation = UIDevice.current.orientation

    if orientation.isLandscape {
        self.columns = 4
        self.rows = 3
    } else {
        self.columns = 3
        self.rows = 4
    }

    // invalidate the top collection view inside the coordinator animate function
    // then inside the animation completion handler, we invalidate each cell's colleciton view
    coordinator.animate(alongsideTransition: { (context) in

        self.collectionView.collectionViewLayout.invalidateLayout()

    }) { (context) in

        let cells = self.collectionView.visibleCells

        for cell in cells {
            guard let cell = cell as? MyCollectionViewCell else { continue }
            cell.invalidateLayout()
        }
    }
}