Keeping the contentOffset in a UICollectionView while rotating Interface Orientation

Question

I'm trying to handle interface orientation changes in a UICollectionViewController. What I'm trying to achieve is, that I want to have the same contentOffset after an interface rotation. Meaning, that it should be changed corresponding to the ratio of the bounds change.

Starting in portrait with a content offset of {bounds.size.width * 2, 0} …

… should result to the content offset in landscape also with {bounds.size.width * 2, 0} (and vice versa).

Calculating the new offset is not the problem, but don't know, where (or when) to set it, to get a smooth animation. What I'm doing so fare is invalidating the layout in willRotateToInterfaceOrientation:duration: and resetting the content offset in didRotateFromInterfaceOrientation::

- (void)willRotateToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation
                                duration:(NSTimeInterval)duration;
{
    self.scrollPositionBeforeRotation = CGPointMake(self.collectionView.contentOffset.x / self.collectionView.contentSize.width,
                                                    self.collectionView.contentOffset.y / self.collectionView.contentSize.height);
    [self.collectionView.collectionViewLayout invalidateLayout];
}

- (void)didRotateFromInterfaceOrientation:(UIInterfaceOrientation)fromInterfaceOrientation;
{
    CGPoint newContentOffset = CGPointMake(self.scrollPositionBeforeRotation.x * self.collectionView.contentSize.width,
                                           self.scrollPositionBeforeRotation.y * self.collectionView.contentSize.height);
    [self.collectionView newContentOffset animated:YES];
}

This changes the content offset after the rotation.

How can I set it during the rotation? I tried to set the new content offset in willAnimateRotationToInterfaceOrientation:duration: but this results into a very strange behavior.

An example can be found in my Project on GitHub.

Answer 1

You can either do this in the view controller:

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

    guard let collectionView = collectionView else { return }
    let offset = collectionView.contentOffset
    let width = collectionView.bounds.size.width

    let index = round(offset.x / width)
    let newOffset = CGPoint(x: index * size.width, y: offset.y)

    coordinator.animate(alongsideTransition: { (context) in
        collectionView.reloadData()
        collectionView.setContentOffset(newOffset, animated: false)
    }, completion: nil)
}

Or in the layout itself: https://stackoverflow.com/a/54868999/308315