iOS - CollectionView rotation responds slow - ios

Question

I have a collectionView where I am using the following to manage rotation of the cells:
override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
super.viewWillTransition(to: size, with: coordinator)
DispatchQueue.main.async {
coordinator.animate(alongsideTransition: nil, completion: { context in
self.collectionView?.collectionViewLayout.invalidateLayout()
self.collectionView?.collectionViewLayout.finalizeCollectionViewUpdates()
})
}
}
I see that the size change on rotation is very slow ( takes ~ 3- 4 seconds ). Is there a better way to get smoother rotation transition? I have dynamic cell sizing from sizeForItemAtIndexPath, code is :
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
var width = CGFloat()
if traitCollection.userInterfaceIdiom == .phone {
width = collectionView.frame.size.width - 2.0 * margin
}
else if traitCollection.userInterfaceIdiom == .pad {
if UIDevice.current.orientation.isLandscape {
width = floor(collectionView.frame.size.width - 14.0 * margin)
}
else {
width = floor(collectionView.frame.size.width - 6.0 * margin)
}
}
let post = self.postListForLoggedInUserFromRealm?.posts[(indexPath as NSIndexPath).item]
if let description = post?.description {
let rect = NSString(string: description).boundingRect(with: CGSize(width: view.frame.width * 0.8, height: CGFloat.greatestFiniteMagnitude), options: [.usesLineFragmentOrigin], attributes: [.font: UIFont(name: "Avenir", size: CGFloat(titleFont))!], context: nil)
var knownHeight : CGFloat = 64
if(post?.imageURL != nil && post?.imageURL != "") {
guard let imageURL = NSURL(string: (post?.imageURL!)!) else { return CGSize(width: width, height: rect.height + knownHeight )}
if let imageSource = CGImageSourceCreateWithURL((imageURL as CFURL), nil) {
if let imageProperties = CGImageSourceCopyPropertiesAtIndex(imageSource, 0, nil) as Dictionary? {
let pixelHeight = imageProperties[kCGImagePropertyPixelHeight] as! CGFloat
let imageMaxHeight: CGFloat = (CIConstants.DeviceType.DeviceTypePhone == deviceType ) ? 400 : 700
let imageHeight = pixelHeight > imageMaxHeight ? imageMaxHeight : pixelHeight
knownHeight = knownHeight + CGFloat(imageHeight)
return CGSize(width: width, height: rect.height + knownHeight)
}
}
}
return CGSize(width: width, height: rect.height + knownHeight)
}
else if(post?.imageURL != nil && post?.imageURL != "") {
var knownHeight : CGFloat = 10 + 44 + 8 + 44 + 10
guard let imageURL = NSURL(string: (post?.imageURL!)!) else { return CGSize(width: width, height: knownHeight + 70)}
if let imageSource = CGImageSourceCreateWithURL(imageURL as CFURL, nil) {
if let imageProperties = CGImageSourceCopyPropertiesAtIndex(imageSource, 0, nil) as Dictionary? {
let pixelHeight = imageProperties[kCGImagePropertyPixelHeight] as! CGFloat
let imageMaxHeight: CGFloat = (CIConstants.DeviceType.DeviceTypePhone == deviceType ) ? 400 : 700
let imageHeight = imageMaxHeight > 700 ? imageMaxHeight : pixelHeight
knownHeight = knownHeight + CGFloat(imageHeight)
}
}
return CGSize(width: width, height: knownHeight)
}
return CGSize(width: 850, height: 70)
}
Any suggestions would be truly appreciated! Thanks for your time :)
EDIT: When I use this code, the delay is gone but I lose the margins ( on left and right of collectionView) in Portrait mode.
override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
DispatchQueue.main.async {
coordinator.animate(alongsideTransition: { context in
self.collectionView?.collectionViewLayout.invalidateLayout()
self.collectionView?.collectionViewLayout.finalizeCollectionViewUpdates()
})
}
super.viewWillTransition(to: size, with: coordinator)
}