Custom pop animation to a UICollectionViewController doesn't work - ios

Question

I have a collection view called ProfileCollectionViewController for collection of images.
When clicked on an image it presents a HorizontalProfileViewController which displays images in full screen.
When back button is pressed on HorizontalProfileViewController I want the full screen image to animate back to a thumbnail in ProfileViewController. I pass the selected index path from ProfileViewController as initialIndexPath to HorizontalProfileViewController so that the position of thumbnail is known. Below is my transition animation code
import UIKit
class SEHorizontalFeedToProfile: NSObject, UIViewControllerAnimatedTransitioning {
func transitionDuration(transitionContext: UIViewControllerContextTransitioning?) -> NSTimeInterval {
return 0.2
}
func animateTransition(transitionContext: UIViewControllerContextTransitioning) {
if let profileVC = transitionContext.viewControllerForKey(UITransitionContextToViewControllerKey) as? SEProfileGridViewController, horizontalFeedVC = transitionContext.viewControllerForKey(UITransitionContextFromViewControllerKey) as? SEProfileHorizontalViewController, containerView = transitionContext.containerView() {
let duration = transitionDuration(transitionContext)
profileVC.collectionView?.reloadData()
if let indexPath = horizontalFeedVC.initialIndexPath {
let cell = profileVC.collectionView?.cellForItemAtIndexPath(indexPath)
print(indexPath)
let imageSnapshot = horizontalFeedVC.view.snapshotViewAfterScreenUpdates(false)
let snapshotFrame = containerView.convertRect(horizontalFeedVC.view.frame, fromView: horizontalFeedVC.view)
imageSnapshot.frame = snapshotFrame
horizontalFeedVC.view.hidden = true
profileVC.view.frame = transitionContext.finalFrameForViewController(profileVC)
containerView.insertSubview(profileVC.view, belowSubview: horizontalFeedVC.view)
containerView.addSubview(imageSnapshot)
UIView.animateWithDuration(duration, animations: {
var cellFrame = CGRectMake(0, 0, 0, 0)
if let theFrame = cell?.frame {
cellFrame = theFrame
}
let frame = containerView.convertRect(cellFrame, fromView: profileVC.collectionView)
imageSnapshot.frame = frame
}, completion: { (succeed) in
if succeed {
horizontalFeedVC.view.hidden = false
// cell.contentView.hidden = false
imageSnapshot.removeFromSuperview()
transitionContext.completeTransition(!transitionContext.transitionWasCancelled())
}
})
}
}
}
}
I put breakpoints and found out that in the code
let cell = profileVC.collectionView?.cellForItemAtIndexPath(indexPath)
the cell is nil. I don't understand why it would be nil. Please help me. I thank you in advance.
The profileVC is a subclass of UICollectionViewController
PS: Please check out the following project that does exactly the same thing without any issues. I tried to mimic it but it doesn't work on mine.
https://github.com/PeteC/InteractiveViewControllerTransitions

Answer 1

As was pointed out in the comments on the question, the cell is set to nil because the cell is not visible. Since your HorizontalProfileViewController allows scrolling to other pictures it is possible to scroll to an image that has not yet had a UICollectionViewCell created for it in your ProfileCollectionViewController.
You can remedy this by scrolling to where the current image should be on the UICollectionView. You say that initialIndexPath is initially the indexPath of the image that was first selected. Assuming you update that as the user scrolls to different images so that it still accurately represents the on screen image's indexPath, we can use that to force the UICollectionView to scroll to the current cell if necessary.
With a few tweaks to your code this should give you the desired effect:
import UIKit
class SEHorizontalFeedToProfile: NSObject, UIViewControllerAnimatedTransitioning {
func transitionDuration(transitionContext: UIViewControllerContextTransitioning?) -> NSTimeInterval {
return 0.2
}
func animateTransition(transitionContext: UIViewControllerContextTransitioning) {
if let profileVC = transitionContext.viewControllerForKey(UITransitionContextToViewControllerKey) as? SEProfileGridViewController, horizontalFeedVC = transitionContext.viewControllerForKey(UITransitionContextFromViewControllerKey) as? SEProfileHorizontalViewController, containerView = transitionContext.containerView() {
let duration = transitionDuration(transitionContext)
//profileVC.collectionView?.reloadData() //Remove this
if let indexPath = horizontalFeedVC.initialIndexPath {
var cell = profileVC.collectionView?.cellForItemAtIndexPath(indexPath) //make cell a var so it can be reassigned if necessary
print(indexPath)
if cell == nil{ //This if block is added. Again it is crucial that initialIndexPath was updated as the user scrolled.
profileVC.collectionView?.scrollToItemAtIndexPath(horizontalFeedVC.initialIndexPath, atScrollPosition: UICollectionViewScrollPosition.CenteredVertically, animated: false)
profileVC.collectionView?.layoutIfNeeded() //This is necessary to force the collectionView to layout any necessary new cells after scrolling
cell = profileVC.collectionView?.cellForItemAtIndexPath(indexPath)
}
let imageSnapshot = horizontalFeedVC.view.snapshotViewAfterScreenUpdates(false)
let snapshotFrame = containerView.convertRect(horizontalFeedVC.view.frame, fromView: horizontalFeedVC.view)
imageSnapshot.frame = snapshotFrame
horizontalFeedVC.view.hidden = true
profileVC.view.frame = transitionContext.finalFrameForViewController(profileVC)
containerView.insertSubview(profileVC.view, belowSubview: horizontalFeedVC.view)
containerView.addSubview(imageSnapshot)
UIView.animateWithDuration(duration, animations: {
var cellFrame = CGRectMake(0, 0, 0, 0)
if let theFrame = cell?.frame {
cellFrame = theFrame
}
let frame = containerView.convertRect(cellFrame, fromView: profileVC.collectionView)
imageSnapshot.frame = frame
}, completion: { (succeed) in
if succeed {
horizontalFeedVC.view.hidden = false
// cell.contentView.hidden = false
imageSnapshot.removeFromSuperview()
transitionContext.completeTransition(!transitionContext.transitionWasCancelled())
}
})
}
}
}
Now if the current UICollectionViewCell is not yet created it:
Scrolls so the indexPath of the cell is centered on the view
Forces the UICollectionView to layout its subviews which creates any UITableViewCells that are necessary
Asks for the UITableViewCell again, but this time it should not be nil