UICollectionView does not scroll after it has been initialized - ios

Question

I have a subclass of UICollectionViewController that is nested inside a UINavigationController. The collection contains several cells (currently, 3) and each cell is as big as the full screen.
When the whole thing is shown, the collection view initally scrolls to a specific cell (which works flawlessly for each cell):
override func viewDidLayoutSubviews() {
super.viewDidLayoutSubviews()
if let path = currentlyPresentedPhotoCellIndexPath { // this is set in the beginning
collectionView?.scrollToItemAtIndexPath(path, atScrollPosition: UICollectionViewScrollPosition.CenteredHorizontally, animated: false)
}
}
However, the collection view refuses to scroll horizontally, hereafter, as if the user interaction was disabled. I am not sure what is happening, but this is what I have checked so far:
user interaction is enabled for the collection view
the next cell (right or left, depending on the scroll direction) is requested correctly which I found out by inspecting collectionView:cellForItemAtIndexPath:
the requested imagePath is the right one
scrollToItemAtIndexPath... does not work either if I try to trigger a scroll programmatically after everything has been loaded (nothing happens)
scrollRectToVisible... does neither
setting collectionView?.contentInset = UIEdgeInsetsZero before the programmatic scroll attempts take place does not change anything
the content size of the collection view is 3072x768 (= 3 screens, i.e. 3 cells)
Which bullet points are missing, here?

Answer 1

Although the post did not precisely tackle the root of my problem it forced me to ponder the code that I posted. If you look at it you will see that it basically says: Whenever the views need to be layouted, scroll to the cell at position currentlyPresentedPhotoCellIndexPath. However, and this you cannot see without any context, this variable is only set once, when the whole controller is being initialized. Thus, when you try to scroll, the layout changes, the controller then jumps back to the initial cell and it looks like nothing happens, at all.
To change this, you just have to enforce a single scroll, e.g. by doing this:
override func viewDidLayoutSubviews() {
super.viewDidLayoutSubviews()
if let path = currentlyPresentedPhotoCellIndexPath { // only once possible
collectionView?.scrollToItemAtIndexPath(path, atScrollPosition: UICollectionViewScrollPosition.CenteredHorizontally, animated: false)
currentlyPresentedPhotoCellIndexPath = nil // because of this line
// "initiallyPresentedPhotoCellIndexPath" would probably a better name
}
}
A big thanks to Mr.T!