xcode CollectionViewController scrollToItemAtIndexPath not working

Question

I have created a CollectionView Control and filled it with images. Now I want to scroll to item at a particular index on start. I have tried out scrollToItemA

Answer 1

As of iOS 9.3, Xcode 8.2.1, Swift 3:

Calling scrollToItem(at:) from viewWillAppear() is still broken, particularly if you are using Section Headers/Footers, Auto Layout, Section Insets.

Even if you call setNeedsLayout() and layoutIfNeeded() on the collectionView, the behavior is still borked. Putting the scrolling code in to an animation block doesn't work reliably.

As indicated in the other answers, the solution is to only call scrollToItem(at:) once you are sure everything has been laid out. i.e. in viewDidLayoutSubviews().

However, you need to be selective; you don't want to perform scrolling every time viewWillLayoutSubviews() is called. So a solution is to set a flag in viewWillAppear(), and act it on it in viewDidLayoutSubviews().

i.e.

fileprivate var needsDelayedScrolling = false

override func viewWillAppear(_ animated: Bool)
{
    super.viewWillAppear(animated)
    self.needsDelayedScrolling = true
    // ...
}

override func viewDidLayoutSubviews()
{
    super.viewDidLayoutSubviews()

    if self.needsDelayedScrolling {
        self.needsDelayedScrolling = false
        self.collectionView!.scrollToItem(at: someIndexPath,
                at: .centeredVertically,
                animated: false)
        }
    }
}