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

Also remember that you need to use proper UICollectionviewScrollPosition value. Please see code below for clarification:

typedef NS_OPTIONS(NSUInteger, UICollectionViewScrollPosition) {
UICollectionViewScrollPositionNone                 = 0,

/* For a view with vertical scrolling */
// The vertical positions are mutually exclusive to each other, but are bitwise or-able with the horizontal scroll positions.
// Combining positions from the same grouping (horizontal or vertical) will result in an NSInvalidArgumentException.
UICollectionViewScrollPositionTop                  = 1 << 0,
UICollectionViewScrollPositionCenteredVertically   = 1 << 1,
UICollectionViewScrollPositionBottom               = 1 << 2,

/* For a view with horizontal scrolling */
// Likewise, the horizontal positions are mutually exclusive to each other.
UICollectionViewScrollPositionLeft                 = 1 << 3,
UICollectionViewScrollPositionCenteredHorizontally = 1 << 4,
UICollectionViewScrollPositionRight                = 1 << 5

};

Answer 2

Sometimes collectionView(_:didSelectItemAt:) is either not called on the main thread, or blocks it, causing scrollToItem(at:at:animated:) to not do anything.

Work around this by doing:

DispatchQueue.main.async {
  collectionView.scrollToItem(at: indexPath, at: .centeredHorizontally, animated: true)
}

Answer 3

This is based on @Womble's answer, all credit goes to them:

The method viewDidLayoutSubviews() gets called repeatedly. For me (iOS 11.2) the first time it gets called the collectionView.contentSize is {0,0}. The second time, the contentSize is correct. Therefore, I had to add a check for this:

var needsDelayedScrolling = false

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

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()
    if self.needsDelayedScrolling && collectionView.contentSize.width > 0 {
        self.needsDelayedScrolling = false
        self.collectionView!.scrollToItem(at: someIndexPath,
                at: .centeredVertically,
                animated: false)
        }
    }
}

After adding that extra && collectionView.contentSize.width > 0 it works beautifully.