Can NSCollectionView autoresize the width of its subviews to display one column

The true answer is to set the maxItemSize to 0,0(NSZeroSize). Otherwise, it is computed.

[self.collectionView setMaxItemSize:NSZeroSize];

This can be set in awakeFromNib.

I know this is a very late response but I got the same problem and hope my solution will help somebody. Solution is to access bounds of enclosing scroll view not of collection view itself. So to solve it you need to replace first line with:

CGFloat width = collectionView.enclosingScrollView.bounds.size.width;

I couldn't get this to work with a default layout - but it is fairly easy to implement a custom layout:

/// Simple single column layout, assuming only one section
class SingleColumnLayout: NSCollectionViewLayout {

    /// Height of each view in the collection
    var height:CGFloat = 100

    /// Padding is wrapped round each item, with double an extra bottom padding above the top item, and an extra top padding beneath the bottom
    var padding = EdgeInsets.init(top: 5, left: 10, bottom: 5, right: 10)

    var itemCount:Int {
        guard let collectionView = collectionView else {
            return 0
        }

        return collectionView.numberOfItems(inSection:0)
    }

    override func shouldInvalidateLayout(forBoundsChange newBounds: NSRect) -> Bool {
        return true
    }

    override open func layoutAttributesForItem(at indexPath: IndexPath) -> NSCollectionViewLayoutAttributes? {
        let attributes = NSCollectionViewLayoutAttributes(forItemWith: indexPath)
        guard let collectionView = collectionView else {
            return attributes
        }

        let bounds = collectionView.bounds
        let itemHeightWithPadding = height + padding.top + padding.bottom
        let row = indexPath.item

        attributes.frame = NSRect(x: padding.left, y: itemHeightWithPadding * CGFloat(row) + padding.top + padding.bottom , width: bounds.width - padding.left - padding.right , height: height)
        attributes.zIndex = row

        return attributes
    }

    //If you have lots of items, then you should probably do a 'proper' implementation here
    override open func layoutAttributesForElements(in rect: NSRect) -> [NSCollectionViewLayoutAttributes] {
        var attributes = [NSCollectionViewLayoutAttributes]()

        if (itemCount>0){
            for index in 0...(itemCount-1) {
                if let attribute = layoutAttributesForItem(at: NSIndexPath(forItem: index, inSection: 0) as IndexPath) {
                    attributes.append(attribute)
                }
            }
        }

        return attributes
    }

    override open var collectionViewContentSize: NSSize {

        guard let collectionView = collectionView else {
            return NSSize.zero
        }

        let itemHeightWithPadding = height + padding.top + padding.bottom

        return NSSize.init(width: collectionView.bounds.width, height: CGFloat(itemCount) * itemHeightWithPadding + padding.top + padding.bottom )
    }
}

then all you need is

var layout=SingleColumnLayout()
collectionView.collectionViewLayout = layout