Vertically scrolling UICollectionView with self-sizing cells only displays half the cells - ios

Question

I've set up a UICollectionView (with the default flow layout, and vertical scrolling) with custom cells that load from a xib. The cells contain a couple of elements (a UILabel and a UIView) with constraints on them. I've set up the constraints such that the cell height grows as the label text height increases using the new UICollectionViewFlowLayout property that's available in iOS8, estimatedItemSize:
layout.estimatedItemSize = CGSizeMake(self.collectionView.frame.width, 100)
Everything works like a charm except there's one big problem: the UICollectionView loads only half the items as returned by the numberOfItemsInSection method. So, when the method returns, say, 10, my UICollectionView only displays 5 cells (but displays and lays them out perfectly).
Some relevant findings from my debugging attempts:
I've been able to force the loading of the remaining items by calling invalidateLayout or changing the number of sections from 1 to 2. But those are just debugging hacks.
Separately, everything works like a charm when I replace estimatedItemSize with the itemSize property, i.e. hard-coding the item size. But that defeats the self-sizing functionality I'd like to implement.
I'm assuming there's something wrong with how I'm thinking about self-sized cells. Specifically, I wonder if the problem has something to do with constraints.
I'd appreciate any help here.

Answer 1

I believe this is due to a bug in iOS8 which still exists as of iOS8.3.
Flow layout collection views with self-sizing cells will fail to display some of their content if the estimatedItemSize is too small. This is despite the fact that according to the the API docs the only requirement on the estimatedItemSize is that it be non-zero to trigger the self-sizing behavior.
Only Apple is in a position to fix this. I think the best workaround in the mean time is to put an inflated value into estimatedItemSize`.
This repo demonstrates the issue: https://github.com/algal/SelfSizingCellsDemo. (I've filed a radar at rdar://18078598.)

Answer 2

The obvious non-hacky solution is to implement
flowLayout.collectionView(layout:sizeForItemAtIndexPath:)
but of course that is more code than you presumably have hoped for.
I would suggest you try two more things:
Try to experiment with more plausible estimated sizes. Also try the default value, CGSizeZero.
Check what happens in preferredlLayoutAttributesFittingAttributes() by examining the layoutAttributes parameter. Maybe you find a clue to why the other cells are "crowded out".

Answer 3

In a custom subclass of UICollectionViewFlowLayout
override prepareLayout like so:
- (void)prepareLayout {
[super prepareLayout];
CGSize contentSize = self.collectionView.contentSize;
// Without this line, the scrolling doesn't go all the way to the bottom, right away but takes a while
[super layoutAttributesForElementsInRect:CGRectMake(0.0f, 0.0f, contentSize.width, contentSize.height)];
}

Answer 4

I had this exact same problem.
I was using layout.estimatedItemSize = CGSizeMake(373.0, 500.0) together with
preferredLayoutAttributesFittingAttributes( layoutAttributes: UICollectionViewLayoutAttributes) -> UICollectionViewLayoutAttributes!
and I got exactly the self-sizing behaviour i wanted, except I was only getting half the cells displayed.
The curious thing is when I changed the estimatedItemSize property to: layout.estimatedItemSize = view.bounds.size ALL the cells appear!!
Hope this helps. Good luck.