UICollectionViewFlowLayout: collectionView!.contentSize vs collectionViewContentSize - ios

Question

Update
The issue seems to be resolved when using the designated collectionViewContentSize() method like so:
let contentSize = collectionViewContentSize()
Nevertheless, I would be very interested in an explanation behind this behaviour, so I've updated my question accordingly.
Original Question
I am trying to recreate the steps found in the first example of this article, but using Swift and Storyboards.
I have a custom UICollectionViewFlowLayout subclass with the following content:
import UIKit
class SpringyFlowLayout: UICollectionViewFlowLayout {
private lazy var animator: UIDynamicAnimator = {
return UIDynamicAnimator(collectionViewLayout: self)
}()
override func prepareLayout() {
super.prepareLayout()
let contentSize = collectionView!.contentSize
let items = super.layoutAttributesForElementsInRect(CGRect(origin: CGPointZero, size: contentSize)) as! [UICollectionViewLayoutAttributes]
if animator.behaviors.isEmpty {
for item in items {
let spring = UIAttachmentBehavior(item: item, attachedToAnchor: item.center)
spring.length = 0
spring.damping = 0.8
spring.frequency = 1.0
animator.addBehavior(spring)
}
}
}
override func layoutAttributesForElementsInRect(rect: CGRect) -> [AnyObject]? {
return animator.itemsInRect(rect)
}
override func layoutAttributesForItemAtIndexPath(indexPath: NSIndexPath) -> UICollectionViewLayoutAttributes! {
return animator.layoutAttributesForCellAtIndexPath(indexPath)
}
override func shouldInvalidateLayoutForBoundsChange(newBounds: CGRect) -> Bool {
let delta = newBounds.origin.y - collectionView!.bounds.origin.y
for spring in animator.behaviors {
let items = spring.items as! [UICollectionViewLayoutAttributes]
if let attributes = items.first {
attributes.center.y += delta
animator.updateItemUsingCurrentState(attributes)
}
}
return false
}
I have the correct Layout class set in Storyboards. When I run the app, my Collection View is empty.
I have determined that the issue is in the following snipped:
override func prepareLayout() {
super.prepareLayout()
let contentSize = collectionView!.contentSize
let items = super.layoutAttributesForElementsInRect(CGRect(origin: CGPointZero, size: contentSize)) as! [UICollectionViewLayoutAttributes]
//...
}
Since I subclass UICollectionViewFlowLayout, I thought I can rely on the super implementation to lay out the elements for me, and then modify their attributes. But when I check contentSize, it reports the width correctly, but the height is 0.
This leads to an empty items array -> no behaviors in animator -> animator.itemsInRect(_) returning an empty array -> the empty Collection View.
I just can't seem to find out what I'm missing. There should be no need to override the contentSize() method, since I'm using a flow layout.

Answer 1

The issue it that collectionView doesn't yet have a content size, because it just started to prepare it's layout. I don't believe that calling collectionView!.contentSize will actually compute the size. The reason collectionViewContentSize() works is because it will compute the size using your other layout code.

Answer 2

Here's something you might find interesting. I noticed in Ash Furrow's example in viewDidAppear a call to collectionViewLayout.invalidateLayout. Ash notes this isn't necessary when using storyboards due a difference in timing of the first invocation of prepareLayout versus when storyboards are not used.
I tried invalidating the collection view layout in viewDidAppear along with your code and a storyboard. Here is what I found:
Test Scenarios:
1) collectionView!.contentSize without invalidateLayout = cv is EMPTY
2) collectionViewContentSize() without invalidateLayout = cv WORKS
3) collectionView!.contentSize with invalidateLayout = cv WORKS
4) collectionViewContentSize() with invalidateLayout = cv WORKS
FYI