UICollectionViewFlowLayout Cell with custom anchorPoint reused too soon - ios

Question

I've subclassed a UICollectionViewFlowLayout to achieve a small scaling effect during horizontal scroll. Therefore I had to subclass a UICollectionViewCell as well as to change the layer.anchorPoint of the cell (my scaling is from the bottom left of the cell rather than from the default center). Now all fine and well except the fact that when I am scrolling horizontally , my cell is reused too soon (I still can see the half cell when it suddenly disappear ).
I have the feeling that collection view still bases its calculations for reusing the cell on the anchor point positioned in the center of the cell...
However , this is my collection view . You can see how the item getting bigger as it reaches the left side of the collection view. This is the scaling I wanted.
Now here I am scrolling to the left. You can see how the right item became bigger and the left is getting out of the screen.
And here you see that the left item didn't get off the screen but already dissapeared. Only the right item remeained visible :/
So what I want is , to make the left item disappear only when it's right boundaries reaching the very left of the screen.Simply saying , to dissapear only when I don't see it anymore.
And here is my code:
class SongsCollectionViewCell : UICollectionViewCell {
#IBOutlet weak var imgAlbumCover: UIImageView!
override func applyLayoutAttributes(layoutAttributes: UICollectionViewLayoutAttributes) {
super.applyLayoutAttributes(layoutAttributes)
//we must change the anchor point for propper cells positioning and scaling
self.layer.anchorPoint.x = 0
self.layer.anchorPoint.y = 1
}
}
Here is the layout itself :
class SongsCollectionViewLayout : UICollectionViewFlowLayout {
override func prepareLayout() {
collectionView?.decelerationRate = UIScrollViewDecelerationRateFast
self.scrollDirection = UICollectionViewScrollDirection.Horizontal;
//size of the viewport
let size:CGSize = self.collectionView!.frame.size;
let itemWidth:CGFloat = size.width * 0.7//0.7//3.0;
self.itemSize = CGSizeMake(itemWidth, itemWidth);
self.sectionInset = UIEdgeInsetsMake(0,0,0,0);
self.minimumLineSpacing = 0
self.minimumInteritemSpacing = 0
}
override func shouldInvalidateLayoutForBoundsChange(newBounds: CGRect) -> Bool {
return true
}
override func layoutAttributesForElementsInRect(rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
let attributes:[UICollectionViewLayoutAttributes] = super.layoutAttributesForElementsInRect(rect)!
var visibleRect:CGRect = CGRect()
visibleRect.origin = self.collectionView!.contentOffset;
visibleRect.size = self.collectionView!.bounds.size;
for layoutAttributes in attributes {
if (CGRectIntersectsRect(layoutAttributes.frame, rect)) {
//we must align items to the bottom of the collection view on y axis
let frameHeight = self.collectionView!.bounds.size.height
layoutAttributes.center.y = frameHeight
layoutAttributes.center.x = layoutAttributes.center.x - self.itemSize.width/2
//find where ite, left x is
let itemLeftX:CGFloat = layoutAttributes.center.x
//distance of the item from the left of the viewport
let distanceFromTheLeft:CGFloat = itemLeftX - CGRectGetMinX(visibleRect)
let normalizedDistanceFromTheLeft = abs(distanceFromTheLeft) / self.collectionView!.frame.size.width
//item that is closer to the left is most visible one
layoutAttributes.alpha = 1 - normalizedDistanceFromTheLeft
layoutAttributes.zIndex = abs(Int(layoutAttributes.alpha)) * 10;
//scale items
let scale = min(layoutAttributes.alpha + 0.5, 1)
layoutAttributes.transform = CGAffineTransformMakeScale(scale, scale)
}
}
return attributes;
}
override func targetContentOffsetForProposedContentOffset(proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
// Snap cells to centre
var newOffset = CGPoint()
let layout = collectionView!.collectionViewLayout as! UICollectionViewFlowLayout
let width = layout.itemSize.width + layout.minimumLineSpacing
var offset = proposedContentOffset.x + collectionView!.contentInset.left
if velocity.x > 0 {
//ceil returns next biggest number
offset = width * ceil(offset / width)
} else if velocity.x == 0 { //6
//rounds the argument
offset = width * round(offset / width)
} else if velocity.x < 0 { //7
//removes decimal part of argument
offset = width * floor(offset / width)
}
newOffset.x = offset - collectionView!.contentInset.left
newOffset.y = proposedContentOffset.y //y will always be the same...
return newOffset
}
}

Answer 1

Answering my own question.
So as I suspected , the layout was taking an old center into account that is why I had to correct the center of the cell right after changing the anchor point . So my custom cell now looks like this :
class SongsCollectionViewCell : UICollectionViewCell {
#IBOutlet weak var imgAlbumCover: UIImageView!
override func prepareForReuse() {
imgAlbumCover.hnk_cancelSetImage()
imgAlbumCover.image = nil
}
override func applyLayoutAttributes(layoutAttributes: UICollectionViewLayoutAttributes) {
super.applyLayoutAttributes(layoutAttributes)
//we must change the anchor point for propper cells positioning and scaling
self.layer.anchorPoint.x = 0
self.layer.anchorPoint.y = 1
//we need to adjust a center now
self.center.x = self.center.x - layoutAttributes.size.width/2
}
}
Hope it helps someone