Fast positioning center UICollectionViewCell

Question

I am trying to implement “ binding to an effect ” but could not get it to work, is there something that I am doing wrong?

Subclassing UICollectionViewFlowLayout and overriding the targetContentOffsetForProposedContentOffset function:

 override func targetContentOffsetForProposedContentOffset(proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint { var offsetAdjustment:CGFloat = CGFloat(MAXFLOAT) let verticalCenter:CGFloat = proposedContentOffset.y + (CGRectGetHeight(self.collectionView!.bounds) / 2.0) let proposedRect:CGRect = CGRectMake(proposedContentOffset.x,0.0,self.collectionView!.bounds.size.width,self.collectionView!.bounds.size.height) let array:NSArray = self.layoutAttributesForElementsInRect(proposedRect)! for layoutAttributes : AnyObject in array { if let _layoutAttributes = layoutAttributes as? UICollectionViewLayoutAttributes { if _layoutAttributes.representedElementCategory != UICollectionElementCategory.Cell { continue } let itemVerticalCenter:CGFloat = layoutAttributes.center.y let _verticalCenter = fabsf(Float(itemVerticalCenter) - Float(verticalCenter)) let _offsetAdjustment = fabsf(Float(offsetAdjustment)) if (_verticalCenter < _offsetAdjustment) { offsetAdjustment = itemVerticalCenter - verticalCenter } } } return CGPointMake(proposedContentOffset.x + offsetAdjustment, proposedContentOffset.y) } 

An example of the project here and the effect of binding, the desired result

Answer 1

I managed to find out at the end

 override func targetContentOffsetForProposedContentOffset(proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint { let rectBounds:CGRect = self.collectionView!.bounds let halfHeight:CGFloat = rectBounds.size.height * CGFloat(0.45) let proposedContentOffsetCenterY:CGFloat = proposedContentOffset.y + halfHeight let attributesArray:NSArray = self.layoutAttributesForElementsInRect(rectBounds)! var candidateAttributes:UICollectionViewLayoutAttributes? for layoutAttributes : AnyObject in attributesArray { if let _layoutAttributes = layoutAttributes as? UICollectionViewLayoutAttributes { if _layoutAttributes.representedElementCategory != UICollectionElementCategory.Cell { continue } if candidateAttributes == nil { candidateAttributes = _layoutAttributes self.delegate?.cellCenteredAtIndexPathWithCollectionView(self.collectionView!, layout: self, indexPath: candidateAttributes!.indexPath) continue } if fabsf(Float(_layoutAttributes.center.y) - Float(proposedContentOffsetCenterY)) < fabsf(Float(candidateAttributes!.center.y) - Float(proposedContentOffsetCenterY)) { candidateAttributes = _layoutAttributes self.delegate?.cellCenteredAtIndexPathWithCollectionView(self.collectionView!, layout: self, indexPath: candidateAttributes!.indexPath) } } } if attributesArray.count == 0 { return CGPointMake(proposedContentOffset.x,proposedContentOffset.y - halfHeight * 2) } return CGPointMake(proposedContentOffset.x,candidateAttributes!.center.y - halfHeight) } 

Also do not forget to set the delegate:

 class MyCostomFlow: UICollectionViewFlowLayout, UICollectionViewDelegateFlowLayout { var delegate:MyFlowLayoutDelegate? } 

and the delegate himself:

 protocol MyFlowLayoutDelegate { func cellCenteredAtIndexPathWithCollectionView(collectionView: UICollectionView, layout: UICollectionViewLayout, indexPath: NSIndexPath) -> Void }