swift positionate Zentrum UICollectionViewCell

Question

Ich versuche, die "Snap zu Wirkung zu implementieren", aber konnte es nicht funktionieren, gibt es etwas, das ich falsch mache?

Subclassing UICollectionViewFlowLayout und überwiegendes targetContentOffsetForProposedContentOffset die Funktion:

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) 
} 

Sample project here und snap effect, desired output

Answer 1

Ich habe es geschafft, am Ende, um herauszufinden,

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) 
} 

Vergessen Sie auch nicht einstellen der Delegierte:

class MyCostomFlow: UICollectionViewFlowLayout, UICollectionViewDelegateFlowLayout { 

    var delegate:MyFlowLayoutDelegate? 
} 

und der Delegierte selbst:

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