Get the indexPath of cell in the center of collectionView in swift? - ios

Question

I have tried this to get the index of a cell in the center of a collection view by using an extension of uicollectionview, but I always get nil instead of the index. How can I fix this?
extension UICollectionView {
var centerPoint : CGPoint {
get {
return CGPoint(x: self.center.x + self.contentOffset.x, y: self.center.y + self.contentOffset.y);
}
}
var centerCellIndexPath: NSIndexPath? {
if let centerIndexPath: NSIndexPath = self.indexPathForItemAtPoint(self.centerPoint) {
return centerIndexPath
}
return nil
}
}
then: in a UIViewController in a random method I have this:
if let centerCellIndexPath: NSIndexPath = collectionTemp!.centerCellIndexPath {
print(centerCellIndexPath)
} else {
println("nil")
}
the index path is always nil, and I don't get it why, because the cells display in order, everything is fine except this.

Answer 1

I've managed to solve my problem by using a custom layout that always keeps a cell in the center. Using this the indexPath in the center can never be nil.
class CenterFlowLayout: UICollectionViewFlowLayout {
override func targetContentOffsetForProposedContentOffset(proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
if let cv = self.collectionView {
let cvBounds = cv.bounds
let halfWidth = cvBounds.size.width * 0.5
let proposedContentOffsetCenterX = proposedContentOffset.x + halfWidth
if let attributesForVisibleCells = self.layoutAttributesForElementsInRect(cvBounds) as [UICollectionViewLayoutAttributes]! {
var candidateAttributes: UICollectionViewLayoutAttributes?
for attributes in attributesForVisibleCells {
// == Skip comparison with non-cell items (headers and footers) == //
if attributes.representedElementCategory != UICollectionElementCategory.Cell {
continue
}
if let candAttrs = candidateAttributes {
let a = attributes.center.x - proposedContentOffsetCenterX
let b = candAttrs.center.x - proposedContentOffsetCenterX
if fabsf(Float(a)) < fabsf(Float(b)) {
candidateAttributes = attributes
}
} else { // == First time in the loop == //
candidateAttributes = attributes
continue
}
}
return CGPoint(x : candidateAttributes!.center.x - halfWidth, y : proposedContentOffset.y)
}
}
// Fallback
return super.targetContentOffsetForProposedContentOffset(proposedContentOffset)
}
}
Add this as the subclass of the UICollectionFlowLayout.