Centered Paging with Preview Cells on UICollectionView

uicollectionview+centerPaging.md

Centered Paging with Preview Cells on UICollectionView

The proposed offset is where the collection view would stop without our intervention. We peek into this area by finding its centre as proposedContentOffsetCenterX and examine our currently visible cells to see which one’s centre is closer to the centre of that area.

import UIKit
 
class CenterCellCollectionViewFlowLayout: UICollectionViewFlowLayout {
    
    var mostRecentOffset : CGPoint = CGPoint()
    
    override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
        
        if velocity.x == 0 {
            return mostRecentOffset
        }
        
        if let cv = self.collectionView {
            
            let cvBounds = cv.bounds
            let halfWidth = cvBounds.size.width * 0.5;
            
            
            if let attributesForVisibleCells = self.layoutAttributesForElements(in: cvBounds) {
                
                var candidateAttributes : UICollectionViewLayoutAttributes?
                for attributes in attributesForVisibleCells {
                    
                    // == Skip comparison with non-cell items (headers and footers) == //
                    if attributes.representedElementCategory != UICollectionElementCategory.cell {
                        continue
                    }
                    
                    if (attributes.center.x == 0) || (attributes.center.x > (cv.contentOffset.x + halfWidth) && velocity.x < 0) {
                        continue
                    }
                    candidateAttributes = attributes 
                }
                
                // Beautification step , I don't know why it works!
                if(proposedContentOffset.x == -(cv.contentInset.left)) {
                    return proposedContentOffset
                }
                
                guard let _ = candidateAttributes else {
                    return mostRecentOffset
                }
                mostRecentOffset = CGPoint(x: floor(candidateAttributes!.center.x - halfWidth), y: proposedContentOffset.y)
                return mostRecentOffset
                
            }
        }
        
        // fallback
        mostRecentOffset = super.targetContentOffset(forProposedContentOffset: proposedContentOffset)
        return mostRecentOffset
    }
 
   
}