Horizontal paging for collections based on the content size

ContentSizePagingCollectionDelegate.swift

//
//  ContentSizePagingCollectionDelegate.swift
//  CardsCarousel
//
//  Created by Daniel Carlos Souza Carvalho on 2/20/21.
//

import UIKit

/// Horizontal paging for collections based on the content size
///
/// While isPagingEnabled stops on multiples of the scroll view’s **bounds**  when the user scrolls,
/// this calculates the offset based on the resulting **content size**, given the cell width and the cell distances.
///
/// Make sure to set isPagingEnabled to false in the collection
///
/// After reloading the data, call collection.layoutIfNeeded().
/// That way collection.contentSize = collection.collectionViewLayout.collectionViewContentSize
class ContentSizePagingCollectionDelegate: NSObject, UICollectionViewDelegateFlowLayout {
    
    private let cellWitdhPercentage: CGFloat = 0.85
    private let idealCellDistance: CGFloat = 16
    
    private var currentPositionIndex = 0
    
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        let cellWidth = cellWitdh(forCollectionViewWidth: collectionView.frame.width)
        return CGSize(width: cellWidth, height: collectionView.frame.height)
    }
    
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat {
        return cellDistance(forCollectionViewWidth: collectionView.frame.width)
    }
    
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
        let collectionViewWidth = collectionView.frame.width
        let sides = collectionViewWidth - cellWitdh(forCollectionViewWidth: collectionViewWidth)
        let horizontalInset: CGFloat = sides/2
        return UIEdgeInsets(top: 0, left: horizontalInset, bottom: 0, right: horizontalInset)
    }
    
    func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {
        let offsetWidthForOneItem = caculateContentOffsetForOneItem(scrollView)
        let offsetForCurrentItem = { CGPoint(x: offsetWidthForOneItem * CGFloat(self.currentPositionIndex), y: targetContentOffset.pointee.y) }

        enum HorizontalDirection { case left, right }
        let horizontalDirection: HorizontalDirection = velocity.x > 0 ? .right : .left
        
        switch horizontalDirection {
        case .left:
            let isFirstItem = currentPositionIndex <= 0
            guard isFirstItem == false else {
                targetContentOffset.pointee = offsetForCurrentItem()
                return
            }
            
            currentPositionIndex -= 1
            targetContentOffset.pointee = offsetForCurrentItem()
            
        case .right:
            let isLastItem = (scrollView.contentOffset.x + offsetWidthForOneItem >= scrollView.contentSize.width)
            guard isLastItem == false else {
                targetContentOffset.pointee = offsetForCurrentItem()
                return
            }
            
            currentPositionIndex += 1
            targetContentOffset.pointee = offsetForCurrentItem()
        }
    }
    
    private func cellWitdh(forCollectionViewWidth collectionViewWidth: CGFloat) -> CGFloat {
        let itemWidth = collectionViewWidth * cellWitdhPercentage
        return itemWidth
    }
    
    private func cellDistance(forCollectionViewWidth collectionViewWidth: CGFloat) -> CGFloat {
        let sides = collectionViewWidth - cellWitdh(forCollectionViewWidth: collectionViewWidth)
        let oneSide = sides/2
        let final = min(idealCellDistance, oneSide/2)
        return final
    }

    private func caculateContentOffsetForOneItem(_ scrollView: UIScrollView) -> CGFloat {
        let cellItemWidth = cellWitdh(forCollectionViewWidth: scrollView.frame.width)
        let sides = scrollView.frame.width - cellItemWidth
        let oneSide: CGFloat = sides/2
        let nextItemVisiblePart = scrollView.frame.width - (oneSide + cellItemWidth + cellDistance(forCollectionViewWidth: scrollView.frame.width))
        return oneSide + (cellItemWidth - nextItemVisiblePart)
    }
}