CenterAlignmentFlowLayout

CenterAlignmentFlowLayout.swift

//
//  CenterAlignmentFlowLayout.swift
//  Play
//
//  Created by Tbxark on 15/12/28.
//  Copyright © 2015年 TBXark. All rights reserved.
//

import UIKit

class CenterAlignmentFlowLayout: UICollectionViewFlowLayout {
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        guard let superAttributes = super.layoutAttributesForElements(in: rect) else {
            return nil
        }
        var rowCollections = [CGFloat: [UICollectionViewLayoutAttributes]]()
        let flowDelegate = collectionView?.delegate as? UICollectionViewDelegateFlowLayout
        let delegateSupportsInteritemSpacing = flowDelegate?.responds(to: #selector(UICollectionViewDelegateFlowLayout.collectionView(_:layout:minimumInteritemSpacingForSectionAt:))) ?? false
        for itemAttributes in superAttributes {
            let midYRound = CGFloat(roundf(Float(itemAttributes.frame.midY)))
            let midYPlus = midYRound + 1
            let midYMinus = midYRound - 1
            var key: CGFloat? = nil
            if rowCollections.keys.contains(midYPlus) {
                key = midYPlus
            }
            if rowCollections.keys.contains(midYMinus) {
                key = midYMinus
            }

            let realKey = key ?? midYMinus
            if !rowCollections.keys.contains(realKey) {
                rowCollections[realKey] = [UICollectionViewLayoutAttributes]()
            }
            rowCollections[realKey]?.append(itemAttributes)
        }
        guard let collection = collectionView else {
            return nil
        }
        let collectionViewWidth: CGFloat = collection.bounds.width - collection.contentInset.left - collection.contentInset.right
        for (_, itemAttributesCollection) in rowCollections {
            let itemsInRow = itemAttributesCollection.count
            var interitemSpacing = minimumInteritemSpacing
            if delegateSupportsInteritemSpacing && itemsInRow > 0 {
                let section = (itemAttributesCollection[0].indexPath as NSIndexPath).section
                if let temp = flowDelegate?.collectionView?(collection, layout: self, minimumInteritemSpacingForSectionAt: section) {
                    interitemSpacing = temp
                }
            }
            let aggregateInteritemSpacing = interitemSpacing * CGFloat(itemsInRow - 1)
            var aggregateItemWidths: CGFloat = 0
            for itemAttributes in itemAttributesCollection {
                aggregateItemWidths += itemAttributes.frame.width
            }

            let alignmentWidth = aggregateItemWidths + aggregateInteritemSpacing
            let alignmentXOffset = (collectionViewWidth - alignmentWidth) / 2

            var previousFrame: CGRect = CGRect.zero
            for itemAttributes in itemAttributesCollection {
                var itemFrame: CGRect = itemAttributes.frame
                if previousFrame.equalTo(CGRect.zero) {
                    itemFrame.origin.x = alignmentXOffset
                } else {
                    itemFrame.origin.x = previousFrame.maxX + interitemSpacing
                }
                itemAttributes.frame = itemFrame
                previousFrame = itemFrame
            }
        }
        return superAttributes

    }
}