iOS Swift UICollectionView横向分页滚动，cell左右排版问题详解

// 保存所有item的attributes
fileprivate var attributesArr: [UICollectionViewLayoutAttributes] = []

// MARK:- 重新布局
override func prepare() {
 super.prepare() 
 let itemWH: CGFloat = kScreenW / CGFloat(kEmotionCellNumberOfOneRow) 
 // 设置itemSize
 itemSize = CGSize(width: itemWH, height: itemWH)
 minimumLineSpacing = 0
 minimumInteritemSpacing = 0
 scrollDirection = .horizontal 
 // 设置collectionView属性
 collectionView?.isPagingEnabled = true
 collectionView?.showsHorizontalScrollIndicator = false
 collectionView?.showsVerticalScrollIndicator = true
 let insertMargin = (collectionView!.bounds.height - 3 * itemWH) * 0.5
 collectionView?.contentInset = UIEdgeInsetsMake(insertMargin, 0, insertMargin, 0) 
 /// 重点在这里
 var page = 0
 let itemsCount = collectionView?.numberOfItems(inSection: 0) ?? 0
 for itemIndex in 0..<itemsCount {
  let indexPath = IndexPath(item: itemIndex, section: 0)
  let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)  
  page = itemIndex / (kEmotionCellNumberOfOneRow * kEmotionCellRow)
  // 通过一系列计算, 得到x, y值
  let x = itemSize.width * CGFloat(itemIndex % Int(kEmotionCellNumberOfOneRow)) + (CGFloat(page) * kScreenW)
  let y = itemSize.height * CGFloat((itemIndex - page * kEmotionCellRow * kEmotionCellNumberOfOneRow) / kEmotionCellNumberOfOneRow)  
  attributes.frame = CGRect(x: x, y: y, width: itemSize.width, height: itemSize.height)
  // 把每一个新的属性保存起来
  attributesArr.append(attributes)
 }
}

override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
 var rectAttributes: [UICollectionViewLayoutAttributes] = []
 _ = attributesArr.map({
  if rect.contains($0.frame) {
   rectAttributes.append($0)
  }
 })
 return rectAttributes
}

import UIKit
let kEmotionCellNumberOfOneRow = 8
let kEmotionCellRow = 3
class LXFChatEmotionCollectionLayout: UICollectionViewFlowLayout {
 // 保存所有item
 fileprivate var attributesArr: [UICollectionViewLayoutAttributes] = [] 
 // MARK:- 重新布局
 override func prepare() {
  super.prepare()  
  let itemWH: CGFloat = kScreenW / CGFloat(kEmotionCellNumberOfOneRow)  
  // 设置itemSize
  itemSize = CGSize(width: itemWH, height: itemWH)
  minimumLineSpacing = 0
  minimumInteritemSpacing = 0
  scrollDirection = .horizontal  
  // 设置collectionView属性
  collectionView?.isPagingEnabled = true  collectionView?.showsHorizontalScrollIndicator = false
  collectionView?.showsVerticalScrollIndicator = true
  let insertMargin = (collectionView!.bounds.height - 3 * itemWH) * 0.5
  collectionView?.contentInset = UIEdgeInsetsMake(insertMargin, 0, insertMargin, 0)  
  var page = 0
  let itemsCount = collectionView?.numberOfItems(inSection: 0) ?? 0
  for itemIndex in 0..<itemsCount {
   let indexPath = IndexPath(item: itemIndex, section: 0)
   let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)   
   page = itemIndex / (kEmotionCellNumberOfOneRow * kEmotionCellRow)
   // 通过一系列计算, 得到x, y值
   let x = itemSize.width * CGFloat(itemIndex % Int(kEmotionCellNumberOfOneRow)) + (CGFloat(page) * kScreenW)
   let y = itemSize.height * CGFloat((itemIndex - page * kEmotionCellRow * kEmotionCellNumberOfOneRow) / kEmotionCellNumberOfOneRow)   
   attributes.frame = CGRect(x: x, y: y, width: itemSize.width, height: itemSize.height)
   // 把每一个新的属性保存起来
   attributesArr.append(attributes)
  }  
 } 
 override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
  var rectAttributes: [UICollectionViewLayoutAttributes] = []
  _ = attributesArr.map({
   if rect.contains($0.frame) {
    rectAttributes.append($0)
   }
  })
  return rectAttributes
 } 
}