iOS Swift UICollectionView横向分页滚动，cell左右排版

状况

最近在作表情键盘时遇到一个问题，我用UICollectionView来布局表情，使用横向分页滚动，但在最后一页出现了如图所示的状况

状况分析图

是的，如今的item分布就是这个鬼样子

如今想要作的，就是将如今这个鬼样子变成另一种样子，如图

那怎么办？只好从新布局item了

解决方案

我是自定了一个Layout(LXFChatEmotionCollectionLayout)，让UICollectionView在建立的时候使用了它

在 LXFChatEmotionCollectionLayout.swift 中

添加一个属性来保存全部item的attributes

// 保存全部item的attributes
fileprivate var attributesArr: [UICollectionViewLayoutAttributes] = []
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从新布局

// MARK:- 从新布局
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)
        // 经过一系列计算, 获得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)
        // 把每个新的属性保存起来
        attributesArr.append(attributes)
    }
}
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返回全部当前可见的Attributes

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

完整代码

import UIKit

let kEmotionCellNumberOfOneRow = 8
let kEmotionCellRow = 3

class LXFChatEmotionCollectionLayout: UICollectionViewFlowLayout {
    // 保存全部item
    fileprivate var attributesArr: [UICollectionViewLayoutAttributes] = []
    
    // MARK:- 从新布局
    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)
            // 经过一系列计算, 获得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
    }
    
}
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