出列单元格比默认 UICollectionView 出列单元格多

Question

有没有办法让比默认 UICollectionView 出列单元格更多的单元格出列？ 例如在下图中 UICollectionView 出列显示单元格，一前一后。 有没有办法让超过这三个单元格出列？ func collectionView(_ collectionView: UICollectionView, prefetchItemsAt indexPaths: [IndexPath]) { print("Prefetch: \\(indexPaths)") }

之后只给我一个索引路径。 我需要的是在显示单元格之前和之前创建四个或五个单元格，以在显示单元格之前实现我的功能。

Answer 1

官方 Apple 示例代码展示了一种您可能会发现有用的技术。 您可以观察滚动并根据contentOffset值，而不是依赖UICollectionView来接收回调并根据需要加载数据，您将知道需要多少向前看。

如果将来的读者无法使用链接，请在此处复制粘贴相关部分。 所有这些都存在于UICollectionViewController子类中——它通常可以适应任何UIScrollView 。

func scrollViewDidScroll(_ scrollView: UIScrollView) {
    // This is your trigger point
    updateCachedAssets()
}

var fetchResult: PHFetchResult<PHAsset>!
fileprivate let imageManager = PHCachingImageManager()
fileprivate let thumbnailSize: CGSize = CGSize(width: 100, height: 100)
fileprivate var previousPreheatRect = CGRect.zero
fileprivate func updateCachedAssets() {
    // Update only if the view is visible.
    guard isViewLoaded && view.window != nil else { return }
    
    // The window you prepare ahead of time is twice the height of the visible rect.
    let visibleRect = CGRect(origin: collectionView!.contentOffset, size: collectionView!.bounds.size)
    let preheatRect = visibleRect.insetBy(dx: 0, dy: -0.5 * visibleRect.height)
        
    // Update only if the visible area is significantly different from the last preheated area.
    let delta = abs(preheatRect.midY - previousPreheatRect.midY)
    guard delta > view.bounds.height / 3 else { return }
    
    // Compute the assets to start and stop caching.
    let (addedRects, removedRects) = differencesBetweenRects(previousPreheatRect, preheatRect)
    let addedAssets = addedRects
        .flatMap { rect in collectionView!.indexPathsForElements(in: rect) }
        .map { indexPath in fetchResult.object(at: indexPath.item) }
    let removedAssets = removedRects
        .flatMap { rect in collectionView!.indexPathsForElements(in: rect) }
        .map { indexPath in fetchResult.object(at: indexPath.item) }
        
    // Update the assets the PHCachingImageManager is caching.
    imageManager.startCachingImages(for: addedAssets,
                                    targetSize: thumbnailSize, contentMode: .aspectFill, options: nil)
    imageManager.stopCachingImages(for: removedAssets,
                                   targetSize: thumbnailSize, contentMode: .aspectFill, options: nil)
    // Store the computed rectangle for future comparison.
    previousPreheatRect = preheatRect
}

fileprivate func differencesBetweenRects(_ old: CGRect, _ new: CGRect) -> (added: [CGRect], removed: [CGRect]) {
    if old.intersects(new) {
        var added = [CGRect]()
        if new.maxY > old.maxY {
            added += [CGRect(x: new.origin.x, y: old.maxY,
                             width: new.width, height: new.maxY - old.maxY)]
        }
        if old.minY > new.minY {
            added += [CGRect(x: new.origin.x, y: new.minY,
                             width: new.width, height: old.minY - new.minY)]
        }
        var removed = [CGRect]()
        if new.maxY < old.maxY {
            removed += [CGRect(x: new.origin.x, y: new.maxY,
                               width: new.width, height: old.maxY - new.maxY)]
        }
        if old.minY < new.minY {
            removed += [CGRect(x: new.origin.x, y: old.minY,
                               width: new.width, height: new.minY - old.minY)]
        }
        return (added, removed)
    } else {
        return ([new], [old])
    }
}

笔记

0. 该代码已有数年历史，您需要根据自己的要求对其进行调整。
1. 您可以将 preheatRect 计算调整为适合您的需求。

 // The window you prepare ahead of time is twice the height of the visible rect. let visibleRect = CGRect(origin: collectionView!.contentOffset, size: collectionView!.bounds.size) let preheatRect = visibleRect.insetBy(dx: 0, dy: -0.5 * visibleRect.height)

2. 该示例实现了这个preheatRect逻辑来预加载PHAsset缩略图。 您需要将您的数据源调整为类似的样式并开始/停止加载您的资源。