UICollectionView 将在调用“reloadItems”方法时重新创建单元格

Posted 2023-03-14

【中文标题】UICollectionView 将在调用“reloadItems”方法时重新创建单元格

【英文标题】：UICollectionView will recreate cells when calling the "reloadItems" method

【发布时间】：2018-11-09 03:51:16

【问题描述】：

我的单元格有一个从网络下载的图像，因此我需要将单元格的高度设置为动态。 图片下载完成后，我将调用self.collectionView.reloadItems(at: [indexPath])触发设置新高度的委托方法。

但似乎reloadItems 方法会重新创建一个单元格，而不仅仅是重新布局一个原来的重用单元格。

我该如何解决这个问题？是苹果的 UICollectionView 上的错误还是我做错了什么？

完整代码：

// code from ViewController
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell 

    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: AnnounmentWallCollectionViewCell.cellIdentifier, for: indexPath) as! AnnounmentWallCollectionViewCell

    let announcement = announcements[indexPath.row]
    cell.collectionView = collectionView
    cell.setBanner(from: announcement.banner, indexPath: indexPath, completion:  [unowned self] (height) in
        self.bannersHeight[indexPath.row] = height
    )
    cell.sethtmlContent(announcement.content)
    contentsHeight[indexPath.row] = cell.htmlContentSize.height
    printD("indexPath: \(indexPath)")
    return cell


// code from cell
func setBanner(from url: URL?, indexPath: IndexPath, completion: @escaping (_ height: CGFloat)->()) 
    // URL(string: "https://i.imgur.com/qzY7BJ9.jpg")
    if let url = url 
        if let banner = SDImageCache.shared().imageFromDiskCache(forKey: url.absoluteString) 
            self.bannerView.isHidden = false
            self.bannerView.image = banner.scaleWidth(to: self.bounds.width - 32) // leading + trailling
            self.bannerHeight.constant = self.bannerView.image?.size.height ?? 1
            completion(self.bannerHeight.constant)
            printD("NO Download: \(indexPath)")
            let animationsEnabled = UIView.areAnimationsEnabled
            UIView.setAnimationsEnabled(false)
            self.collectionView.reloadItems(at: [indexPath])
            UIView.setAnimationsEnabled(animationsEnabled)
         else 
            DispatchQueue.global().async 
                SDWebImageDownloader.shared().downloadImage(with: url, options: .useNSURLCache, progress: nil)  (banner, data, error, finished) in
                    DispatchQueue.main.async 
                        if let banner = banner 
                            SDImageCache.shared().store(banner, forKey: url.absoluteString, toDisk: true)
                            self.bannerView.isHidden = false
                            self.bannerHeight.constant = banner.scaleWidth(to: self.bounds.width - 32)?.size.height ?? 1
                            completion(self.bannerHeight.constant)
                            self.collectionView.reloadData()
                            printD("Download: \(indexPath): \(self.bannerHeight.constant)")
                         else 
                            self.bannerView.isHidden = true
                            self.bannerHeight.constant = 1
                            completion(self.bannerHeight.constant)
                        
                    
                
            
        
     else 
        bannerView.isHidden = true
        bannerHeight.constant = 1
        completion(bannerHeight.constant)
    


// code from delegate 
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize 
    let width = self.view.bounds.width
    let height = bannersHeight[indexPath.row] + contentsHeight[indexPath.row]
        + 1 // sticker
        + 11 // banner top
    printD("indexPath: \(indexPath): \(height)")
    return CGSize(width: width, height: height)

【参考方案1】：

这不是错误。这就是为给定索引路径重新加载单元格的方式。如果你只想更新布局也可以试试

[self.collectionView.collectionViewLayout invalidateLayout]

而不是

self.collectionView.reloadItems(at: [indexPath])

然后在委托方法中返回正确的大小。

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath 
return //whatever size that you want to return. 

我还强烈建议您缓存您的图像大小，以便您下次可以使用它，而不是过度下载/计算......

【讨论】：

谢谢。顺便说一句，我听说您建议我缓存调整大小的图像而不是原始图像，并从缓存图像中获取高度。但似乎我从 SDImageCache 获得了具有错误比例属性的图像。我将针对该问题打开另一个问题。

如果您调用使布局无效，这将使整个布局无效。如果有人只想重新布局特定单元格怎么办？