UICollectionView will recreate cells when calling the "reloadItems" method

Question

My cell has an image which be downloaded from network, therefore I need to set the height of cell as dynamic. When an image download is finished, I am going to call self.collectionView.reloadItems(at: [indexPath]) to trigger the delegate method for setting a new height.

But it seems that the reloadItems method will recreate a cell, not just re-layout an original reuse cell.

How can I solve this problem? Is it a bug on UICollectionView from apple or something wrong I did?

Whole code:

// 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)
}

Answer 1

That's not a bug. That's how you reload a cell for a given index path. If you only want to update the layout you can also try

[self.collectionView.collectionViewLayout invalidateLayout]

instead of

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

and then return the proper size in the delegate method.

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

}

I would also highly suggest you to cache your image sizes so that you can use that for the next time, instead of downloading/calculating over an over...