Download and cache images in UITableViewCell

问题:

Note: Please no libraries. This is important for me to learn. Also, there are a variety of answers on this but none that I found solves the issue nicely. Please don't mark as duplicate. Thanks in advance!

The problem I have is that if you scroll really fast in the table, you will see old images and flickering.

• The solution from the questions I read is to cancel the URLSession data request. But I do not know how to do that at the correct place and time. There might be other solutions but not sure.

This is what I have so far:

Image cache class

class Cache {

    static let shared = Cache()

    private let cache = NSCache<NSString, UIImage>()
    var task = URLSessionDataTask()
    var session = URLSession.shared

    func imageFor(url: URL, completionHandler: @escaping (image: Image? error: Error?) -> Void) {
            if let imageInCache = self.cache.object(forKey: url.absoluteString as NSString)  {
                completionHandler(image: imageInCache, error: nil)
                return
            }

            self.task = self.session.dataTask(with: url) { data, response, error in

                if let error = error {
                    completionHandler(image: nil, error: Error)
                    return
                }

                let image = UIImage(data: data!)

                    self.cache.setObject(image, forKey: url.absoluteString as NSString)
                    completionHandler(image: image, error: nil)
                }

            self.task.resume()
    }
}

Usage

func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: "cell", for: indexPath)
    let myImage = images[indexPath.row]

    if let imageURL = URL(string: myImage.urlString) {
        photoImageView.setImage(from: imageURL)
    }
    return cell
}

Any thoughts?

回答1:

A couple of issues:

1. One possible source of flickering is that while you're updating the image asynchronously, you really want to clear the image view first, so you don't see images for prior row of reused/dequeued table view cell. Make sure to set the image view's image to nil before initiating the asynchronous image retrieval. Or, perhaps combine that with "placeholder" logic that you'll see in lots of UIImageView sync image retrieval categories.

   For example:

   extension UIImageView {
   
       func setImage(from url: URL, placeholder: UIImage? = nil) {
           image = placeholder               // use placeholder (or if `nil`, remove any old image, before initiating asynchronous retrieval
   
           ImageCache.shared.image(for: url) { [weak self] result in
               switch result {
               case .success(let image):
                   self?.image = image
   
               case .failure:
                   break
               }
           }
       }
   }
   

2. The other issue is that if you scroll very quickly, the reused image view may have an old image retrieval request still in progress. You really should, when you call your UIImageView category's async retrieval method, you should cancel and prior request associated with that cell.

   The trick here is that if you're doing this in a UIImageView extension, you can't just create new stored property to keep track of the old request. So you'd often use "associated values" to keep track of prior requests.

回答2:

Swift 3:

Flickering can be avoided by this way:

Use the following code in public func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell

cell.photoImageView.image = nil //or keep any placeholder here
cell.tag = indexPath.row
let task = URLSession.shared.dataTask(with: imageURL!) { data, response, error in
    guard let data = data, error == nil else { return }

    DispatchQueue.main.async() {
        if cell.tag == indexPath.row{
            cell.photoImageView.image = UIImage(data: data) 
        }
    }
}
task.resume()

By checking cell.tag == indexPath.row, we are assuring that the imageview whose image we are changing, is the same row for which the image is meant to be. Hope it helps!