How to Add Caching to Asyncimage

downloading and caching images from url asynchronously

I know you found your problem and it was unrelated to the above code, yet I still have an observation. Specifically, your asynchronous requests will carry on, even if the cell (and therefore the image view) have been subsequently reused for another index path. This results in two problems:

1. If you quickly scroll to the 100th row, you are going to have to wait for the images for the first 99 rows to be retrieved before you see the images for the visible cells. This can result in really long delays before images start popping in.

2. If that cell for the 100th row was reused several times (e.g. for row 0, for row 9, for row 18, etc.), you may see the image appear to flicker from one image to the next until you get to the image retrieval for the 100th row.

Now, you might not immediately notice either of these are problems because they will only manifest themselves when the image retrieval has a hard time keeping up with the user's scrolling (the combination of slow network and fast scrolling). As an aside, you should always test your app using the network link conditioner, which can simulate poor connections, which makes it easier to manifest these bugs.

Anyway, the solution is to keep track of (a) the current URLSessionTask associated with the last request; and (b) the current URL being requested. You can then (a) when starting a new request, make sure to cancel any prior request; and (b) when updating the image view, make sure the URL associated with the image matches what the current URL is.

The trick, though, is when writing an extension, you cannot just add new stored properties. So you have to use the associated object API to associate these two new stored values with the UIImageView object. I personally wrap this associated value API with a computed property, so that the code for retrieving the images does not get too buried with this sort of stuff. Anyway, that yields:

extension UIImageView {
    private static var taskKey = 0
    private static var urlKey = 0

    private var currentTask: URLSessionTask? {
        get { objc_getAssociatedObject(self, &UIImageView.taskKey) as? URLSessionTask }
        set { objc_setAssociatedObject(self, &UIImageView.taskKey, newValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC) }
    }

    private var currentURL: URL? {
        get { objc_getAssociatedObject(self, &UIImageView.urlKey) as? URL }
        set { objc_setAssociatedObject(self, &UIImageView.urlKey, newValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC) }
    }

    func loadImageAsync(with urlString: String?, placeholder: UIImage? = nil) {
        // cancel prior task, if any

        weak var oldTask = currentTask
        currentTask = nil
        oldTask?.cancel()

        // reset image view’s image

        self.image = placeholder

        // allow supplying of `nil` to remove old image and then return immediately

        guard let urlString = urlString else { return }

        // check cache

        if let cachedImage = ImageCache.shared.image(forKey: urlString) {
            self.image = cachedImage
            return
        }

        // download

        let url = URL(string: urlString)!
        currentURL = url
        let task = URLSession.shared.dataTask(with: url) { [weak self] data, response, error in
            self?.currentTask = nil

            // error handling

            if let error = error {
                // don't bother reporting cancelation errors

                if (error as? URLError)?.code == .cancelled {
                    return
                }

                print(error)
                return
            }

            guard let data = data, let downloadedImage = UIImage(data: data) else {
                print("unable to extract image")
                return
            }

            ImageCache.shared.save(image: downloadedImage, forKey: urlString)

            if url == self?.currentURL {
                DispatchQueue.main.async {
                    self?.image = downloadedImage
                }
            }
        }

        // save and start new task

        currentTask = task
        task.resume()
    }
}

Also, note that you were referencing some imageCache variable (a global?). I would suggest an image cache singleton, which, in addition to offering the basic caching mechanism, also observes memory warnings and purges itself in memory pressure situations:

class ImageCache {
    private let cache = NSCache<NSString, UIImage>()
    private var observer: NSObjectProtocol?

    static let shared = ImageCache()

    private init() {
        // make sure to purge cache on memory pressure

        observer = NotificationCenter.default.addObserver(
            forName: UIApplication.didReceiveMemoryWarningNotification,
            object: nil,
            queue: nil
        ) { [weak self] notification in
            self?.cache.removeAllObjects()
        }
    }

    deinit {
        NotificationCenter.default.removeObserver(observer!)
    }

    func image(forKey key: String) -> UIImage? {
        return cache.object(forKey: key as NSString)
    }

    func save(image: UIImage, forKey key: String) {
        cache.setObject(image, forKey: key as NSString)
    }
}

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A bigger, more architectural, observation: One really should decouple the image retrieval from the image view. Imagine you have a table where you have a dozen cells using the same image. Do you really want to retrieve the same image a dozen times just because the second image view scrolled into view before the first one finished its retrieval? No.

Also, what if you wanted to retrieve the image outside of the context of an image view? Perhaps a button? Or perhaps for some other reason, such as to download images to store in the user’s photos library. There are tons of possible image interactions above and beyond image views.

Bottom line, fetching images is not a method of an image view, but rather a generalized mechanism of which an image view would like to avail itself. An asynchronous image retrieval/caching mechanism should generally be incorporated in a separate “image manager” object. It can then detect redundant requests and be used from contexts other than an image view.

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As you can see, the asynchronous retrieval and caching is starting to get a little more complicated, and this is why we generally advise considering established asynchronous image retrieval mechanisms like AlamofireImage or Kingfisher or SDWebImage. These guys have spent a lot of time tackling the above issues, and others, and are reasonably robust. But if you are going to “roll your own,” I would suggest something like the above at a bare minimum.

Using VStack with AsyncImage inside LazyVStack causes images to reload on scrolling

I fully agree with you that this is a weird bug. I would guess that the reason it's happening has something to do with the way LazyVGrid chooses to layout views, and that using a VStack here gives it the impression there is more than one view to show. This is a poor job on Apple's part, but this is how I solved it: just put the VStacks internal to the AsyncImage. I'm not entirely sure what the original error is, but I do know that this fixes it.

struct MyTestView: View {
    let url = URL(string: "https://picsum.photos/200/300")
    let columns: [GridItem] = [.init(.fixed(110)),.init(.fixed(110)),.init(.fixed(110))]

    var body: some View {
        ScrollView {
            LazyVGrid(columns: columns) {
                ForEach(0..<20) { i in
                    AsyncImage(url: url) { image in
                        VStack {
                            image
                                .resizable()
                                .aspectRatio(contentMode: .fit)
                            
                            Text("label \(i)")
                        }
                    } placeholder: {
                        VStack {
                            Image(systemName: "photo")
                                .imageScale(.large)
                                .frame(width: 110, height: 110)
                            
                            Text("image broken")
                        }
                    }
                }
            }
        }
    }
}

Coil image caching

Try disabling the cache headers support.

val imageLoader = ImageLoader.Builder(context)
        .respectCacheHeaders(false)
        .build()
Coil.setImageLoader(imageLoader)

Coil image caching not working with Jetpack Compose

I used logger(DebugLogger()) in ImageLoader to figure out what was going on and I found out that app was running into HTTP 504 error when Coil was trying to load image offline. So I added .respectCacheHeaders(false) to the ImageLoader. That seemed to do the trick for me.

Hope this helps someone else running into similar problem.

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