Making a constructor swift that relies on network (making UIImage from url)

Question

Based on https://stackoverflow.com/a/27712427/4166655, I found that I can create a UIImage with this code

func downloadImage(from url: URL) {
    print("Download Started")
    getData(from: url) { data, response, error in
        guard let data = data, error == nil else { return }
        print(response?.suggestedFilename ?? url.lastPathComponent)
        print("Download Finished")
        DispatchQueue.main.async() {
            self.imageView.image = UIImage(data: data)
        }
    }
}

I saw many UIImageView extensions incorporating this code, but I wondered if I could make this work as a direct init for UIImage.

I came up with this —

extension UIImage {
    convenience init?(url: String) {
        guard let urlObj = URL(string: url) else { return nil }
        self.init(url: urlObj)
    }

    convenience init?(url: URL) {
        let semaphore = DispatchSemaphore(value: 0)
        var imageData: Data? = nil

        URLSession.shared.dataTask(with: url) { (data, response, err) in
            // breakpoint put here
            guard let data = data else { return }
            imageData = data
            semaphore.signal()
        }
        semaphore.wait()

        if let data = imageData {
            self.init(data: data)
        } else {
            return nil
        }
    }
}

But this just times out, and my breakpoint (noted in the code) never triggers.

What's wrong with this approach?

Waiting on a completion handler like this defeats the entire point of the completion handler: you're blocking the main thread, and freezing the UI, to wait for a web request to finish (which it may never do). — Alexander, Feb 13 '20 at 23:05
Fair point - at this point I'm just curious why this doesn't even work — Ronak Shah, Feb 13 '20 at 23:07

score 2 · Accepted Answer · answered Feb 13 '20 at 23:22

my breakpoint (noted in the code) never triggers

Because you never actually started the URLSession data task (by telling it to resume):

    URLSession.shared.dataTask(with: url) { (data, response, err) in
        // whatever
    }.resume() // <-- this is what's missing

However, your entire approach here is wrong:

You cannot "pause" during an initializer and "wait" for some asynchronous operation to complete while blocking the main queue. You need to return the initialized instance right now.
If this were not an initializer and were happening on a background queue, the best way to "wait" for another background operation to finish would be with a DispatchQueue.

Making a constructor swift that relies on network (making UIImage from url)

1 Answers1