Data contentsOf yields nil, but the URL is valid - ios

Question

I am using this code
let url = URL(string: "http://image.tmdb.org/t/p/w185" + movie.poster_path!) // https://www.themoviedb.org/talk/568e3711c3a36858fc002384
print(url!)
DispatchQueue.global().async {
let data = try? Data(contentsOf: url!) //make sure your image in this url does exist, otherwise unwrap in a if let check / try-catch
DispatchQueue.main.async {
self?.movieImage.image = UIImage(data: data!)
}
}
from this stack overflow post. I have a URL with an image on it, I would like to use that URL to bring the image into my app and have it show up in a
#IBOutlet weak var movieImage: UIImageView!
but for some reason, I am getting an error saying that data is nil. Why would data be nil if the URL is valid? Is this an issue with the contentsOf function or am I doing something wrong here?

Answer 1

If you try changing your URL declaration to be: let url = URL(string: "http://image.tmdb.org/t/p/w185//nBNZadXqJSdt05SHLqgT0HuC5Gm.jpg") it works as expected. So perhaps you are not assembling the URL correctly?
I would print whatever URL you're creating and try visiting the website to see if it is actually correct

Answer 2

I’d suggest not using try? (which discards any meaningful error data) and instead use try wrapped in a do-catch block, and in the catch block, examine what the error is. Right now, you’re flying blind.
Or, better, use URLSession.shared.dataTask(with:) and look at the error in the completion handler.
You asked:
... but why is this such a bad thing [to use Data(contentsOf:)] if it is the background thread?
Yes, by dispatching this to a global queue you’ve mitigated the “don’t block the main thread” problem. But Data(contentsOf:) doesn’t provide much diagnostic information about why it failed. Also, it ties up one of the very limited number of worker threads that GCD draws upon. If you exhaust the worker thread pool, then GCD won’t be able to do anything else until it’s freed up. Using URLSession offers the chance to do more meaningful diagnostics and avoids blocking GCD worker threads.
So, I would suggest removing all of those ! forced unwrapped operators and not using Data(contentsOf:). Thus, I might suggest something like:
guard
let path = movie.poster_path,
let baseURL = URL(string: "http://image.tmdb.org/t/p/w185")
else {
print("problem getting path/URL")
return
}
let url = baseURL.appendingPathComponent(path)
URLSession.shared.dataTask(with: url) { [weak self] data, response, error in
guard
let data = data,
let response = response as? HTTPURLResponse,
error == nil
else {
print("network error:", error ?? "Unknown error")
return
}
guard 200..<300 ~= response.statusCode else {
print("invalid status code, expected 2xx, received", response.statusCode)
}
guard let image = UIImage(data: data) else {
print("Not valid image")
return
}
DispatchQueue.main.async {
self?.movieImage.image = image
}
}.resume()
Then, by displaying the error, if any, we’ll see what the problem was. FWIW, the above network request identifies three types of errors, which might be helpful for diagnostic purposes:
Basic network errors
HTTP errors
Content errors (not an image)