swift3 - Swift 3: Display Image from URL

Question

In Swift 3, I am trying to capture an image from the internet, and have these lines of code:

var catPictureURL = NSURL(fileURLWithPath: "http://i.imgur.com/w5rkSIj.jpg")
var catPictureData = NSData(contentsOf: catPictureURL as URL) // nil
var catPicture = UIImage(data: catPictureData as! Data)

What am I doing wrong here?

Answer 1

There's a few things with your code as it stands:

1. You are using a lot of casting, which is not needed.
2. You are treating your URL as a local file URL, which is not the case.
3. You are never downloading the URL to be used by your image.

The first thing we are going to do is to declare your variable as let, as we are not going to modify it later.

let catPictureURL = URL(string: "http://i.imgur.com/w5rkSIj.jpg")! // We can force unwrap because we are 100% certain the constructor will not return nil in this case.

Then we need to download the contents of that URL. We can do this with the URLSession object. When the completion handler is called, we will have a UIImage downloaded from the web.

// Creating a session object with the default configuration.
// You can read more about it here https://developer.apple.com/reference/foundation/urlsessionconfiguration
let session = URLSession(configuration: .default)

// Define a download task. The download task will download the contents of the URL as a Data object and then you can do what you wish with that data.
let downloadPicTask = session.dataTask(with: catPictureURL) { (data, response, error) in
    // The download has finished.
    if let e = error {
        print("Error downloading cat picture: (e)")
    } else {
        // No errors found.
        // It would be weird if we didn't have a response, so check for that too.
        if let res = response as? HTTPURLResponse {
            print("Downloaded cat picture with response code (res.statusCode)")
            if let imageData = data {
                // Finally convert that Data into an image and do what you wish with it.
                let image = UIImage(data: imageData)
                // Do something with your image.
            } else {
                print("Couldn't get image: Image is nil")
            }
        } else {
            print("Couldn't get response code for some reason")
        }
    }
}

Finally you need to call resume on the download task, otherwise your task will never start:

downloadPicTask.resume().

All this code may look a bit intimidating at first, but the URLSession APIs are block based so they can work asynchronously - If you block your UI thread for a few seconds, the OS will kill your app.

Your full code should look like this:

let catPictureURL = URL(string: "http://i.imgur.com/w5rkSIj.jpg")!

// Creating a session object with the default configuration.
// You can read more about it here https://developer.apple.com/reference/foundation/urlsessionconfiguration
let session = URLSession(configuration: .default)

// Define a download task. The download task will download the contents of the URL as a Data object and then you can do what you wish with that data.
let downloadPicTask = session.dataTask(with: catPictureURL) { (data, response, error) in
    // The download has finished.
    if let e = error {
        print("Error downloading cat picture: (e)")
    } else {
        // No errors found.
        // It would be weird if we didn't have a response, so check for that too.
        if let res = response as? HTTPURLResponse {
            print("Downloaded cat picture with response code (res.statusCode)")
            if let imageData = data {
                // Finally convert that Data into an image and do what you wish with it.
                let image = UIImage(data: imageData)
                // Do something with your image.
            } else {
                print("Couldn't get image: Image is nil")
            }
        } else {
            print("Couldn't get response code for some reason")
        }
    }
}

downloadPicTask.resume()