How to download files with URLSession and downloadTask()

Swift version: 5.1

Paul Hudson @twostraws May 28th 2019

URLSession is designed to make network transfers as easy as possible, and a great example of that is its downloadTask()` method. This fetches the contents of a URL you specify, saves it to a local file, then calls a completion handler so you can manipulate the file – all in one.

To demonstrate this, here’s some code to download the source code to the apple.com homepage:

let url = URL(string: "https://www.apple.com")!

let task = URLSession.shared.downloadTask(with: url) { localURL, urlResponse, error in
    if let localURL = localURL {
        if let string = try? String(contentsOf: localURL) {
            print(string)
        }
    }
}

task.resume()

There are a few important things to note in there:

Your completion handler gets called with a local URL, which is where the data was saved locally. This is optional, so you need to unwrap it carefully.
If something went wrong – e.g. if the network was down – then you’ll get an error passed to you explaining what happened.
When you have created your download task you should call resume() on it to make it happen.
You don’t need to worry about storing the download task somewhere while it happens – it’s being tracked by the shared URLSession on your behalf.