Programming iOS 10

Appendix C. How Asynchronous Works

A very common beginner mistake is rooted in a failure to understand what it means for code to run asynchronously. It means that your code runs out of order. Consider:

func doSomeNetworking() {
    // ... prepare url ...
    let session = URLSession.shared 
    let task = session.downloadTask(with:url) { loc, resp, err in 
        // ... completion function body goes here ... 
    }
    task.resume() 
}

The method downloadTask(with:completionHandler:) calls its completion function asynchronously — because it calls it when the networking finishes, and networking takes time. The order in which the chunks of code run is the numerical order of the numbered lines:

The code before the call.

The call itself.

The code after the call, including the return from the surrounding function doSomeNetworking ...