How do I dispatch functions in Swift the right way?

Question

I\'ve kept trying but I just don\'t get it. I\'m rather new to programming so almost every new step is an experiment. Whereas I have no problems dispatching normal closures

Answer 1

Let's say you had some function like so:

func calculate(foo: String, bar: Int) -> Int {
    // slow calculations performed here

    return result
}

If you wanted to do that asynchronously, you could wrap it in something like this:

func calculate(foo: String, bar: Int, completionHandler: @escaping (Int) -> Void) {
    DispatchQueue.global().async {
        // slow calculations performed here

        completionHandler(result)
    }
}

Or, alternatively, if you want to ensure the completion handler is always called on the main queue, you could have this do that for you, too:

func calculate(foo: String, bar: Int, completionHandler: @escaping (Int) -> Void) {
    DispatchQueue.global().async {
        // slow calculations performed here

        DispatchQueue.main.async {           
            completionHandler(result)
        }
    }
}

For the work being performed in the background, you may use a different priority background queue, or your might use your own custom queue or your own operation queue. But those details aren't really material to the question at hand.

What is relevant is that this function, itself, doesn't return any value even though the underlying synchronous function does. Instead, this asynchronous rendition is passing the value back via the completionHandler closure. Thus, you would use it like so:

calculate(foo: "life", bar: 42) { result in
    // we can use the `result` here (e.g. update model or UI accordingly)

    print("the result is = \(result)")
}

// but don't try to use `result` here, because we get here immediately, before
// the above slow, asynchronous process is done

(FYI, all of the above examples are Swift 3. For Swift 2.3 rendition, see previous version of this answer.)