iPhone UIActivityIndicatorView not starting or stopping

Question

When I call startAnimating on a UIActivityIndicatorView, it doesn\'t start. Why is this?

[This is a blog-style self-answered question. The solution below works for

Answer 1

If needed, swift 3 version:

func doSomething() {
    activityIndicator.startAnimating()
    DispatchQueue.global(qos: .background).async {
        //do some processing intensive stuff
        DispatchQueue.main.async {
            self.activityIndicator.stopAnimating()
        }
    }
}