The run loop

June 12, 2010 Adam Milligan

When I started writing Objective-C for iPhone and iPad projects I started to hear about these mystical and all-powerful run loops objects. Everything seems to relate to run loops, all the class references talk about how one object or another depends on run loops, the most basic iPhone application documentation even talks about how the device starts and maintains the main run loop for you because it’s so important. However, I found few straightforward descriptions of what a run loop is or how it works.

So, I read Apple’s Threading Programming Guide. I read about timers and ports and sources, about the starting and stopping and timing out of run loops. After this reading, and a bit of thought, I realized that all the sound and fury surrounding run loops blinded me to the fact that they are actually very, very simple. To wit, run loops are a mechanism for single-threaded asynchronous programming. Sound familiar? Sounds to me a lot like JavaScript.

A lot was made during the recent WWDC of the difference between synchronous and asynchronous network programming, so as an example let’s consider an AJAX request in JavaScript. To get some information from a server you’d send off a request in one function, passing in another function (or functions) for the system to call when it receives a response.

makeARequest({onSuccess: function(response) {
  // do some success stuff.

Your initial function exits, and while the request is making its merry way through the series of tubes between the client and your server the system is happily going about its business. But, it has its digital ear to the ground for your response, and when that comes back it calls your function. Voilà, asynchronicity.

That’s a run loop. It has event sources, like some form of network socket on which it listens for your server’s response; it has timers, in the form of setTimeout() and clearTimeout(). The Apple Core Foundation run loops take a fair bit more setup to use — the network request performed by those three lines of JS would involve NSURLConnection, NSURLRequest, some form of delegate object, etc. — but it’s exactly the same idea. It’s waiting for something to happen while you take a nap; when that something happens it tells you about it.

There is one extremely significant difference between the JavaScript implementation and the Core Foundation implementation, at least in my mind: testing. In JavaScript this type of asynchronous code isn’t a run loop, it’s just JavaScript. As such, when you run your tests your code does everything you’d expect it to do; it’s the same interpreter running the tests as the production code, after all. So, when your test setup reaches into your AJAX library and stubs out the part that actually talks to the network everything continues to hum along. If you want, you step behind the curtain and send fake responses back through the AJAX object.

In comparison, a Core Foundation run loop is a special construct that the system sets up for you. When you run an app on your iPhone the device takes care of all the setup, care, and feeding of the main run loop. So, in tests, you if you want to run code that depends on a run loop you have to start the run loop yourself. Worse, starting a run loop blocks the execution of your tests until you stop the run loop. So, if you stub out the network code that actually talks to the network your code may sit and wait forever for a response that can’t possibly come. If you set up your code to return a fake response before starting the run loop your code will receive that response, but the run loop will continue to run, potentially blocking your test code forever.

This is, in my opinion, a significant problem for testing Cocoa projects. Asynchronous programming using run loops is simpler than mucking about with threads, and largely endorsed by Apple via their frameworks. But, there’s no reasonably simple way to test code written this way. You could set up a test run loop with test-specific run loop sources or timeouts to prevent it from blocking forever, but that seems immensely complex when compared to the JavaScript implementation. I would very much like to see some support from Apple in the form of a stub run loop for test environments that provides mechanisms for passing fake data to run loop sources, freezing and incrementing time to test timers and delayed invocations, etc.

I’m interested to hear other ideas you’ve had for how to get around this, or test mechanisms you’d like to see from Apple.

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