Interpret Objective C scripts at runtime on iPhone?

Question

Is there anyway to load an objective c script at runtime, and run it against the classes/methods/objects/functions in the current iPhone app?

MAJOR NOTE: The major reason I'd like to do this is to allow me to rapidly prototype an application, and then, after I'm done a major prototyping phase, take the scripts I was writing and compile them at build time instead. I don't ever plan on shipping an app with an objective c interpreter in it.

The reason i ask is that I've been playing around with iPhone wax, a lua interpreter that can be embedded in an iPhone app, and it works very nicely, in the sense that any object/method/function that's publically available in your Objective C code is automatically bridged, and available in lua.

This allows you to rapidly prototype applications by simply making the core of your app be lua files that are in the users documents directory. Just reload the app, and you can test out changes to your lua files without needing to rebuild the app in XCode - a big time saver!

But, with Apples recent 3.1.3 SDK stuff, it got me thinking that the safest approach for doing this type of rapid prototypeing would be if you could use Objective C as the interpreted code... That way, worst case scenario, you could just compile it into your app before your release instead. I have heard that the lua source can be compiled to byte code, and linked in at build time, but I think the ultimate safe thing would be if the scripted source was in objective c, not lua. That way your source is always in objective c, regardless.

This leads me to wondering (i've searched, but come up with nothing) if there are any examples on how to embed an Objective C Interpreter in an iPhone app? This would allow you to rapidly prototype your app against the current classes that are built into your binary, and, when your about to deploy your app, instead of running the classes through the in app interpreter, you compile them in instead.

With the iPad and OS 4, bluetooth and virtual keyboards can work with iPhones and iPads... That's going to make this type of rapid prototyping something much more useful, at least for dev time. If you have an interpreter built into your app and have it on your iPad, for example, you can code against the interpreter while on the road, without XCode. And to me, the most useful way to get the source back to an "apple approved" state would be if the scripts were Objective C.

Answer 1

Objective-C is really just C with a runtime and some syntactic sugar. It's an inherently compiled language (I don't think there are any production-ready interpreters for C, although I might be wrong).

Xcode used to have a feature called ZeroLink to speed up compile time, but removed it in Xcode 3 because it caused too many bugs.

Answer 2

Well, there's a couple worthwhile points to bring up:

1. Why interpret Objective-C code when you can compile it? I understand the "rapid prototyping" idea, but part of the reason to do that in, e.g., Lua, is because Lua is a much terser language than Objective-C. I don't know if interpreting Objective-C will have as much of a bonus.
2. If you want to have plugins or dynamically-loadable modules in your app, you can always compile them as a separate bundle and load them, using NSBundle or a similar mechanism.
3. All that said, I don't know of any Objective-C interpreters. You'd likely have to write your own. I'm not sure if it would violate Apple's guidelines or not: it'd still be Objective-C code, but I thought they had rules against interpreted code, too. (I suppose they never envisioned a hypothetical scenario in which Objective-C was interpreted, though.)

Answer 3

It's not exactly impossible, but it wouldn't be easy enough to be worth it. Objective-C isn't normally an interpreted language. This isn't insurmountable — interpreted vs. compiled is just an implementation choice in most cases. For example, Ruby is traditionally considered an interpreted scripting language, but MacRuby compiles it down to code just like Objective-C produces. So it would be possible to write an interpreter for Objective-C, but nobody has done this. You would have to write it yourself.

Also, the rules forbid interpreters other than Apple's Javascript interpreter. So far this hasn't been enforced on anyone, but if you're trying to be a very straight arrow, interpreted code is unfortunately out as well.

Answer 4

Take a look at the documentation for objc_msgSend() and other parts of the Objective-C Runtime Reference. You can essentially parse text and send it to the runtime.