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iOS and JavaScript - for Real this Time!

Less talk, more action – Apple just approved two of my JavaScript games for the AppStore: Biolab Disaster and Drop. Both are free; go check them out. You can also play them in your browser here and here.

Both games are pretty simple (the source for Drop is only 300 odd lines long) and written with my JavaScript Game Engine Impact.

These are certainly not the first games written in JavaScript to be available in the AppStore. Tools like AppMobi, PhoneGap or Titanium make it easy to bundle some HTML pages and JavaScript together in an App and display them in a UIWebView, which is basically just a browser window (correction: Titanium doesn't use a UIWebView but instead has some native bindings). Games written with Impact already work okay-ish in the iPhone's browser and thus also in AppMobi and PhoneGap.

So what's so special about these two games now? They don't use PhoneGap or Titanium. They don't even use a UIWebView. Instead, they bypass the iPhone's browser altogether and use Apple's JavaScript interpreter (JavaScriptCore) directly. All graphics are rendered with OpenGL instead of in a browser window and all sound and music is played back with OpenAL instead of… well, having no sound at all.

What makes this possible is a compatibility layer that mimics the HTML5 Canvas and Audio APIs but is implemented with OpenGL and OpenAL behind the scenes. Think of it as a browser that can only display a Canvas element and play Audio elements, but does not render generic HTML pages. A browser perfectly suited for HTML5 games.

This means you can take your JavaScript games written for Impact and run them on iOS with perfect sound and touch input and way better drawing performance than with Mobile Safari. Now, to be clear, I only implemented a bare minimum of the Canvas API - just enough to be able to run Impact. The whole thing is still in a very experimental state.

If you have a license for Impact you will find the complete source code for this all on your download page. I also wrote some basic documentation to get you started. In theory, you don't have to know anything about Objective-C to use this, but at this stage some Objective-C knowledge will sure come in handy. Again, this is very experimental. Don't expect it to work at all.

If you've been following this blog for a while, you might remember a post from October 2010 where I attempted the exact same thing. Back then I used the JavaScriptCore library that is already available on iOS and used by Apple in Mobile Safari. The problem was, that this library is "private", meaning that Apple does not want you to use it. And since you can't publish anything in the AppStore that uses private libraries I abandoned the idea.

However I recently revisited the project, because there was still a chance to make this work: JavaScriptCore is a part of the open source WebKit project. Instead of using the private library that comes with iOS, you theoretically could compile your own version of this library and bundle it together with your App. Which is exactly what I did.

Since Apple does not provide any project files to compile JavaScriptCore for iOS (presumably to annoy us) and JavaScriptCore itself uses some of iOS' private APIs, compiling this beast into a static library - in an App Store compatible fashion - took me a few days.

I also had to make a small sacrifice: JavaScriptCore uses libicu to sort strings according to a unicode locale. Sadly, libicu is also private on iOS and bundling it is not an option because of its size. So I got rid of libicu completely. This means that only ASCII strings are now sorted correctly (e.g. the umlaut "Ä" will come after "Z", not after "A" as it should). Other than that, the JavaScript library should behave exactly as the private one that comes with iOS.

Also, the JavaScriptCore library bundled with iOSImpact does not use the JIT compiler (Nitro). You can't allocate executable memory on the iPhone and if Apple doesn't lift that restriction, there's nothing I can do about it. However, Apple recently made an exception for Mobile Safari – if they would make their JavaScriptCore API public, they probably could enable the JIT for everyone. That's a big if though; I don't see it happening, because Apple loves native code (Objective-C) and hates scripting languages.

I was afraid Apple would reject the two games for some obscure reason, but they didn't. Which leaves me to wonder why the JavaScriptCore library that comes with iOS is private in the first place. Bundling JavaScriptCore with your App adds about 2MB in size – not much, but I fail to see how this can be in Apple's interest.

Anyway, the performance of both games is pretty good. I still get some occasional slowdowns (~20fps) on my iPhone3GS in Biolab Disaster when there are too many particles on the screen, but it remains playable at all times. It's also nice to have perfectly working sound on iOS now - something even some desktop browsers still struggle with.

Wednesday, April 27th 2011
— Dominic Szablewski, @phoboslab