For the better part of 2 years, RoboVM has been my life. Today, RoboVM announced its discontinuation, abruptly ending that chapter. The amount of things I learned during that short period has easily overshadowed anything I learned during previous jobs, both on the technical and non-technical side. I’ll deeply miss this daily experience.
So, what does this mean for libGDX? There’s good and bad news. Let’s get the bad news out of the way.
RoboVM is dead, what now?
Quite a few libGDX folks have free RoboVM license keys to deploy their libGDX app on iOS. These license keys will continue to work until the 17th of April 2017. However, there will be no further updates to RoboVM, be it new features or bug fixes. The same is true for all the RoboPods the community came to rely on. Moreover, new sign-ups for free RoboVM Indie licenses will also no longer be fulfilled.
In short, if you have a game using RoboVM already, you can continue using RoboVM until the license expiration date. If you start a new game, RoboVM is not an option. In either case, you should move to an alternative as soon as possible.
What are the alternatives?
There are many alternatives to RoboVM, however, none comes quite close to what RoboVM offered. Let’s examine all the available options. We’ll look at them from various viewpoints: tooling support (IDEs, Maven, Gradle), bindings support, binary size, performance and Bitcode compatibility.
Before we dive in, let me elaborate on the Bitcode compatibility, as it is probably the most important feature. Apple recently introduced a requirement that prohibits submission of apps in raw machine code form for watchOS and tvOS. Instead, Apple wants you to submit your apps as Bitcode. Bitcode is a (not quite) machine independent intermediate language from which Apple generates actual machine code on app submission. Any solution that has no clear path to Bitcode is hence at a big disadvantage.
Mobile OpenJDK 9
Oracle recently announced OpenJDK 9 availability for both iOS and Android. While this seems like the most sensible option, it has a metric ton of down-sides as of this writing.
- Tooling: No integration with IDEs, Maven or Gradle
- Bindings: No bindings for Objective-C/Swift APIs. No easy consumption of Objective-C/Swift APIs, everything goes through JNI
- Binary size: The generated binaries include all of OpenJDK, both native code and the class libraries. Apps are huge
- Performance: It uses Zero VM on iOS, which is essentially an interpreter. Say goodbye to performance
- Bitcode: Uncertainty about Bitcode compatibility, the interpreter relies on some assembler and signals, both of which don’t work in Bitcode land
None of this is likely to change in the short- to midterm future, so OpenJDK 9 is out.
A while ago, Google open-sourced J2Objc. It falls under the category of “transpiler” meaning it translates one high-level source (Java) to another one (Objective-C). Google is successfully using it for a few of their apps, like Google Inbox. Conceptually, it’s very close to GWT, which we employ to get your libGDX games running on the web.
- Tooling: Essentially a command line tool. There exists a Gradle plugin that takes away some of the pain, but no other tooling is provided
- Bindings: J2ObjC is mean to allow you to share business logic while you write your UI in ObjC/Swift. As such, there are no bindings. Also notable is the lack of support for most of the Java SE APIs. This makes including 3rd party Java libraries a no-go.
- Binary size: very good, mostly due to the fact that it doesn’t have to ship a proper managed environment or Java SE APIs.
- Performance: very good, it’s essentially Objective-C
- Bitcode: Out-of-the-box, as things are compiled to Objective-C
With a lack of support for Java SE level APIs, the use of reference counting instead of GC, and the fact it only works for Java source code, J2ObjC is not fit for use with libGDX. One GWT based backend is enough
Avian is a great piece of OSS tech. JMonkeyEngine chose Avian as their driver on iOS a while ago, so there is some precedent. Avian let’s you chose whether you want to use the Android class library or OpenJDK’s class library, definitely a plus. However, Avian suffers from a few minor and one major shortcomingy.
- Tooling: No integration with IDEs, Maven or Gradle, save for the integration JMonkeyEngine did with NetBeans
- Bindings: No bindings for platform APIs, no easy consumption of Platform APIs, JNI hell it is
- Binary size: Not terrible when using Proguard
- Performance: The AOT mode essentially takes the JIT mode output and bakes it into an executable. The JIT is rather primitive, and performance isn’t where it should be
- Bitcode: No path forward to be compatible with Bitcode, as the AOT directly generates machine code. This is the big one
Avian has always been a favorite of mine, as the engineering behind it is top notch. However, using it in production is just a major pita, and the missing path to Bitcode rules it out completely.
Xamarin + IKVM
Back in the olden days, Michael Bayne of PlayN fame took it upon himself to create a somewhat functional path for Java bytecode to live on iOS. The solution consisted of compiling Java bytecode to .NET CIL via IKVM and shipping a heavily cut down version of the OpenJDK class library. I added support for JNI so we could also use it with libGDX. Suffice it to say, it was a Frankensteinian monster of epic proportions. But it worked. Until Xamarin released a new version, in which case non-trivial bug fixes had to be made. Let’s give this a fair shake:
- Tooling: No integration with IDEs, Maven or Gradle. Integrates with Xamarin Studio and Visual Studio though, including debugging. Otherwise it’s a bunch of shell scripts.
- Bindings: Excellent, you are able to call into pretty much all native APIs through the IKVM C#/Java bridge
- Binary size: Good, were it not for the fact that you also need to ship a huge Java class library next to the Xamarin runtime.
- Performance: Pretty good, although IKVM introduces some suboptimal paths, which makes this solution slower than RoboVM
- Bitcode: clear path forward, as Xamarin is based on LLVM
Note that the IKVM fork Michael and I worked on is mostly defunct. The cut down class library is also a huge problem, with things like java.net completely missing. Were it not for the last alternative in this list, this solution would probably win.
Intel Multi-OS Engine
Formerly known as Migeran, Multi-OS Engine is a pretty nice piece of tech that let’s you run Java bytecode on iOS. Under the hood, Multi-OS Engine is powered by ART, Android’s virtual machine, which has an AOT compilation mode Multi-OS Engine exploits.
- Tooling: Integrates with Android Studio, IntelliJ IDEA and Gradle. Used to have support for Eclipse, but it seems that’s no longer available. Also let’s you build and deploy from a Windows machine (using a Mac as the build slave).
- Bindings: Provides a custom bridge called Nat/J, which is quite similar to RoboVM’s Bro, albeit quite a bit more cumbersome. It appears that all native APIs are accessible from Java via Nat/J, and 3rd party libraries can be bound (semi-)automatically.
- Binary size: OK but not great. Bigger than comparable RoboVM binaries.
- Performance: Good, faster than RoboVM in some workloads, slower in others.
- Bitcode: Currently unsupported, but apparently Intel is working on it.
Multi-OS Engine comes closest to RoboVM along all these axes.
What is the future of libGDX on iOS?
Tomski has already written a new libGDX backend for Multi-OS engine. Here’s the roadmap for libGDX:
- Clean-up the Multi-OS backend, integrate it with the libGDX Setup UI, and update the documentation. We are at a 95% status with this, i hope to have the code drop sometime this weekend
- Release a new libGDX version next week, that will make Multi-OS engine the default iOS backend for newly created libGDX projects
- Begin creating bindings for popular 3rd party iOS libraries. This is where we hope the community can jump in
- Keep the RoboVM backend around until the licenses expire (April 17 2017). We’ll try our best to fix any bugs in the backend, however we won’t be able to fix bugs in RoboVM itself
The roadmap of your existing apps should look something like this:
- Keep publishing updates using RoboVM 1.14.0 and the corresponding backend
- Immediately start adding a Multi-OS engine based version of your app for iOS
- Test it and report bugs on our issue tracker
- Switch over to Multi-OS Engine as soon as possible
There will be some rough edges with the new Multi-OS Engine backend. We are in contact with Intel, who have been very reactive in fixing issues we identified while creating the new iOS backend. Please help test the new backend and report issues, so Intel is able to fix problems on their end.
Multi-OS Engine is free to use, but not OSS. Before you guys start screaming for my head again, I’d like you to read the above analysis carefully one more time. None of the OSS options are even close to production ready. I will however happily merge any and all backend based on those OSS options.
I’d be grateful if everyone funnels their emotional energy created by this announcement into helping out with the new backend, e.g. testing, bindings, reporting bugs etc. I believe this is a much better use of everyone’s time. If you feel the need to vent, I understand. But please don’t expect me to vent with you. The winding down of RoboVM is already emotionally taxing enough for me. My goal is it to keep things running smoothly for everyone, I have no intention to waste time on a screaming contest.