Current status in NDK usage?

[Aditya Vaidyam]

This isn't so much an issue as it is a usage question:

I have a small Swift framework that I'd rather not port into Java. It's only using NSObject for the setValue:forKey: magic, and that could be easily replaced. If it's written in pure Swift, how would I interface it from the Java side of things?

[zhuowei]

Currently @ephemer is working on https://github.com/SwiftAndroid/swift-jni, which allows access to Java JNI. Then, one can write wrapper functions that convert JNI arguments to/from native Swift datatypes, and use them as native methods from Java. A very simple proof of concept can be found at https://github.com/SwiftAndroid/swift-android-samples/blob/master/swifthello/src/main/swift/Sources/main.swift

I would suggest that you also look at other projects for Swift on Android if you don't need full compatibility with the reference toolchain, such as the (more mature) RemObjects Silver.

[Aditya Vaidyam]

The problem is that I do need compatibility with the reference toolchain. I'll take a look at the other projects; thanks for the info!

[Geordie J]

What do you mean exactly by compatibility?

@zhuowei has made a gradle plugin that will compile your swift code alongside other native code. Personally I've just used swiftc-android to build shared libraries which can be loaded / linked against by any other native come you compile (regardless of toolchain)

Either way you'll very likely have to deal with the JNI/NDK, unless you use RemObjects Silver or the like (which may or may not have its own issues- I don't know how it works under the hood, I did read some question marks re: Arrays being passed by reference rather than by value, but I suspect the actual issues there will be more along the lines of interfacing with other native / Java code down the line)

[Aditya Vaidyam]

@ephemer @zhuowei So if I have a pure Swift project, understandably I would end up dealing with the JNI or NDK (which is C++, so how is Swift doing that?). Is there a way for me to expose all the library functions/classes/structs to the Java side of things and call them directly?

[Geordie J]

@avaidyam, you expose certain top-level (non-class) functions to Java, currently using some undocumented (but very easy to use) compiler features. In a nutshell, that is what the JNI does.

Maybe I'm missing something but from what I can see the JNI and NDK have C (in addition to C++) APIs, which is no problem for Swift. What I've done is wrap those C APIs to make them look closer to their C++ equivalents and more idiomatically "Swift-like" in their usage.

[Aditya Vaidyam]

@ephemer Got it, thanks! 👍