Hi,
First of all thanks to Joe Groff for his insight, pull request, and guidance re
the @_cdecl attribute, which is important for what follows below.
I’ve been working on a JNI (Java Native Interface) integration for the Android
port of Swift (github.com/SwiftAndroid) with good results so far.
Java->Swift->Java returns and method calls / passing data in either direction
all seem fine, but require a lot of code mangling and weird constructs like
'env.memory.memory' that are very un-Swift-like. So I’m wondering how one might
go about wrapping the JNI API in some kind of automated way.
To provide some context, the Java VM calls a native (Swift) function, passing a
number of parameters, almost all of which are pointers or pointers to pointers
(hence memory.memory). All of the functions again take the pointer to itself as
the first parameter like this:
env.memory.memory.GetJavaVM(env, &_emptyJavaVMPointerToPointer)
Note the ‘env’ first parameter. Also these are almost all pointer operations,
which are inherently unsafe and become tiresome in Swift pretty quickly for
good reasons.
Questions:
Is there a way of wrapping env (which is of type
UnsafeMutablePointer<UnsafePointer<JNINativeInterface>>) to have some sane
calling standards like
let vm = swiftEnv.getJavaVM()
without manually writting a SwiftJNIEnv Class and wrapping each function
individually? I understand the OSX / iOS SDKs were automatically wrapped in a
way comparable to this.
If the best solution currently involves automation via .gyb files, is this
option available to the Swift Package Manager, or would it make more sense to
build a JNI module along with the stdlib for Android (in the same way Glibc is
built and bundled with the stdlib on Linux)?
As an aside: I understand the JNI works this way because C natively has no
concept of objects. When working with C++, Java is able to directly pass
objects to the native code that have much saner method signatures. Is there any
scope to accept an object in this way via a @convenction( c ) function or are
we stuck with this pointer pattern until C++ compatibility arrives (which is
presumably still a year or two away at least)?
Thanks
-Geordie
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