Nim Hello JNI Callback is an Android sample that uses JNI to call C code from a Android Java Activity, with the C code generated from the Nim computer language instead of directly from C source files; with the added ability to call Java class static and instance methods from native code.
This has been adapted from the sample hello-jnicallback android project
The changes have been to add the required .nim files, adding nim biuld script files (.cmd for windows and sh for linux), changing to using ndk-build instead of CMake, and thus adding an Android.mk file to do the native code building.
Notable discoveries were that the nimbase.h file needs to be made available for the native build process to work and that NimMain needs to be called before anything can be done with Nim strings as the Android and Nim Garbage Collection (GC) and memory management systems otherwise conflict (unless some alternate GC is used, but the best of those are still in development and not really recommended; using --gc:none will cause memory leaks if used extensively without a great deal of extra complexity in creating cstring's); these issues are documented various places in the Nim documentation and in the Nim forum, but this is the only place that everything is brought together. As well, since the Nim produced .c files are hard coded for cpu architecture, we use separate directories for each cpu and make the build system select the correct one depending on the currently building ANDROID_ABI architecture. Further, the slickest way to run the NimMain proc is to run it in a JNI_OnLoad routine so it will automatically be run just once when the dynamic library file is loaded.
The build system used here where Nim is used to (-c) compile to .c files only and letting the build system turn those files into shared library files seems slightly simpler than using Nim to compile all the way to shared dynamic libraries to be just included by the build system as third party libraries, although doing that would also work and would eliminate the need for the nimbase.h file in the build files system. However, the second method of generating the .so files from Nim would require locating the version of the Android NDK to use, which is done automatically for us by Gradle.
- Android Studio 2.2+ with NDK bundle.
- Install Nim for your platform.
- Download Android Studio
- Install Android Studio as per the instructions.
- Launch Android Studio.
- Open Tools/SDK Manager and be sure the Android 10.0 (Q) SDK Platform, 29.0.2 SDK Build tools and NDK (Side by side) are installed.
- Download this repository as a .zip file and extract it to a location of your choice.
- Open that project location with Android Studio to open the project.
- From the Android Studio terminal run "buildnim.cmd" (for Windows) or "buildnim.sh" (for Linux/OSX) to compile the Nim files to .c files.
- Click Tools/Android/Sync Project with Gradle Files.
- Click Run/Run 'app'.
- Alternatively, you can use "Build/Generate Signed Bundle/APK..." to generate an APK file that can be transfered to an Android phone and "side-loaded".
- The above involves generating a signing store and key (one time), which can be done almost automatically with Android Studio.
- You could store the signing store key file in this directory.
There is a prebuilt signed (with a generated signing key not provided) APK installation file available in the app/release directory that can by installed (side-loaded) on Android smartphones if one trusts that it's contents reflect the source provided here; if one doesn't trust it, generate a new one as described by the alternate process described above.
Copyright (C) 2020 W. Gordon Goodsman (GordonBGood). All rights reserved.
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