[CMake] CMake + Gradle for Android

[Jom O'Fisher]

Gradle does introspection on the CMake build to find .so targets and those
get packaged.
There is also a special case for stl/runtime .so files from the NDK.
Any additional .so files need to specified in build.gradle using jniDirs

On Mon, Aug 21, 2017 at 7:30 AM, Robert Dailey <rcdailey.lists at gmail.com>
wrote:

> How exactly does Gradle package *.so files in an APK? I know that ANT
> used to do this for any libs under "libs/<ABI>". Does Gradle do some
> introspection into CMake targets to see if outputs are *.so, and copy
> those to some location if needed? What about libraries like
> libgnustl_shared.so that come with the NDK? I'd like to know if any
> manual copy steps are needed in CMake to put outputs in proper
> locations for the APK build step. I had to do this when using ANT.
>
> On Mon, Aug 7, 2017 at 6:16 PM, Jom O'Fisher <jomofisher at gmail.com> wrote:
> > 1) There is a folder created for each ABI under the project module folder
> > (so unique per module per ABI)
> > 2) Gradle doesn't specify language level though you can choose to
> specify it
> > yourself from the build.gradle. This doc does a pretty good job of
> > explaining which variables are set by Gradle:
> > https://developer.android.com/ndk/guides/cmake.html#variables.
> > Philosophically, we try to set as little as we can get away with. In
> > particular, the section titled "Understanding the CMake build command"
> lays
> > out exactly what we set. You can also see the folders we specify (one per
> > module per ABI)
> > 3) Not sure I understand this.
> >
> > The other document worth taking a look at (if you haven't already) is:
> > https://developer.android.com/studio/projects/add-native-code.html
> >
> >
> > On Mon, Aug 7, 2017 at 3:35 PM, Robert Dailey <rcdailey.lists at gmail.com>
> > wrote:
> >>
> >> Thanks Jom
> >>
> >> Honestly, I prefer option 1 to work simply because that's how Google's
> >> officially supporting CMake. But it also has debugging which is the #1
> >> reason for me.
> >>
> >> However, I'd like to understand a lot more about how the integration
> >> really happens. For example, I have these questions:
> >>
> >> 1) How, internally, are CMake build directories managed? Do you
> >> generate 1 per unique android project? What about for each specific
> >> platform (x86, armeabi-v7a, etc)?
> >> 2) Last time I looked into CMake integration, things defined inside
> >> the CMake scripts were ignored because they are specified at the
> >> command line. Namely, all of those settings that are driven by the
> >> Gradle configuration (CXX language level was one in particular I
> >> think; I specify C++14 support via CMake, but I recall this being
> >> overridden from outside)?
> >> 3) How redundant is it to configure individual libraries via the
> >> gradle scripts? In my previous attempts, I wanted to define common
> >> stuff for CMake / native code at the root gradle or settings file, and
> >> only define the differences in the actual gradle build files for each
> >> corresponding Java target (like, defining the name of the native
> >> (shared library) target in Gradle, but the command line invocation, -D
> >> CMake settings, etc would all be common and defined at the root).
> >>
> >> The TLDR is, the closer we can stay to CMake's way of doing things and
> >> keep CMake-related settings self-contained to the CMake scripts
> >> themselves, the better. This also makes cross-platform easier (we
> >> build the native code in Windows, for example, so having settings
> >> specified in the gradle files do not carry over to other platforms.
> >> Namely, settings that are not platform specific like the C++ language
> >> level).
> >>
> >> If there's a detailed document / wiki I can read on the intrinsics of
> >> CMake integration in Gradle / Android Studio, I'd love to read it.
> >> Otherwise, I hope you won't mind if I pick your brain as questions
> >> come up. I think I'm going to try option 1 for now and see how it
> >> goes. It's just black box for me because unlike option 2, I have very
> >> little control over what happens after building the shared libraries,
> >> and to make up for that I need to really get a deep understanding of
> >> how it works so I can make sure I code my CMake scripts properly for
> >> not only Android, but my other platforms as well (non-Android
> >> platforms).
> >>
> >> Thanks again.
> >>
> >> On Mon, Aug 7, 2017 at 5:12 PM, Jom O'Fisher <jomofisher at gmail.com>
> wrote:
> >> > Either option can work fine. Disclosure: I work on Android Studio and
> >> > was
> >> > the one that added CMake support.
> >> >
> >> > Option (1) is the way it's designed to work and we're working toward
> >> > getting
> >> > rid of the need for the CMake fork. I can't really say when that will
> >> > happen
> >> > but if you can get away with an older CMake for now then I'd go this
> >> > way.
> >> > As you mentioned, option (1) will allow you to view your source file
> >> > structure in Android Studio, edit files, and debug using the built-in
> >> > debugging support.
> >> >
> >> > To get option (2) to work, you can use jniDirs setting to tell Android
> >> > Gradle where to pick up your built .so files (see
> >> >
> >> > https://stackoverflow.com/questions/21255125/how-can-i-
> add-so-files-to-an-android-library-project-using-gradle-0-7).
> >> > I'm not aware of any projects that use this approach but it should
> work
> >> > in
> >> > principal.
> >> >
> >> > I hope this helps,
> >> > Jomo
> >> >
> >> >
> >> > On Mon, Aug 7, 2017 at 11:09 AM, Robert Dailey
> >> > <rcdailey.lists at gmail.com>
> >> > wrote:
> >> >>
> >> >> Right now I have custom targets set to execute the "ant release"
> >> >> command after my native targets are built. Part of that command
> >> >> involves copying *.so files to the libs/armeabi-v7a directory so they
> >> >> get packaged in an APK.
> >> >>
> >> >> When switching to gradle, I have two options:
> >> >>
> >> >> 1. Gradle drives CMake: This means using Android Studio and being
> >> >> locked down to Google's fork of CMake which is a few major releases
> >> >> behind. I see that as a negative.
> >> >>
> >> >> 2. CMake drives Gradle: This would be the same or similar to what I'm
> >> >> already doing: The custom targets I have would execute gradle as a
> >> >> separate build step, instead of running ant commands. I'm not too
> >> >> familiar with Gradle, so I'm not sure how you tell it where your
> >> >> shared libraries are for the APK packaging steps.
> >> >>
> >> >> Which does everyone recommend? Is anyone using one of these setups
> >> >> successfully? The downside to option 2 is probably no on-device
> native
> >> >> debugging since Android Studio probably can't handle gradle projects
> >> >> without any external CMake builds set up.
> >> >>
> >> >> Would like some general direction & advice before I move away from
> >> >> ANT. Thanks in advance.
> >> >> --
> >> >>
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