[CMake] CMake + Gradle for Android

[Robert Dailey]

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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