[CMake] CMake + Gradle for Android

Robert Dailey rcdailey.lists at gmail.com
Mon Aug 21 12:33:05 EDT 2017 

Basically, yes. We have this sort of structure:

<Root of git clone>/
    Applications/
        App1/
            build.gradle
            CMakeLists.txt
        App2/
            build.gradle
            CMakeLists.txt
        App3/
            build.gradle
            CMakeLists.txt
    CommonLib/
        build.gradle
        CMakeLists.txt
    CMakeLists.txt

The libs are defined as follows:

* CommonLib is a static library (java code builds into a library)
    * No dependencies of its own
* App1 is a shared library (java code builds into a library)
    * Dependencies (both java & native): CommonLib
* App2 is a shared library (java code builds into an APK)
   * Dependencies (both java & native): App1, CommonLib
* App3 is a shared library (java code builds into an APK)
   * Dependencies (both java & native): CommonLib

In all cases, CMake must be invoked starting at the root
CMakeLists.txt 1 time. Each target can be built from the same binary
directory after that. Previously with ANT, I was building all native
targets first, then moved libs to appropriate directories so that the
'ant' command would package the libs.

For gradle, I wanted to avoid redundantly specifying the root
directory in each leaf-level project directory. Using the example
above, the leaf-level directories in this case would be App1, App2,
App3, and CommonLib. However I think we only specify the native CMake
stuff for the java targets that actually output an APK (that would be
App2 and App3 only).

The ultimate goal is to specify stuff that doesn't change per
independent "module" of ours at the top level so it is transitive /
inherited. Then only specify the differences (e.g. the native CMake
target to build) in the leaf build gradle files. However you indicated
this isn't possible.



On Mon, Aug 21, 2017 at 11:11 AM, Jom O'Fisher <jomofisher at gmail.com> wrote:
> What you're doing already sounds correct. You can't directly specify
> CMakeLists.txt from the top-level build.gradle. Recommendation is that it
> should be specified from the build.gradle of the module of the APK. Is the
> issue that you have multiple APK modules that all reference the same CMake
> libraries?
>
> On Mon, Aug 21, 2017 at 9:00 AM, Robert Dailey <rcdailey.lists at gmail.com>
> wrote:
>>
>> Thanks this is very helpful. The other question I have is: Is there a
>> place to centrally specify the root CMakeLists.txt? Basically, I want
>> to specify the CMake root in 1 place, and have targets (defined
>> further down in subdirectories) that require APK packaging to specify
>> only the native target name that should be built & packaged.
>>
>> At the moment we specify the root CMakeLists.txt by walking up the
>> tree, paths like "../../../../CMakeLists.txt". I think this should be
>> put at the top-level build gradle file if possible. Is this doable at
>> the moment? What is the recommended setup?
>>
>> On Mon, Aug 21, 2017 at 9:37 AM, Jom O'Fisher <jomofisher at gmail.com>
>> wrote:
>> > 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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