magnum/doc/platforms-android.dox

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              Vladimír Vondruš <mosra@centrum.cz>

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namespace Magnum {

/** @page platforms-android Android
@brief Building and deploying Android projects

@tableofcontents
@m_footernavigation

@todoc code coverage
@todoc static plugins

The following guide explains how to build Android projects using minimal
command-line tools, without Android Studio involved.

At the very least you need to have Android SDK and Android NDK installed.
Running console utilities and tests on the device don't need much more, in case
you want to develop actual applications, you need also Gradle and SDK platform +
SDK platform build tools for version of your choice. Gradle is able to download
all the dependencies on its own, however it's also possible to install system
packages for a cleaner setup.

@note On ArchLinux it's the `gradle` package and the following AUR packages,
    adapt the version numbers as necessary:
@note
    -   [android-sdk](https://aur.archlinux.org/packages/android-sdk/)
    -   [android-ndk](https://aur.archlinux.org/packages/android-ndk/)
    -   [android-sdk-build-tools](https://aur.archlinux.org/packages/android-sdk-build-tools/)
    -   [android-sdk-platform-tools](https://aur.archlinux.org/packages/android-sdk-platform-tools/)
    -   [android-platform-22](https://aur.archlinux.org/packages/android-platform-22/)
    -   [android-sdk-cmake](https://aur.archlinux.org/packages/android-sdk-cmake/)

Gradle requires Android SDK version of CMake, which is currently at version
3.6. See below for an experimental way to @ref platforms-android-system-cmake "use the system CMake"
instead.

@section platforms-android-console Building and running console applications

Android allows to run arbitrary console utilities and tests via ADB. Assuming
you have Magnum installed in the NDK path as described in @ref building-cross-android,
build your project simply as this (adapt version numbers and ABIs as needed):

@code{.sh}
mkdir build-android-arm64 && cd build-android-arm64
cmake .. \
    -DCMAKE_SYSTEM_NAME=Android \
    -DCMAKE_SYSTEM_VERSION=22 \
    -DCMAKE_ANDROID_ARCH_ABI=arm64-v8a \
    -DCMAKE_ANDROID_NDK_TOOLCHAIN_VERSION=clang \
    -DCMAKE_ANDROID_STL_TYPE=c++_static \
    -DCMAKE_BUILD_TYPE=Release
cmake --build .
@endcode

After that you can use ADB to upload your executable to the device and run it
there. The global temporary directory is `/data/local/tmp` and while the parent
directories often don't have permissions, it's possible to @cb{.sh} cd @ce into
it and create arbitrary files there. Assuming you built an executable in
`build-android-arm64/src/my-application`, the workflow would be like this:

@code{.sh}
adb push build-android-arm64/src/my-application /data/local/tmp
adb shell /data/local/tmp/my-application
@endcode

You can also use @cb{.sh} adb shell @ce to enter the device shell directly and
continue from there. Besides plain command-line apps it's also possible to
create an EGL context without any extra setup using
@ref Platform::WindowlessEglApplication. See also @ref GL::OpenGLTester for
information about OpenGL testing.

@section platforms-android-apps Building and installing graphics apps

Building of graphics applications is managed fully using Gradle, which also
builds your CMake project internally. It's possible to use other means such as
`ndk-build`, but CMake is the officially preferred way. The following guide
assumes you have Gradle installed in a system-wide location available in
@cb{.sh} $PATH @ce. See the [Gradle installation docs](https://docs.gradle.org/current/userguide/installation.html)
for more information, @ref platforms-android-gradlew "see below" if you want to
use the `gradlew` wrappers instead.

In case you don't have an OpenGL ES build set up yet, you need to copy
`FindEGL.cmake` and `FindOpenGLES2.cmake` (or `FindOpenGLES3.cmake`) from the
[modules/](https://github.com/mosra/magnum/tree/master/modules) directory in
Magnum source to the `modules/` dir in your project so it is able to find EGL
and OpenGL ES libraries.

Magnum provides Android application wrapper in @ref Platform::AndroidApplication.
See its documentation for more information about general usage. You can also
use the Android Native Activity directly or any other way.

@note The @ref Platform::AndroidApplication also contains a fully configured
    bootstrap project that's ready to build and deploy. Check its documentation
    for details.

The first thing you need compared to building an app for other platforms is
creating a shared library instead of an executable:

@code{.cmake}
if(NOT CORRADE_TARGET_ANDROID)
    add_executable(my-application MyApplication.cpp)
else()
    add_library(my-application SHARED MyApplication.cpp)
endif()
@endcode

Then you need to create a `build.gradle` file that references your root
`CMakeLists.txt`. Assuming it's saved right next to your root `CMakeLists.txt`,
the most minimal version might look like this:

@code{.gradle}
buildscript {
    repositories {
        jcenter()
        google()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.0.1'
    }
}

apply plugin: 'com.android.application'

android {
    compileSdkVersion 25

    defaultConfig {
        minSdkVersion 22

        externalNativeBuild {
            cmake {
                arguments '-DANDROID_STL=c++_static'
            }
        }
        ndk {
            abiFilters 'arm64-v8a'
        }
    }

    externalNativeBuild {
        cmake {
            path 'CMakeLists.txt'
        }
    }
}
@endcode

Important things are @cb{.gradle} compileSdkVersion @ce and
@cb{.gradle} minSdkVersion @ce, which set SDK version that will be used to
compile the project and minimal SDK version that the app can run on. You can
add further CMake parameters in the @cb{.gradle} arguments @ce line (here it's
just requesting to use static libc++) and the @cb{.gradle} abiFilters @ce allow
you to restrict which ABIs will the project be built for --- Gradle by default
builds for both 32 and 64-bit ARM, MIPS and x86, which might be quite annoying
to wait for (during development at least). The @cb{.gradle} path @ce then
references your `CMakeLists.txt` file. Gradle by default bundles all shared
library targets defined in the CMake project, so there's no need to specify a
particular library name.

The [official documentation](https://developer.android.com/studio/projects/gradle-external-native-builds.html#configure-gradle)
contains a more complete overview of all possibilities.

Another important file is `src/main/AndroidManifest.xml`, which says some
properties about the Android package. The location is also important, it has to
be placed inside `src/main` subdirectory, *not* straight besides the
`build.gradle` file. A minimal stripped-down version is:

@code{.xml-jinja}
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android" package="{{ package }}" android:versionCode="1" android:versionName="1.0">
  <uses-feature android:glEsVersion="0x00020000" />
  <application android:label="{{ app_name }}" android:hasCode="false">
    <activity android:name="android.app.NativeActivity" android:label="{{ app_name }}">
      <meta-data android:name="android.app.lib_name" android:value="{{ lib_name }}" />
      <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
      </intent-filter>
    </activity>
  </application>
</manifest>
@endcode

Replace @cb{.jinja} {{ package }} @ce with Java-like package name for your app
(in this case it could be e.g. @cpp "cz.mosra.magnum.my_application" @ce, for
example), @cb{.jinja} {{ app_name }} @ce with human-readable app name that's
displayed in the system (so e.g. @cpp "My Application" @ce) and finally the
@cb{.jinja} {{ lib_name }} @ce is name of the library that you compiled with
CMake, which in this case would be @cpp "my-application" @ce.

The @cb{.xml} <uses-feature android:glEsVersion="0x00020000" /> @ce says that
the minimal OpenGL ES version is 2.0, change it in case you require a different
version. Consult [the Android developer documentation](https://developer.android.com/guide/topics/manifest/manifest-intro.html)
for further information about the manifest file.

With everything set up, you are now ready to build the project by simply
executing the following from the directory with your `build.gradle`. During the
first run, Gradle will download a huge amount of random stuff when building
even the simplest thing. Close your eyes and ignore that it happened.

@code{.sh}
gradle build
@endcode

Installing on a connected device or emulator is then a matter of

@code{.sh}
gradle installDebug
@endcode

after which you can launch the app from your home screen. See the
@ref platforms-android-troubleshooting section below if you ran into problems.

@section platforms-android-multiple-abis Building for multiple ABIs and system versions

The above guide simplifies things a bit and builds for just a single ARM64 ABI.
In order to support multiple platforms, you need to separately build and
install the dependencies for all ABIs of choice --- create separate build
directories and run CMake with different `CMAKE_ANDROID_ARCH_ABI` and
corresponding `CMAKE_INSTALL_PREFIX`. Similarly with SDK versions, adapt
`CMAKE_SYSTEM_VERSION` and `CMAKE_INSTALL_PREFIX` to a desired version. The
headers are shared and should be always installed into `&lt;ndk&gt;/sysroot/usr`
regardless of ABI or SDK version. The supported ABI values are:

ABI         | Corresponding install prefix
----------- | ----------------------------
armeabi-v7a | &lt;nk&gt;/platforms/android-&lt;version&gt;/arch-arm/usr
arm64-v8a   | &lt;nk&gt;/platforms/android-&lt;version&gt;/arch-arm64/usr
x86         | &lt;nk&gt;/platforms/android-&lt;version&gt;/arch-x86/usr
x86_64      | &lt;nk&gt;/platforms/android-&lt;version&gt;/arch-x86_64/usr

After that, you can add the additional ABIs to the `abiFilters` list in your
`build.gradle`.

For example, building Magnum for 32-bit and 64-bit ARM with SDK version 24
could look like this:

@code{.sh}
mkdir build-android-arm && cd build-android-arm
cmake .. \
    -DCMAKE_SYSTEM_NAME=Android \
    -DCMAKE_SYSTEM_VERSION=24 \
    -DCMAKE_ANDROID_ARCH_ABI=armeabi-v7a \
    -DCMAKE_ANDROID_NDK_TOOLCHAIN_VERSION=clang \
    -DCMAKE_ANDROID_STL_TYPE=c++_static \
    -DCMAKE_BUILD_TYPE=Release \
    -DCMAKE_INSTALL_PREFIX=<ndk>/platforms/android-24/arch-arm/usr \
    -DMAGNUM_INCLUDE_INSTALL_PREFIX=<ndk>/sysroot/usr
cmake --build . --target install

cd ..

mkdir build-android-arm64 && cd build-android-arm64
cmake .. \
    -DCMAKE_SYSTEM_NAME=Android \
    -DCMAKE_SYSTEM_VERSION=24 \
    -DCMAKE_ANDROID_ARCH_ABI=arm64-v8a \
    -DCMAKE_ANDROID_NDK_TOOLCHAIN_VERSION=clang \
    -DCMAKE_ANDROID_STL_TYPE=c++_static \
    -DCMAKE_BUILD_TYPE=Release \
    -DCMAKE_INSTALL_PREFIX=<ndk>/platforms/android-24/arch-arm64/usr \
    -DMAGNUM_INCLUDE_INSTALL_PREFIX=<ndk>/sysroot/usr
cmake --build . --target install
@endcode

And the `build.gradle` for your app then looks like this:

@code{.gradle}
buildscript {
    repositories {
        jcenter()
        google()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.0.1'
    }
}

apply plugin: 'com.android.application'

android {
    compileSdkVersion 25

    defaultConfig {
        minSdkVersion 24

        externalNativeBuild {
            cmake {
                arguments '-DANDROID_STL=c++_static'
            }
        }
        ndk {
            abiFilters 'armeabi-v7a', 'arm64-v8a'
        }
    }

    externalNativeBuild {
        cmake {
            path 'CMakeLists.txt'
        }
    }
}
@endcode

See [the official documentation about ABIs](https://developer.android.com/ndk/guides/abis.html)
for more information.

@section platforms-android-output-redirection Redirecting output to Android log buffer

While printing to standard output and standard error output "just works" with
command-line apps, you might want to redirect your @ref Corrade::Utility::Debug "Debug",
@ref Corrade::Utility::Warning "Warning" and @ref Corrade::Utility::Error "Error"
output to Android log buffer. so it can be accessed through the @cb{.sh} adb logcat @ce
utility. See @ref Corrade::Utility::AndroidLogStreamBuffer for more
information.

The @ref Platform::AndroidApplication sets this up implicitly with a `magnum`
tag, you can then filter it out like this, for example:

@code{.shell-session}
$ adb logcat *:S magnum
...
03-16 17:35:26.703 17726 17745 I magnum  : Renderer: Mali-G71 by ARM
03-16 17:35:26.703 17726 17745 I magnum  : OpenGL version: OpenGL ES 3.2 v1.r2p0
03-16 17:35:26.703 17726 17745 I magnum  : Using optional features:
03-16 17:35:26.703 17726 17745 I magnum  :     GL_EXT_robustness
@endcode

@section platforms-android-system-cmake Using system-wide CMake installation

According to the [official documentation](https://developer.android.com/studio/projects/add-native-code.html#vanilla_cmake),
it's possible to use system CMake installation without needing to install
Android SDK version of CMake 3.6. Simply update the
@cb{.gradle} externalNativeBuild @ce in your `build.gradle` file to specify
CMake version that you have installed in your system, for example:

@code{.gradle}
android {
    ...
    externalNativeBuild {
        cmake {
            path 'CMakeLists.txt'
            ...
            version '3.10.2'
        }
    }
}
@endcode

However, be aware that this is an experimental feature and may be broken. At
the time of writing (March 2018), it didn't work for me with NDK r16b, Android
buid plugin 3.0.1 and CMake 3.10.

@section platforms-android-gradlew Using gradlew wrappers instead of a system installation

It's possible to bundle Gradle in the project itself as opposed to requiring a
pre-existing system installation. It has the downside of having a bit more
boilerplate files in your project, though.

First, add the following to your `build.gradle` file:

@code{.gradle}
task wrapper(type: Wrapper) {
    gradleVersion = '4.0'
}
@endcode

Then run this on a system that has Gradle installed:

@code{.sh}
gradle wrapper
@endcode

This will generate the following files that you can then add to version
control:

-   `gradlew` shell script for Unix-like systems
-   `gradle.bat` batch script for Windows
-   `gradle/` directory with wrapper binaries

With this in place, you can just use @cb{.sh} gradlew @ce instead of
@cb{.sh} gradle @ce.

@section platforms-android-travis Setting up Android build on Travis CI

For simple compilation tests, add the following to your `.travis.yml` matrix
builds. According to the [Travis Android documentation](https://docs.travis-ci.com/user/languages/android/),
`build-tools-22.0.1` and `android-22` are always present, so your builds
shouldn't get any extra delay when requesting them. The @cb{.sh} $TARGET @ce
environment variable is used here only to disambiguate later, you might or
might not need it.

@code{.yml}
matrix:
  include:
  # ...
  - language: android
    os: linux
    dist: trusty
    env:
    - TARGET=android
    android:
      components:
      - build-tools-22.0.1
      - android-22
@endcode

At the time of writing (March 2018), while the generic Ubuntu 14.04 images
already have CMake 3.9.2, for some reason the Android Ubuntu 14.04 images have
just CMake 3.2. Android support is builtin since version 3.7, but
[an important fix](https://gitlab.kitware.com/cmake/cmake/issues/17253) for the
LLVM toolchain was merged as late as in 3.9.2, so you may want to grab that
version. Example `.travis.yml` setup that downloads the binary and extracts it
to @cb{.sh} $HOME/cmake @ce, with @cb{.sh} $PATH @ce setup and caching:

@code{.yml}
cache:
  directories:
  - $HOME/cmake

install:
- >
  if [ "$TARGET" == "android" ] && [ ! -e "$HOME/cmake/bin" ]; then
    cd $HOME ;
    wget https://cmake.org/files/v3.9/cmake-3.9.2-Linux-x86_64.tar.gz &&
    mkdir -p cmake &&
    cd cmake &&
    tar --strip-components=1 -xzf ../cmake-3.9.2-Linux-x86_64.tar.gz &&
    cd $TRAVIS_BUILD_DIR ;
  fi
- >
  if [ "$TARGET" == "android" ]; then
    export PATH=$HOME/cmake/bin:$PATH &&
    cmake --version ;
  fi
@endcode

The NDK can be fetched as a simple `*.zip` file. However, version r16b has over
800 MB, so you might want to explore creation of a
[Standalone Toolchain](https://developer.android.com/ndk/guides/standalone_toolchain.html)
with only the things you need to speed up the build. Again, downlading it into
@cb{.sh} $HOME/android-ndk-r16b @ce is a matter of adding this into your
@cb{.yml} install: @ce section:

@code{.yml}
- >
  if [ "$TARGET" == "android" ]; then
    cd $HOME ;
    wget https://dl.google.com/android/repository/android-ndk-r16b-linux-x86_64.zip &&
    unzip -q android-*.zip &&
    cd $TRAVIS_BUILD_DIR ;
  fi
@endcode

Travis CI discourages caching the NDK, as downloading the cache will take
roughly the same amount of time as downloading it from upstream.

Building your actual code is just a matter of setting up a correct NDK path.
You can install the dependencies to any location as long as you specify the
same location in `CMAKE_PREFIX_PATH` and `CMAKE_FIND_ROOT_PATH` in depending
projects. Using `armeabi-v7a` instead of `arm64-v8a` ensures that you can run
the code in a preinstalled emulator later, see below.

@code{.sh}
mkdir build-android-arm && cd build-android-arm
cmake .. \
    -DCMAKE_ANDROID_NDK=$HOME/android-ndk-r16b \
    -DCMAKE_SYSTEM_NAME=Android \
    -DCMAKE_SYSTEM_VERSION=22 \
    -DCMAKE_ANDROID_ARCH_ABI=armeabi-v7a \
    -DCMAKE_ANDROID_NDK_TOOLCHAIN_VERSION=clang \
    -DCMAKE_ANDROID_STL_TYPE=c++_static \
    -DCMAKE_PREFIX_PATH=$HOME/deps \
    -DCMAKE_FIND_ROOT_PATH=$HOME/deps \
    ...
@endcode

@subsection platforms-android-travis-run Running tests on the emulator

In order to run your tests on the emulator, you need to request some system
image. Again, `sys-img-armeabi-v7a-android-22` is part of the default
installation, so it shouldn't add any extra time to your build:

@code{.yml}
matrix:
  include:
  - language: android
    # ...
    android:
      components:
      # ...
      - sys-img-armeabi-v7a-android-22
@endcode

As described [in the Travis documentation](https://docs.travis-ci.com/user/languages/android/#How-to-Create-and-Start-an-Emulator),
create a system image and wait  for the emulator to start (be prepared, it can
easily take up *minutes*). Assuming you use
the @ref TestSuite-Tester-running-cmake "Corrade::TestSuite Android integration",
simply run your tests via `ctest` and optionally enable colored output for
extra clarity:

@code{.sh}
echo no | android create avd --force -n test -t android-22 --abi armeabi-v7a
emulator -avd test -no-audio -no-window &
android-wait-for-emulator
CORRADE_TEST_COLOR=ON ctest -V
@endcode

@subsection platforms-android-travis-bundle APK bundle creation

At the time of writing (March 2018), Travis Ubuntu 14.04 has Gradle 4.0,
however the Android build plugin 3.0 requires at least Gradle 4.1, so you need
to backport `gradle.build` to plugin version 2.3.3 compared to the
@ref platforms-android-apps "template above". In particular, the `classpath`
needs to be updated, `compileSdkVersion`  and `minSdkVersion` adapted to
versions defined in @cb{.yml} components: @ce in your `travis.yml` file and the
`buildToolsVersion` explicitly specified, because that's needed in plugin
versions before 3.0:

@code{.gradle}
buildscript {
    // ...
    dependencies {
        classpath 'com.android.tools.build:gradle:2.3.3'
    }
}

// ...

android {
    compileSdkVersion 22
    buildToolsVersion '26.0.2'

    defaultConfig {
        minSdkVersion 22
        // ...
@endcode

Gradle bundles its own CMake 3.6, downloading it on-demand and then failing
because SDK licenses are not signed. Solution is to install CMake and sign its
license explicitly beforehand. Add the following to your `.travis.yml`:

@code{.yml}
before_install:
- if [ "$TARGET" == "android" ]; then yes | sdkmanager "cmake;3.6.4111459"; fi
@endcode

Unlike above, and especially if you build for multiple ABIs, it's better to
install all dependencies where Gradle expects them. In particular, in case of
Corrade and ARM64 ABI and NDK being in @cb{.sh} $HOME/android-ndk-r16b @ce, the
install prefixes look like this:

@code{.sh}
cmake .. \
    -DCMAKE_INSTALL_PREFIX=$HOME/android-ndk-r16b/platforms/android-22/arch-arm64/usr \
    -DCORRADE_INCLUDE_INSTALL_PREFIX=$HOME/android-ndk-r16b/sysroot/usr \
    ...
@endcode

Finally, you need to tell Gradle where the NDK is located and where to look for
native binaries (for example the `corrade-rc` executable) using environment
variables. At last, execute `gradle build` in the directory where
`build.gradle` is:

@code{.sh}
export ANDROID_NDK_HOME=$HOME/android-ndk-r16b
export CMAKE_PREFIX_PATH=$HOME/deps-native/

gradle build
@endcode

@section platforms-android-troubleshooting Troubleshooting

@subsection platforms-android-troubleshooting-cant-find Gradle CMake can't find dependencies

Gradle by default searches only in the NDK install path. If you have your
dependencies installed somewhere else (this goes especially for the *native*
`corrade-rc` executable), you might want to point the `CMAKE_PREFIX_PATH`
environment variable to your install location:

@code{.sh}
export CMAKE_PREFIX_PATH=<path-where-your-dependencies-are-installed>
gradle build
@endcode

If you have the dependencies installed in the NDK path, but it still fails,
check that you installed for the same SDK version as in `minSdkVersion` and all
ABIs mentioned in `abiFilters` inside your `build.gradle` file --- Gradle runs
CMake once for each entry in the list so it might happen that it finds them for
all but one ABI. See @ref platforms-android-multiple-abis above for more
information.

@subsection platforms-android-troubleshooting-cant-launch App can't launch

If your application can't launch (or it just blinks and then disappears), you
can inspect @cb{.sh} adb logcat @ce output to see what went wrong, but be
quick, the log is spitting out a lot of info all the time. Possible causes:

-   Mismatch between actual library name and library referenced from
    `AndroidManifest.xml`, causing Java to fail loading it
-   The device has an ABI for which the app was not compiled (check the
    @cb{.gradle} abiFilters @ce option in `build.gradle`) or the app was
    compiled with SDK version that's not supported by the device yet. See the
    [official API level documentation](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)
    for more information.
-   The device doesn't support OpenGL ES 3.0 yet. Rebuild Magnum and its
    dependencies with the `TARGET_GLES2` option enabled. See @ref building-features
    for more information.
-   Loading fails with `ANativeActivity_onCreate` symbol not being found. If
    you are using @ref Platform::AndroidApplication, this issue should be
    prevented, otherwise you need to add `-u ANativeActivity_onCreate` to your
    linker flags or reference the symbol some other way. See
    [android-ndk/ndk#381](https://github.com/android-ndk/ndk/issues/381) for
    details.
-   Additional `*.so` libraries are referenced by the main `*.so` but not
    bundled in the `*.apk`. One option is to switch to static libraries,
    another is explicitly specifying them in the `build.gradle` file. See
    [the official documentation](https://developer.android.com/studio/projects/gradle-external-native-builds.html#jniLibs)
    for details.

@subsection platforms-android-troubleshooting-term Gradle aborting due to termcap

If you see the following output, Gradle is crashing because @cb{.sh} $TERM @ce
is set to `xterm-256color` or `xterm-24`:

@code{.shell-session}
$ gradle build

FAILURE: Build failed with an exception.

* What went wrong:
Could not open terminal for stdout: could not get termcap entry
@endcode

Solution is to set @cb{.sh} TERM=xterm @ce. See
[gradle/gradle#4440](https://github.com/gradle/gradle/issues/4440) for more
information.

@code{.sh}
TERM=xterm gradle build
@endcode

@subsection platforms-android-troubleshooting-licenses Accepting SDK licenses for Gradle

Gradle might refuse to build a project if SDK licenses are not accepted.
Depending on where your SDKs are installed, you might need to execute the
following (assuming you have SDK version 26 at least):

@code{.sh}
sdkmanager --licenses # and then manually accept all of them
@endcode

The tool doesn't provide any diagnostic output if the accepting failed, so be
sure to verify that everything went well by executing @cb{.sh} sdkmanager --licenses @ce
again. If it offers the same licenses again, you might want to force it with
@cb{.sh} sudo @ce.

@subsection platforms-android-troubleshooting-permissions Android SDK directory permissions

Gradle is able to work with system-installed Android SDK. If it complains about
directory permissions such as

@code{.shell-session}
> Failed to install the following SDK components:
  [Android SDK Build-Tools 26.0.2, Android SDK Platform 25]
  The SDK directory (/opt/android-sdk) is not writeable,
  please update the directory permissions.
@endcode

it's often enough to just install such packages. In case of ArchLinux, all
relevant packages are available in AUR.
*/

}