= Building for Android

NOTE: I am not an Android developer, so this work has received only cursory
testing.  As always, _caveat emptor_.

== Pre-Requisites

Android Studio:::

  There are probably other ways to cross-build for Android, but if you're
  not using Android Studio, then you probably don't need much help here.

Android NDK::

  A copy of the Android NDK is required to build native code applications.
  Android Studio has information for downloading it within the app.

== Steps

You need to use the CMake that is included with Android Studio, because
it knows how to build the Gradle targets.  (At least on macOS, the system
default cmake does not have the right generator support.)

You will also use the Android toolchain file for CMake that is included
with the NDK.

NOTE: You *must* use the _NDK_ toolchain, not the one that came with the _SDK_.
The _SDK_ toolchain file is too old and will not work with modern NDKs!

When building for Android, by default we build static libraries, and
we do not build tools or tests.  The tools and tests won't work since
they assume a shell environment, and you don't want to deal with the
dependency nightmare that is dynamic libraries anyway.  Trust us.

The Android NDK includes documentation for how to run
CMake including the options that can be set.  Details are located
here: https://developer.android.com/ndk/guides/cmake

TIP: It is *highly* recommended you review the NDK documentation,
because you will most likely want to change some of the default values
for the ABI or API level.
For the sake of clarity, we are assuming only the defaults here.

Set the following environment variables:

`CMAKE`::

  Path to the Android SDK supplied CMake binary.  For example, on
  our macOS install of Android Studio, we have it located in
  `$HOME/Library/Android/sdk/cmake/3.6.4111459/bin/cmake`.

`NDK`::

  Path to the Android NDK.  In the same installation, on our system,
  it is in `$HOME/Library/Android/sdk/ndk-bundle`

Using the above toolchain file, we can build for Android using
the CMake standard `CMAKE_TOOLCHAIN_FILE` macro, and using the
Android supplied CMake:

If you have checked out this repository in `$SRC`, and your copy of the
Android SDK is located in `$SDK`, the following should work:

[source, sh]
----
% cd $SRC
% mkdir android-build
% cd android-build
% ${CMAKE} -DCMAKE_TOOLCHAIN_FILE=${NDK}/build/cmake/android.toolchain.cmake ..
----

Then you can build using Xcode, or simply use cmake to drive the build:

[source, sh]
----
% ${CMAKE} --build .
----

Extra effort may be required to enable the use of mbedTLS; you will
need to review the mbedTLS documentation for configuring that for use
in Android.
