goandroid

Patches to the Go tools and runtime to enable Android apps to interface directly with a shared library written in Go. Goandroid also includes two demos, hellogl2 and nativeactivity, both written in Go. Goandroid currently runs on ARMv7 or later CPUs.

Running Go code from Android apps is currently not possible, because the Go tools can only output executables while Android requires any foreign code in shared library (.so) format. This repository contains patches for the Go tools and runtime to enable shared library output, including workarounds to Android specific limitations.

Disclaimer: Please note that the patches are unofficial, and while I hope the Go developers will eventually incorporate at least the patches not specific to Android, there's no guarantee they will. Also note that goandroid only includes the bare minimum to let Go run in Android apps. If you need a more comprehensive framework, check out Mandala.

Set up

This guide is tested on linux/amd64 and assumes you have an android device connected through USB and that you meet the requirements for building Go from source.

1. Download and install the NDK at least version r8e. These instructions assumes the NDK is installed in $NDK.

2. Create a standalone NDK toolchain (as described in $NDK/docs/STANDALONE-TOOLCHAIN.html):

   $NDK/build/tools/make-standalone-toolchain.sh --platform=android-9 --install-dir=ndk-toolchain

   You might need to add --system=linux-x86_64 or --system=darwin-x86_64 depending on your system.

   Set $NDK_ROOT to point at the ndk-toolchain directory

3. Clone the golang repository:

   hg clone -u release https://code.google.com/p/go

4. Copy the patches directory to the go/.hg directory:

   cp -a patches go/.hg

5. Enable the mq extension by adding the following lines to go/.hg/hgrc:

   [extensions]  
   mq =
   codereview = !
   
   [ui]  
   username = me<[email protected]>
   

6. In the go/src directory apply the patches and build go:

   cd go/src  
   hg qpush -a  
   CGO_ENABLED=0 GOOS=linux GOARCH=arm ./make.bash \
     CC="$NDK_ROOT/bin/arm-linux-androideabi-gcc" GOOS=linux GOARCH=arm GOARM=7 CGO_ENABLED=1 ../bin/go install -tags android -a -v std  
   cd ../..
   

Building and installing the example apps

If everything is set up correctly, you should be able to cd hello-gl2 and run build.sh to build and copy libgoandroid.so to android/libs. Then, running ant -f android/build.xml clean debug install will build and install the final apk to the connected device. Running the app should display a simple color animated triangle that you can move around the screen with your finger.

A more complicated but also more useful example is nativeactivity. It mimicks the C/C++ native_app_glue library and uses the NativeActivity API to completely avoid Java code and gain control of the input and render loop as well as context creation through EGL. It can be compiled and installed in the same way as hellogl2 with ./build.sh and ant -f android/build.xml clean debug install. The nativeactivity sample requires Android 2.3, which is the version where NativeActivity were introduced.

An interesting artifact of Go apps is that the compile-deploy cycle can be shorter than the ant scripts. If an existing debug apk is already present after an ant -f android/build.xml clean debug install you can use ./upload.sh to replace the Go library and upload the apk to the device. On my system, touch src/nativeactivity/main.go android/AndroidManifest.xml && time ./build.sh && time ant -f android/build.xml debug install takes 17 seconds, while touch src/nativeactivity/main.go && time ./upload.sh takes 10 seconds. This difference will only be more exaggerated if the apk contains resources.

Go patches

All patches except android-tls and android-build-hacks correspond to the patches for linux/arm external linking and shared library support discussed on the golang-nuts mailing list.

The android-tls patch is a workaround for the missing support for the R_ARM_TLS_IE32 relocation in the Android linker.

The android-build-hacks patch contains various changes to account for the difference between a vanilla linux/arm system and Android.