Develop Android app using Golang

HelloAndroid.go
SeongJae Park <sj38.park@gmail.com>

This work by SeongJae Park is licensed under the Creative
Commons Attribution-ShareAlike 3.0 Unported License. To
view a copy of this license, visit http://creativecommons.
org/licenses/by-sa/3.0/.

This slides were presented during
3rd GDG Korea Android Conference
(http://event.android.gdg.kr/3rd-GKAC/)
The talk video is available at:
https://www.youtube.com/watch?v=vMZFjDipaK8&index=2&list=PL_WJkTbDHdBl5QXy6N_bMMBYlKLna5RER

Nice To Meet You
SeongJae Park
sj38.park@gmail.com
golang newbie programmer

Warning
● This speech could be useless for you
○ This is just for fun
http://m.c.lnkd.licdn.com/mpr/mpr/p/4/005/095/091/210ae8c.jpg

Warning
● This speech could be useless for you
○ This is just for fun
● Don’t try this at office
○ The code is not stable yet
http://m.c.lnkd.licdn.com/mpr/mpr/p/4/005/095/091/210ae8c.jpg

golang: Programming Language
● For simple, reliable, and efficient software.
http://blog.golang.org/5years/gophers5th.jpg

golang: Programming Language
● For simple, reliable, and efficient software.
● Could it be used for simple, reliable, and
efficient Android application?
http://blog.golang.org/5years/gophers5th.jpg

Golang and Android
● Golang supports Android from v1.4
○ Though it’s still unstable

Golang and Android
● Golang supports Android from v1.4
○ Though it’s still unstable
● https://github.com/golang/mobile
○ Packages and build tools for using Go on Android

Goal of This Speak
● Showing how we can use golang on Android
○ By exploring example code

Goal of This Speak
● Showing how we can use golang on Android
○ By exploring example code
● Just for fun, rather than profit

Pre-requisites
● Basic development environment(vim, git,
gcc, java, …)
http://www.theprospect.net/wp-content/uploads/2014/02/one-does-not-simply-skip-a-step.jpg

Pre-requisites
gcc, java, …)
● Android SDK & NDK

Pre-requisites
gcc, java, …)
● Android SDK & NDK
● Golang 1.4 or higher cross-compiled for
GOOS=android

Pure Golang Android App
NO JAVA!

Main Idea: Use NDK
● c / c++ only apk is available
using NativeActivity
○ Golang be compiled to native binary, too. Why not?
http://www.android.pk/images/android-ndk.jpg

Main Idea: Use NDK
Plan is...
● Build golang program as .so file
○ ELF shared object
● Implement every callbacks using OpenGL

Main Idea: Use NDK
Plan is...
● Build golang program as .so file
○ ELF shared object
● Implement every callbacks using OpenGL
● Build NativeActivity apk using NDK / SDK

Example Code
https://github.
com/golang/mobile/tree/master/example/basic

Example Code
https://github.
com/golang/mobile/tree/master/example/basic
$ tree
.
├── all.bash
├── all.bat
├── AndroidManifest.xml
├── build.xml
├── jni
│ └── Android.mk
├── main.go
├── make.bash
└── make.bat
1 directory, 8 files

main.go: Register Callbacks
Register callbacks from golang entrypoint
func main() {
app.Run(app.Callbacks{
Start: start,
Stop: stop,
Draw: draw,
Touch: touch,
})
}
(mobile/example/basic/main.go)

main.go: Use OpenGL
func draw() {
gl.ClearColor(1, 0, 0, 1)
...
green += 0.01
if green > 1 {
green = 0
}
gl.Uniform4f(color, 0, green, 0, 1)
...
debug.DrawFPS()
}
(mobile/example/basic/main.go)

NativeActivity
NativeActivity only application doesn’t need
JAVA
<application android:label="Basic" android:hasCode="false">
<activity android:name="android.app.NativeActivity"
android:label="Basic"
android:configChanges="orientation|keyboardHidden">
<meta-data android:name="android.app.lib_name" android:value="basic" />
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
(mobile/example/basic/AndroidManifest.xml)

Build Process
Build golang code into ELF shared object for
ARM
mkdir -p jni/armeabi
CGO_ENABLED=1 GOOS=android GOARCH=arm GOARM=7
go build -ldflags="-shared" -o jni/armeabi/libbasic.so .
ndk-build NDK_DEBUG=1
ant debug
(mobile/example/basic/make.bash)

Build Process
ARM
NDK to add the so file
ant debug

Build Process
ARM
NDK to add the so file
SDK to build apk file
ant debug

Pure Golang Android App
Pros: No more JAVA! Yay!!!
Cons: Should I learn OpenGL to show a cat?

Golang as a Library
Cooperate Java and Golang

Java and C language connected via JNI
Main Idea: Use JNI-like way
Java
CPP
JNI

C language and Golang connected via cgo
Java
CPP
GO
JNI CGO

...But, JNI and then cgo looks tedious
Java
CPP
GO
JNI CGO

...But, JNI and then cgo looks tedious
Golang supports Java-Golang bind
Java
CPP
GO
JNI CGO
bind/seq

Example Code
https://github.
com/golang/mobile/tree/
master/example/libhello

Example Code
https://github.
com/golang/mobile/tree/
master/example/libhello
$ tree
.
├── all.bash
├── all.bat
├── AndroidManifest.xml
├── build.xml
├── hi
│ ├── go_hi
│ │ └── go_hi.go
│ └── hi.go
├── main.go
├── make.bash
├── make.bat
├── README
└── src
├── com
│ └── example
│ └── hello
│ └── MainActivity.java
└── go
└── hi
└── Hi.java
8 directories, 12 files

Callee in Go
Golang code is implementing Hello() function
func Hello(name string) {
fmt.Printf("Hello, %s!n", name)
}
(mobile/example/libhello/hi/hi.go)

Caller in JAVA
Java code is calling Golang function, Hello()
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Go.init(getApplicationContext());
Hi.Hello("world");
}
(mobile/example/libhello/src/com/example/hello/MainActivity.java)

gobind
generate language bindings that make it
possible to call Go code and pass objects from
Java
go install golang.org/x/mobile/cmd/gobind
gobind -lang=go github.com/libhello/hi > hi/go_hi/go_hi.go
gobind -lang=java github.com/libhello/hi > src/go/hi/Hi.java

gobind: Generated .java
Provides wrapper function for golang calling
code
public static void Hello(String name) {
go.Seq _in = new go.Seq();
go.Seq _out = new go.Seq();
_in.writeUTF16(name);
Seq.send(DESCRIPTOR, CALL_Hello, _in, _out);
}
private static final int CALL_Hello = 1;
private static final String DESCRIPTOR = "hi";
(mobile/example/libhello/src/go/hi/Hi.java)

gobind: Generated .go
Provides proxy and registering for the exported
function
func proxy_Hello(out, in *seq.Buffer) {
param_name := in.ReadUTF16()
hi.Hello(param_name)
}
func init() {
seq.Register("hi", 1, proxy_Hello)
}
(mobile/example/libhello/hi/go_hi/go_hi.go)

Golang as a Library
Pros: JAVA for UI, Golang for background
(Looks efficient enough)

Golang as a Library
Pros: JAVA for UI, Golang for background
(Looks efficient enough)
Cons: bind/seq is not so efficient, yet

Inter-Process
Communication
Philosophy of Unix

Main Idea: Android is a Linux
http://www.kitguru.net/wp-content/uploads/2012/03/linux-android.png
Android is a variant of Linux system with ARM
x86 Androids exist, though...

Go supports ARM & Linux Officially
with static-linking

Go supports ARM & Linux Officially
with static-linking
Remember the philosophy of Unix

Example Code
https://github.com/sjp38/goOnAndroid
https://github.com/sjp38/goOnAndroidFA

Example Code
https://github.com/sjp38/goOnAndroid
https://github.com/sjp38/goOnAndroidFA
Were demonstrated by live-coding from
GDG Korea DevFair 2014 (http://devfair2014.
gdg.kr/) and
GDG Golang Seoul Meetup 2015 (https:
//developers.google.
com/events/5381849181323264/)

Golang Process on Android: Plan
1. Cross compile Go program as ARM / Linux

2. Include the binary in assets/ of Android app

3. Copy the binary in private space of the app

4. Give execute permission to the binary
/data/data/com.example.goRunner/files # ls -al
-rwxrwxrwx u0_a55 u0_a55 4512840 2014-11-28 17:45 gobin

4. Give execute permission to the binary
5. Execute it
/data/data/com.example.goRunner/files # ls -al
-rwxrwxrwx u0_a55 u0_a55 4512840 2014-11-28 17:45 gobin

Go bin Loading
Load golang program from assets to private dir
private void copyGoBinary() {
String dstFile = getBaseContext().getFilesDir().getAbsolutePath() + "/verChecker.bin";
try {
InputStream is = getAssets().open("go.bin");
FileOutputStream fos = getBaseContext().openFileOutput(
"verChecker.bin", MODE_PRIVATE);
byte[] buf = new byte[8192];
int offset;
while ((offset = is.read(buf)) > 0) {
fos.write(buf, 0, offset);
}
Runtime.getRuntime().exec("chmod 0777 " + dstFile);
} catch (IOException e) { }
}

Execute Go process
Spawn new process for the program and
communicates using stdio
ProcessBuilder pb = new ProcessBuilder();
pb.command(goBinPath());
pb.redirectErrorStream(false);
goProcess = pb.start();
new CopyToAndroidLogThread("stderr",
goProcess.getErrorStream())
.start();

Inter Process Communication
Pros: Just normal unix way

Golang team is using this for Camlistore
(https://github.com/camlistore/camlistore)

Golang team using this for Camlistore
(https://github.com/camlistore/camlistore)
Cons: Hacky, a little

Summary
You can use Golang for Android
though it’s not stable yet
Just for fun

This work by SeongJae Park is licensed under the
Creative Commons Attribution-ShareAlike 3.0 Unported
License. To view a copy of this license, visit http:
//creativecommons.org/licenses/by-sa/3.0/.

Develop Android app using Golang

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