Develop Android/iOS app using golang

HelloMobile.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
GopherCon Korea 2015
(https://plus.google.com/u/0/events/c4b79ocq4k9ac11bpb5b2govkes)

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

Warning
● This speech could be useless for you
○ The speaker is doing this just for fun
https://github.com/golang/mobile/raw/master/doc/caution.png

Warning
● This speech could be useless for you
○ The speaker is doing this just for fun
● Don’t try this at office carelessly
○ It’s an experiment yet
https://github.com/golang/mobile/raw/master/doc/caution.png

Java is Dangerous, Now
● Oracle may take Java away from Android
● Swift is open source now, but...
● We need alternatives
http://img.talkandroid.com/uploads/2011/10/OracleGoogle.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, efficient,
and free mobile application?
http://blog.golang.org/5years/gophers5th.jpg

Golang on Mobile
● Golang supports Android from v1.4
● Golang supports iOS from v1.5
○ Though it’s still in experimental stage, there were
many improvements especially in tools
● https://github.com/golang/mobile
○ Repo of packages and tools for Go on Mobile

Goal of This Speak
● Showing how we can use golang on Mobile
○ Focus on Android, rather than iOS
(Speaker has no iPhone…)
○ By exploring example code

Goal of This Speak
● Showing how we can use golang on Mobile
○ Focus on Android, rather than iOS
(Speaker has no iPhone…)
○ By exploring example code
● We will explore code based on go1.4, go1.5
○ Code based on go1.4 shows naked face of build
process
■ based on go1.4: https://github.com/golang/mobile/tree/40f92f7d9f9092072ea68570fd6f5725a5cbcd6b
■ based on go1.5: https://github.com/golang/mobile/tree/25faf494e186f45f8d9704fb53a6c6c555366d20

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

Pre-requisites
● Android SDK & NDK

Pre-requisites
● Android SDK & NDK
● Golang 1.4 cross-compiled for GOOS=android
or higher versions

Pure Golang Mobile App
NO JAVA!

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

Main Idea: NDK
● c / c++ only apk is available
using NativeActivity
○ Golang is a compiled language, too. Why not?
Plan is as below:
● Build golang program as .so file
○ ELF shared object
● Process events(draw, touch, …) via OpenGL
● Build NativeActivity apk using NDK / SDK
http://www.android.pk/images/android-ndk.jpg

Example Code
https://github.
com/golang/mobile/tree/40f92f7d9f9092072ea68570fd6f57
25a5cbcd6b/example/basic for 1.4 based
https://github.
com/golang/mobile/tree/25faf494e186f45f8d9704fb53a6c6c
555366d20/example/basic for 1.5 based

Example Code (1.4 based)
https://github.
com/golang/mobile/tree/40f92f7d9f9092072ea68570fd6f57
25a5cbcd6b/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
(After 1.5, programming model changed a little)
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

https://github.
555366d20/example/basic
gomobile command do build
$ tree
.
└── main.go
0 directories, 1 file

https://github.
$ go get golang.org/x/mobile/cmd/gomobile
$ tree
.
└── main.go

https://github.
$ gomobile init # install ndk if needed
$ tree
.
└── main.go

https://github.
$ gomobile init # install ndk if needed
$ gomobile build -target=android golang.org/x/mobile/example/basic
$ tree
.
└── main.go

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

Main Idea: Gives binding
Java and C language connected via JNI
Java
C
JNI

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

ObjC(Objective C) is a super-set of C language
Java
C GO
JNI
CGO
ObjC

ObjC(Objective C) is a super-set of C language
Golang supports Java/ObjC-Golang bind
(Uses Cgo internally)
Java
C GO
JNI
CGO
bind
ObjC
bind

https://github.
com/golang/mobile/tree/40f92
f7d9f9092072ea68570fd6f572
5a5cbcd6b/example/libhello

https://github.
com/golang/mobile/tree/40f92
f7d9f9092072ea68570fd6f572
5a5cbcd6b/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 # stub
$ gobind -lang=java github.com/libhello/hi > src/go/hi/Hi.java # proxy

gobind: Generated Proxy java Code
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 Stub go Code
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)

https://github.
com/golang/mobile/tree/25faf
494e186f45f8d9704fb53a6c6
c555366d20/example/bind
gomobile do binding
automatically
$ gomobile bind
-target=android
golang.org/x/mobile/example/bind/hello
$ tree
.
├── android
│ ├── README
├── ...
│ └── settings.gradle
├── hello
│ └── hello.go
└── ios
├── bind
│ ├── AppDelegate.h
│ ├── AppDelegate.m
│ ├── Base.lproj
│ │ ├── LaunchScreen.xib
│ │ └── Main.storyboard
│ ├── Info.plist
│ ├── main.m
│ ├── ViewController.h
│ └── ViewController.m
├── bind.xcodeproj
│ └── project.pbxproj
└── README

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: Binding supports only subset of Go types

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
Not tested for iOS

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 as 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 is using this for Camlistore
(https://github.com/camlistore/camlistore)
Cons: Hacky, a little
x/mobile is comfortable enough, now
Not sure whether it works on iOS

Summary
● Go can run on Mobile
○ As a native application,
○ as a library,
○ or, as a process
● Though it’s an experiment yet, it’s fairly
comfortable

Develop Android/iOS app using golang

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