I talked this presentation in GopherCon 2016. Go Mobile (golang.org/x/mobile) is a project which shows new possibilities for mobile apps development. Especially, for mobile games which requires high performance processing, Go can be expected to be an alternative to C and C++. This session explains how to develop game apps using some packages such as app, event and sprite provided by Go Mobile project. Furthermore the session introduces a way to call Android API from Go codes, and solutions for some problems which occurs when we distribute apps on Google Play. I would like to show current capabilities and future potentials of Go Mobile.

1. The Go gopher was designed by Renée French.
The gopher stickers was made by Takuya Ueda.
Licensed under the Creative Commons 3.0 Attributions license.
Go for Mobile Games
GopherCon 2016, Denver
11th July 2016
Takuya Ueda @tenntenn
KLab Inc.
Slide URL: https://goo.gl/cIvTb5

2. Slide URL: https://goo.gl/cIvTb5
What is this talk about?
● The Basics of Go Mobile
○ Cross-compile and Android device
○ SDK Apps and Native Apps
● Go for Mobile Game
○ Event handling
○ 2D scene graph
● Advanced Topics
○ Distribute apps on Google Play
○ How to use Android API from Go
2

3. Slide URL: https://goo.gl/cIvTb5
The Basics of Go Mobile
3
● Run on Android Devices
● Cross-compile and cgo
● SDK Apps and Native Apps

4. Slide URL: https://goo.gl/cIvTb5
Cross-compile
● GOOS and GOARCH
○ Go can cross-compile
○ GOOS indicates target OS
○ GOARCH indicates target architecture
4
# Build for 32bit Windows
$ GOOS=windows GOARCH=386 go build
# Build for arm Linux
$ GOOS=linux GOARCH=arm go build
A linux/arm binary also works on android devices.
The Basics of Go Mobile / Cross-compile and cgo

5. Slide URL: https://goo.gl/cIvTb5
Webserver on Android Devices
Watch at Youtube Source Code
Android
Shell on Mac
adb shell
The Basics of Go Mobile / Webserver on Androind Devices 5

6. Slide URL: https://goo.gl/cIvTb5
cgo
● C codes into Go codes
import "unsafe"
/*
#include <stdio.h>
#include <stdlib.h>
void hello(char *s) { printf("Hello, %sn", s); }
*/
import "C"
func main() {
str := C.CString("GopherCon")
C.hello(str)
C.free(unsafe.Pointer(str))
}
Comments before import "C"
would be built as C codes
Call C’s function from Go code
The Basics of Go Mobile / Cross Compile and cgo 6

7. Slide URL: https://goo.gl/cIvTb5
cgo for Android
● cgo codes also can be cross-compiled
7
$ CGO_ENABLED=1
CC=arm-linux-androideabi-gcc
GOOS=android
GOARCH=arm
GOARM=7
go build -buildmode=pie hellocgo.go
$ adb push hellocgo /data/local/tmp
$ chmod 755 /data/local/tmp/hellocgo
$ /data/local/tmp/hellocgo
Hello, GopherCon
GOOS should be android
when CGO_ENABLED is 1.
Enable cgo at cross-compiling
adb shell
PC
The Basics of Go Mobile / Cross Compile and cgo

8. Slide URL: https://goo.gl/cIvTb5
buildmode
● Change output formats
○ archive, c-archive
■ build to C archive (.a file)
○ shared, c-shared
■ build to shared library (.so file)
○ exe
■ build to executable file
○ pie
■ build to PIE style executable file
archive and shared ignore
main package
Go can build to .so files for Android
8The Basics of Go Mobile / Cross Compile and cgo

9. Slide URL: https://goo.gl/cIvTb5
Go Mobile
● What is Go Mobile?
○ Go Mobile is a toolkit for Mobile Platform (Android and
iOS) with Go.
● How Go Mobile works?
○ Go Mobile provides bindings of Android and iOS
through cgo.
9
Go C
Java
Obj-C
JNIcgo
Android
iOS
The Basics of Go Mobile / Go Mobile

10. Slide URL: https://goo.gl/cIvTb5
Go Mobile
The Basics of Go Mobile / Go Mobile 10
https://github.com/golang/mobile

11. Slide URL: https://goo.gl/cIvTb5
Installation
● Install gomobile comand
● Initialize the build tool chain
○ gomobile init initializes the build tool chain for
mobile apps. Android NDK is also installed.
11
$ gomobile init -v
$ ls $GOPATH/pkg/gomobile
android-ndk-r12 pkg_android_386 pkg_android_arm
pkg_darwin_amd64 pkg_darwin_arm64 dl
pkg_android_amd64 pkg_android_arm64 pkg_darwin_arm version
$ go get golang.org/x/mobile/cmd/gomobile
The Basics of Go Mobile / Installation

12. Slide URL: https://goo.gl/cIvTb5
gomobile command
gomobile command provides sub-commands.
● Sub-commands
12
bind build a library for Android and iOS
build compile Android APK and iOS app
clean remove object files and cached gomobile files
init install android compiler toolchain
install compile android APK and install on device
version print version
The Basics of Go Mobile / gomobile command

13. Slide URL: https://goo.gl/cIvTb5
SDK Apps and Native Apps
Go Mobile provides two ways to develop mobile
apps.
■ SDK Apps
● Write common funcations in Go as a library
● Write UI and platform dependent functions in Java for
Android and Objective-C/Swift for iOS
■ Native Apps
● Write UI and all codes in Go
13The Basics of Go Mobile / SDK Apps and Native Apps

14. Slide URL: https://goo.gl/cIvTb5
Go
aar file
SDK Apps and Native Apps
Binding Classes (Java)
Shared library (.so)
Java
● SDK Apps for Android
● Native Apps for Android
apk file
Go
GoNativeActivity
Shared library (.so)
UI, IAB, ...
As a library
UI, audio, ...
gomobile bind
gomobile build
The Basics of Go Mobile / SDK Apps and Native Apps 14

15. Slide URL: https://goo.gl/cIvTb5
Go
aar file
SDK Apps and Native Apps
Binding Classes (Java)
Shared library (.so)
Java
● SDK Apps for Android
● Native Apps for Android
apk file
Go
GoNativeActivity
Shared library (.so)
UI, IAB, ...
As a library
UI, audio, ...
gomobile bind
gomobile build
The Basics of Go Mobile / SDK Apps and Native Apps 15

16. Slide URL: https://goo.gl/cIvTb5
SDK App example: Ivy
● Ivy big number calculator (source code)
○ Interpriter for APL-like language
○ Android App and iOS App use a same engine
○ The engine is written in Go by Rob Pike
16
Google Play App Store
The Basics of Go Mobile / SDK App example: Ivy

17. Slide URL: https://goo.gl/cIvTb5
gomobile bind
● Generate an Android Archive (.aar)
○ Including a shared library (.so) written in Go
○ Including a JAR file which is bult Java bindings
● Develop with Android Studio Plugin
○ Runs gomobile bind
○ Links to a generated .aar file
$ gomobile bind [-target ios|android] mypkg
17The Basics of Go Mobile / gomobile bind

18. Slide URL: https://goo.gl/cIvTb5
Contents of AAR
18The Basics of Go Mobile / Contents of AAR
$ gomobile bind sample
$ unzip -Z1 sample.aar
AndroidManifest.xml
proguard.txt
classes.jar
jni/armeabi-v7a/libgojni.so
jni/arm64-v8a/libgojni.so
jni/x86/libgojni.so
jni/x86_64/libgojni.so
R.txt
res/
Compiled Java code
Compiled
Go/C code

19. Slide URL: https://goo.gl/cIvTb5
Binding
Use SDK from Application
19The Basics of Go Mobile / Use SDK from Application
Java code
Application Code
(Java)
C code
Go/cgo
JNI Generated by
gomobile bind
SDK Code
(Go)
cgo

20. Slide URL: https://goo.gl/cIvTb5
Binding Go and Java
20The Basics of Go Mobile / Binding Go and Java
Package Abstrct Class
Struct Inner Class
Struct Field
Getter／Setter
(Native)
Method Method
(Native)
Pacakge Function Static Method
Go Java

21. Slide URL: https://goo.gl/cIvTb5
Binding Go and Java
21The Basics of Go Mobile / Binding Go and Java
package sample
func Hello() string { return "Hello" }
type MyStruct struct { Str string }
func (s MyStruct) MyMethod() string { return s.Str }
public abstract class Sample {
// ...
private Sample() {} // uninstantiable
public static final class MyStruct extends Seq.Proxy {
public final native String getStr();
public final native void setStr(String v);
public native String MyMethod();
// ...
}
public static native String Hello();
}
Java
Go
Struct
Field
Method
Package Function

22. Slide URL: https://goo.gl/cIvTb5
Type restrictions
The Basics of Go Mobile / 22
● Signed integer and floating point type
● String and boolean type
● Byte slice type
● Any functions
○ parameter and result types must be supported types
○ results are 0, 1 or 2 (2nd result must be an error type)
● Any struct type
○ all fields and methods must be supported types
● Any interface
○ all methods must be supported types

23. Slide URL: https://goo.gl/cIvTb5
Go
aar file
SDK Apps and Native Apps
Binding Classes (Java)
Shared library (.so)
Java
● SDK Apps for Android
● Native Apps for Android
apk file
Go
GoNativeActivity
Shared library (.so)
UI, IAB, ...
As a library
UI, audio, ...
gomobile bind
gomobile build
The Basics of Go Mobile / SDK Apps and Native Apps 23

24. Slide URL: https://goo.gl/cIvTb5
gomobile build and gomobile install
● Generate an .apk file
○ Including a shared library (.so) written in Go
○ Including a dex file which is bult GoNativeActivity
● Build and Install
○ Run gomobile build and adb install
○ Android Only
$ gomobile build [-target ios|android] mainpkg
24The Basics of Go Mobile / gomobile build and gomobile install
$ gomobile install [-target ios|android] mainpkg

25. Slide URL: https://goo.gl/cIvTb5
Contents of APK
25The Basics of Go Mobile / Contents of APK
$ gomobile build golang.org/x/mobile/example/flappy
$ unzip -Z1 flappy.apk
AndroidManifest.xml
classes.dex
lib/armeabi-v7a/libflappy.so
lib/arm64-v8a/libflappy.so
lib/x86/libflappy.so
lib/x86_64/libflappy.so
assets/README
assets/sprite.png
META-INF/MANIFEST.MF
META-INF/CERT.SF
META-INF/CERT.RSA
Compiled GoNativeActivity.java
Compiled
Go/C code

26. Slide URL: https://goo.gl/cIvTb5
x/mobile/app
How Native App works?
26The Basics of Go Mobile / How Native App works?
GoNativeActivity
C code
Go/cgo
JNI
Application Code
(Go)
cgo
exntends
NativeActivity

27. Slide URL: https://goo.gl/cIvTb5
Go for Mobile Game
27
● Basic event loop and paint events
● Size Event and Lifecycle Event
● 2D Scene Graph
● Touch Event

28. Slide URL: https://goo.gl/cIvTb5
Go for Mobile Game
Go for Mobile Game / Go for Mobile Game 28
● Flappy Gopher
○ by Andrew Gerrand
○ for Go Conference 2015 Winter
○ Source Code
● How it works?
○ gomobile build
○ render images and animate
○ handle touch events
$ go get golang.org/x/mobile/cmd/gomobile && gomobile init
$ gomobile install golang.org/x/mobile/example/flappy

29. Slide URL: https://goo.gl/cIvTb5
Packages
● Rendering
○ OpenGL ES2 : gl, exp/gl/glutil
○ 2D Scene Graph : exp/sprite
● Event
○ Touch Event : event/touch
○ Lifecycle Event : event/lifecycle
● Sensors : exp/sensor
○ Accelerometer, Gyroscope, Magnetometer
● Audio : exp/audio
Go for Mobile Game / Packages 29
Package Name

30. Slide URL: https://goo.gl/cIvTb5
app.Main
● app.Main receives an entry point function
Go for Mobile Game / app.Main 30
func main() {
app.Main(func(a app.App) {
// Event Loop
})
}

31. Slide URL: https://goo.gl/cIvTb5
Event Loop
● Receive events through a channel
Go for Mobile Game / Event Loop 31
Event Loop
Paint Event
Touch Event
Life Cycle Event
Switch
by Event Types
CASE
CASE
CASE

32. Slide URL: https://goo.gl/cIvTb5
Event Channel
● Receive events from App.Events() channel
Go for Mobile Game / Event Channel 32
// Events() <-chan interface{}
for e := range a.Events() { // a is app.App
switch e := a.Filter(e).(type) {
case paint.Event:
case touch.Event:
case lifecycle.Event:
}
}

33. Slide URL: https://goo.gl/cIvTb5
Event Types
Go for Mobile Game / Event Types 33
paint.Event Rendering event
touch.Event Screen touch event
lifecycle.Event
App’s lifecycle event
Such as OnStart and OnStop on Android
size.Event
dimension, physical resolution and orientation
changed event
mouse.Event Mouse event
key.Event Only hardware keyboard event

34. Slide URL: https://goo.gl/cIvTb5
How to render images
Go for Mobile Game / How to render images 34
● Construct a scene graph
○ exp/sprite package provides 2D scene graph
● Load a texture
○ asset package loads a image as a texture
● Set a sub-texture to a node
○ A node of scene graph can be set a sub-texture (part of
a texture)
● Render a scene graph
○ sprite.Engine type has Render method

35. Slide URL: https://goo.gl/cIvTb5
2D scene graph
Go for Mobile Game / 2D scene graph 35
scene
gopher
Rendering
Engine
ground1
Node
TextureSub-texture
Game SceneScene Graph Render
Set Load

36. Slide URL: https://goo.gl/cIvTb5
Construct a scene graph
● Create a rendering engine
● Create and register a node
● Append a child node
Go for Mobile Game / Construct a scene graph 36
images = glutil.NewImages(glctx)
eng = glsprite.Engine(images)
node := &sprite.Node{}
eng.Register(node)
parentNode.AppendChild(childNode)
glctx is
a OpenGL context
images maintains
shared state used
by texutres

37. Slide URL: https://goo.gl/cIvTb5
Load a texture
● Open an asset
● Decode an image
● Load a texture to rendering engine
Go for Mobile Game / Load a texture 37
a, err := asset.Open("gopher.png")
img, err := image.Decode(a)
tex, err := eng.LoadTexture(img)
asset files are put into
assets directory
use image package from
standard packages

38. Slide URL: https://goo.gl/cIvTb5
Set a sub-texture
● Create a sub-texture
● Set a sub-texture to a node
Go for Mobile Game / Set a sub-texture 38
subtex := sprite.Subtex {
T:tex,
R:iamge.Rect(128, 0, 128, 128)
}
eng.SetSubTex(node, subtex)
Bounds on a source texture

39. Slide URL: https://goo.gl/cIvTb5
Render a scene graph
● Render receives a root node of a scene graph
Go for Mobile Game / Render a scene graph 39
// compute current frame in 60FPS
since := time.Since(startTime)
now := clock.Time(since * 60 / time.Second)
// scene *sprite.Node
// now clock.Time
// sz event.Size
eng.Render(scene, now, sz)
Root Node
Screen Size and Orientation

40. Slide URL: https://goo.gl/cIvTb5
Handling Events
Go for Mobile Game / Handling Events 40
Handle Size Event
Handle Lifecycle Event
Handle Paint Event
Get Screen Size and
Orientation
Get OpenGL Context
and Construct a Scene Graph
Render a Scene Graph

41. Slide URL: https://goo.gl/cIvTb5
Handle Size Event
Go for Mobile Game / Handle Size Event 41
app.Main(func(a app.App) {
var glctx gl.Context
var sz size.Event
for e := range a.Events() {
switch e := a.Filter(e).(type) {
case size.Event:
sz = e
// ...
}
}
})
type Event struct {
WidthPx, HeightPx int
WidthPt, HeightPt geom.Pt
PixelsPerPt float32
Orientation Orientation
}

42. Slide URL: https://goo.gl/cIvTb5
Handle Lifecycle Event
Go for Mobile Game / Handle Lifecycle Event 42
case lifecycle.Event:
switch e.Crosses(lifecycle.StageVisible) {
case lifecycle.CrossOn:
glctx, _ = e.DrawContext.(gl.Context)
onStart(glctx)
a.Send(paint.Event{})
case lifecycle.CrossOff:
onStop()
glctx = nil
}
Get a OpenGL Context
StageDead
StageAlive
StageVisible
StageFocused

43. Slide URL: https://goo.gl/cIvTb5
Handle Lifecycle Event
Go for Mobile Game / Handle Lifecycle Event 43
StageDead
StageAlive
StageVisible
StageFocused
From
To
type Event struct {
From, To Stage
DrawContext interface{}
}
lifecycle.Event
e.Crosses(StageVisible)
== CrossOn

44. Slide URL: https://goo.gl/cIvTb5
Handle Lifecycle Event
Go for Mobile Game / Handle Lifecycle Event 44
StageDead
StageAlive
StageVisible
StageFocused
To
From
type Event struct {
From, To Stage
DrawContext interface{}
}
lifecycle.Event
e.Crosses(StageAlive)
== CrossOff

45. Slide URL: https://goo.gl/cIvTb5
Handle Paint Event
Go for Mobile Game / Handle Paint Event 45
Arrange Nodes
Set Sub-texture
Render Scene Graph
Set an Affine Transformation
Matrix Relative to Parent Node
Change images for animations

46. Slide URL: https://goo.gl/cIvTb5
Arrange a Scene Graph Node
Go for Mobile Game / Arrange a Scene Graph Node 46
type Arranger interface{
Arrange(e Engine, n *Node, t clock.Time)
}
// For convinience, arrangeFunc implements Arrange interface
type arrangeFunc func(e Engine, n *Node, t clock.Time)
func (f arrangeFunc) Arrange(e Engine, n *Node, t clock.Time) {
f(e, n, t)
}
● sprite.Arrranger has Arrange method
which is called each frame by rendering engine

47. Slide URL: https://goo.gl/cIvTb5
Set Arranger to a Scene Graph Node
Go for Mobile Game / Set Arranger to a Scene Graph Node 47
// Called each frame
func arrange(e sprite.Engine,
n *sprite.Node,
t clock.Time) {
// Node manupulation
}
// Set a Arranger interface to a node
node.Arranger = arrangerFunc(arrange)

48. Slide URL: https://goo.gl/cIvTb5
Affine Transformation Matrix
Go for Mobile Game / Affine Transformation Matrix 48
// Scale(2,2) and Translate(5,5)
eng.SetTransform(node, f32.Affine{
{2, 0, 5},
{0, 2, 5},
})
● A scene graph node can set a affine transform
matrix to compute position, scale and rotation
angle

49. Slide URL: https://goo.gl/cIvTb5
Affine Transform
Go for Mobile Game / Affine Transform 49
● Affine transform from the root node to the child
node
root
n
Translate(5,5)
Scale(2,2)
Translate(10,10)
Scale(100,100)
pos: (5,5)
size: 2pt x 2pt
pos: (25,25)
size: 200pt x 200pt
1pt = 1/72 inch
Base size is 1pt x 1pt
Base position is (0,0)
Base rotation angle is 0

50. Slide URL: https://goo.gl/cIvTb5
Affine Transform
Go for Mobile Game / Affine Transform 50
root
n
pos: (5,5)
size: 2pt x 2pt
pos: (25,25)
size: 200pt x 200pt
eng.SetTransform(root,
f32.Affine{
{2, 0, 5},
{0, 2, 5},
})
eng.SetTransform(n,
f32.Affine{
{100, 0, 10},
{0, 100, 10},
})
pos: (0,0)
size: 1pt x 1pt

51. Slide URL: https://goo.gl/cIvTb5
Handle Touch Event
Go for Mobile Game / Handle Touch Event 51
type Event struct {
// Touch location in pixels.
X, Y float32
// Sequence number for each touch.
Sequence Sequence
// Type is the touch type.
// TypeBegin, TypeMove, TypeEnd
Type Type
}
touch.Event

52. Slide URL: https://goo.gl/cIvTb5
Touch Types and Sequence
Go for Mobile Game / Touch Types and Sequence 52
TypeBegin TypeMove
Same Sequence
TypeEnd
Sequence:1
Multi Touch
Sequence:2

53. Slide URL: https://goo.gl/cIvTb5
Debug on PC
● Go Mobile’s packages also works fine on PC
(Mac, Windows, Linux).
Go for Mobile Game / Debug on PC 53
$ go build mainpkg
$ ./mainpkg
assets directry should be
placed at working directory
Running on Mac

54. Slide URL: https://goo.gl/cIvTb5
Restrictions for Native Apps (Android)
● Cannot set own App’s icon
● Only use a debug keystore
● Cannot distribute on Google Play
○ Need to set own App’s icon
○ Need a release keystore
● Cannot use Platform API
○ Cannot use In-app Billing API
○ Cannot use third party API
Go for Mobile Game / Debug on PC 54

55. Slide URL: https://goo.gl/cIvTb5
Advanced Topics
55
● Distribute apps on Google Play
● Use Platform APIs from Go

56. Slide URL: https://goo.gl/cIvTb5
Contents of APK
56Advanced Topics / Contents of APK
$ gomobile build golang.org/x/mobile/example/flappy
$ unzip -Z1 flappy.apk
AndroidManifest.xml
classes.dex
lib/armeabi-v7a/libflappy.so
lib/arm64-v8a/libflappy.so
lib/x86/libflappy.so
lib/x86_64/libflappy.so
assets/README
assets/sprite.png
META-INF/MANIFEST.MF
META-INF/CERT.SF
META-INF/CERT.RSA
res directory is not
included into APK
Signature Related Files

57. Slide URL: https://goo.gl/cIvTb5
Modify APK for Release
57Advanced Topics / Modify APK for Release
gomobile build● Create an APK file for debug
● Expand the APK file by apktool
● Modify the AndroidManifest
○ Add icon setting and Disable debuggable setting
● Add app’s icons and res directory
● Rearchive an APK file by apktool
● Signature with release keystore
● ZIP Align

58. Slide URL: https://goo.gl/cIvTb5
Expand an APK File
Advanced Topics / Expand an APK File 58
# if you have not installed
$ brew install apktool
$ apktool d flappy.apk # expand
● Use apktool to expand and archive an APK
file

59. Slide URL: https://goo.gl/cIvTb5
Add App’s Icons
Advanced Topics / Add App’s Icons 59
$ tree flappy/res
flappy/res
├── drawable-hdpi
│ └── ic_launcher.png
...
└── drawable-xxxhdpi
└── ic_launcher.png
● Make a res direcotry and add app’s icons
○ under the expanded APK directory
App’s Icons

60. Slide URL: https://goo.gl/cIvTb5
Add icon setting
● Add an icon setting to AndroidManifest
● android:icon sepecify icon file name
Advanced Topics / Add icon setting 60
<?xml version="1.0" encoding="utf-8"?>
...
<application android:label="Flappy"
android:icon="@drawable/ic_launcher"
...

61. Slide URL: https://goo.gl/cIvTb5
Disable debuggable setting
● To upload apk on Google Play, apk should be
built without debuggable="true"
○ We can also use own AndroidManifest.xml by
putting top level directory before building.
Advanced Topics / Disable debuggable setting 61
<?xml version="1.0" encoding="utf-8"?>
...
<application android:label="Flappy" ...
android:debuggable="true"
...

62. Slide URL: https://goo.gl/cIvTb5
Signiture with Release Keystore
62Advanced Topics / Signiture with Release Keystore
● Re-archive the APK file
● Re-signiture the APK file
$ apktool b flappy
$ cd flappy/dist
$ jarsigner ... -keystore release.keystore
flappy.apk flappy
$ zipalign -f -v 4 flappy{,_aligned}.apk
$ ls
flappy.pak flappy_aligned.apk

63. Slide URL: https://goo.gl/cIvTb5
Distribute on Google Play
63Advanced Topics / Distribute on Google Play

64. Slide URL: https://goo.gl/cIvTb5
Restrictions for Native App (Android)
● Cannot set own App’s icon
● Only use a debug keystore
● Cannot distribute on Google Play
○ Need to set own App’s icon
○ Need a release keystore
● Cannot use Platform API
○ Cannot use In-app Billing API
○ Cannot use third party API
Advanced Topics / Debug on PC 64
OK!
OK!
OK!

65. Slide URL: https://goo.gl/cIvTb5
Requirement Mobile Games for Business
● In-app Billing
○ Purchase a items in the game
● SNS connection
○ Facebook, Twitter, ...
● Advertisements
● Analytics
○ Google Analytics, Firebase, Facebook Analytics,...
Advanced Topics / Requirement of Mobile Games for Business 65
These APIs are provided
as Java SDK for Android

66. Slide URL: https://goo.gl/cIvTb5
How to call Android API from Go
Advanced Topics / How to call Android API from 66
● Call Android API through cgo by JNI
○ We should get JavaVM* object in own cgo code
● JavaVM* object is defined in Go Mobile inner
package as current_vm variable
○ x/mobile/internal/mobileinit/ctx_android.go
● We also use current_vm in own cgo code

67. Slide URL: https://goo.gl/cIvTb5
Call android.util.Log.d
Advanced Topics / Call android.util.Log.d 67
RunOnJVM(func(vm, jniEnv, ctx uintptr) error {
env := (*C.JNIEnv)(unsafe.Pointer(jniEnv))
C.logd(env)
return nil
})
void logd(JNIEnv* env) {
jclass log = (*env)->FindClass(env, "android/util/Log");
jmethodID methodD = (*env)->GetStaticMethodID(
env, log, "d", "(Ljava/lang/String;Ljava/lang/String;)I");
jstring tag = (*env)->NewStringUTF(env, "log");
jstring message = (*env)->NewStringUTF(env, "hi! JVM");
(*env)->CallStaticIntMethod(env, log, methodD, tag, message);
}
Go
C
All Source Code
RunOnJVM is copied from
ctx_android.go

68. Slide URL: https://goo.gl/cIvTb5
Call own Java Codes from Go codes
Advanced Topics / Call own Java Codes from Go codes 68
● Create a new Android project
● Copy GoNativeActivity and AndroidManifest
from GoMobile repositry to own project
● Write own Java codes
● Write application code in Go
○ Call own Java codes with RunOnJVM
● Build Go code to a shared library
● Copy shared library to own Android project
● Build the Android project Do all processes by hand!

69. Slide URL: https://goo.gl/cIvTb5
Toast on Go Mobile
Advanced Topics / Toast on Go Mobile 69

70. Slide URL: https://goo.gl/cIvTb5
Plugin System for Game Engine
Advanced Topics / Plugin System for Game Engine 70
● Most Game Engine has a plugin system
○ Unity, Cocos2d-x, ...
○ Facebook Plugin, In-app Billing Plugin, ...
● Go Mobile needs a plugin system
Application Code
(Go)
Own Plugin
Java/Obj-C code
C code
Go/cgo
Go Mobile
Register
Use

71. Slide URL: https://goo.gl/cIvTb5
Summaries
● The Basics of Go Mobile
○ Cross-compile and Android device
○ SDK App and Native App
● Go for Mobile Game
○ Event handling
○ 2D scene graph
● Advanced Topics
○ Distribute apps on Google Play
○ How to use Android API from Go
71

72. Slide URL: https://goo.gl/cIvTb5
Go for Unity Native Plugin
● Unity Native Plugin can be written in Go/cgo
Advanced Topics / Go for Unity Native Plugin 72
Unity
Native Plugin
.so for Android
.a for iOS
Written in GoGo has good packages
to create a communication module!

73. Slide URL: https://goo.gl/cIvTb5
Go for Unity Native Plugin
Advanced Topics / Go for Unity Native Plugin 73
package main
import "C"
//export Hoge
func Hoge() int {return 100}
func main() {}
#if UNITY_IPHONE
[DllImport("__Internal")]
#else
[DllImport("hoge")]
#endif
private static extern int Hoge();
void Start() {Debug.LogFormat("{0}", Hoge());}
Go
Unity (C#)

74. Slide URL: https://goo.gl/cIvTb5
Build a Native Plugin for Android
Advanced Topics / Build a Native Plugin for Android 74
$ CGO_ENABLED=1
CC=arm-linux-androideabi-gcc
GOOS=android
GOARCH=arm
GOARM=7
go build -buildmode=c-shared -pkgdir=
$GOPATH/pkg/gomobile/pkg_android_arm
-o libhoge.so hoge.go

75. Slide URL: https://goo.gl/cIvTb5
Build a Native Plugin for iOS
Advanced Topics / Build a Native Plugin for iOS 75
GOOS=darwin
GOARCH=arm
GOARM=7
CC=`xcrun --sdk iphoneos -f clang`
CXX=`xcrun --sdk iphoneos -f clang`
CGO_CFLAGS="-isysroot `xcrun --sdk iphoneos --show-sdk-
path` -arch armv7 -miphoneos-version-min=6.1"
CGO_LDFLAGS="-isysroot `xcrun --sdk iphoneos --show-sdk-
path` -arch armv7 -miphoneos-version-min=6.1"
CGO_ENABLED=1
go build -pkgdir=$GOPATH/pkg/gomobile/pkg_darwin_arm
-buildmode=c-archive -tags=ios -o hoge.a hoge.go

76. Takuya Ueda
@tenntenn
KLab Inc.
76

77. Takuya Ueda
@tenntenn
KLab Inc.
77

78. Takuya Ueda
@tenntenn
KLab Inc.
78

79. Slide URL: https://goo.gl/cIvTb5
Run on Android Devices
79
● Go can build for Android Devices
Android DevicePC
Cross-compile Run on adb shellpush binary
to Android Deivce
$ adb push cmd /sdcard
The Basics of Go Mobile / Run on Androind Devices

80. Slide URL: https://goo.gl/cIvTb5
Run on Android Devices
Watch at Youtube Source Code
Android
Shell on Mac
adb shell
The Basics of Go Mobile / Run on Androind Devices 80

81. Slide URL: https://goo.gl/cIvTb5
Handle Lifecycle Event
Go for Mobile Game / Handle Lifecycle Event 81
case lifecycle.Event:
switch e.Crosses(lifecycle.StageVisible) {
case lifecycle.CrossOn:
glctx, _ = e.DrawContext.(gl.Context)
onStart(glctx)
a.Send(paint.Event{})
case lifecycle.CrossOff:
onStop()
glctx = nil
}
StageDead
StageAlive
StageVisible
StageFocused