This document summarizes lessons learned from developing the Realm Android library. It discusses challenges such as setting up an Android library project, API design, testing, distribution methods, and issues like annotation processing, bytecode weaving, and native code support. Key points covered are how to start a library project, the importance of testing libraries extensively, and distribution options like Bintray.

1. DEVELOPING ANDROID LIBRARIES:
LESSONS FROM REALM
EMANUELE ZATTIN (@EMANUELEZ)
DEVIEW, SEPTEMBER 14, 2015

2. THANK YOU NAVER!

3. LET me KNOW YOU

4. WHAT IS REALM?

5. REALM IS A
MOBILE
DATABASE

6. REALM IS A
ZERO-COPY
OBJECT STORE

7. REALM FOR ANDROID WAS
FIRST RELEASED ONE YEAR
AGO HERE AT DEVIEW

8. SOME OF OUR USERS*
IN KOREA
*
Find out more at http://realm.io/kr/users

9. WHY WOULD YOU WRITE A
LIBRARY?

10. MODULARITY
The ability to split your code base into several units with well defined
interface. This allows to focus on smaller problems, producing more
testable code

11. REUSABILITY
A consequence of modularity. Now you can use that unit again in several
places in your projects or accross several different ones

12. SHAREABILITY
A consequence or reusability. You found a nice way to solve a particular
issue. Why now helping other developers? Get your name out there!

13. WHY WOULD YOU WRITE AN
ANDROID LIBRARY?

14. ▸ UI
▸ Looper/Handler
▸ Sensors
▸ Native code
▸ Many more!

15. WHY WOULD REALM WRITE
AN ANDROID LIBRARY?

16. ▸ UI
▸ Looper/Handler
▸ Sensors
▸ Native code
▸ Many more!

17. CHALLENGE 1HOW TO START AN ANDROID
LIBRARY PROJECT?

18. THE PROBLEM
ANDROID STUDIO ALLOWS YOU TO:
▸ Create a new Application project
▸ Create a new Library project
▸ Add a new Application module
▸ Add a new Library module

19. SOLUTION 1
From Android Studio:
1. Create a new Application project
2. Add a new Library module
3. Remove the Application module

20. SOLUTION 2
From the command line:
android create lib-project -t 1 -k kr.deview.awesomelib -p . -g -v 1.3.0
-t: target (Use android list targets to get a list of target ids)
-k: package name
-p: path to the project
-g: make it a Gradle project (requires SDK >= 19)
-v: version of the Android Gradle plugin to use

21. CHALLENGE 2API DESIGN

22. SOLUTION
DEFINE WHAT A GOOD API IS

23. ▸ Easy to learn
▸ Easy to use, even without documentation
▸ Hard to misuse
▸ Easy to read and maintain code that uses it
▸ Sufficiently powerful to satisfy requirements
▸ Easy to extend
▸ Appropriate to audience

24. STAND ON THE SHOUDERS OF GIANTS
▸ Effective Java 2 by Joshua Bloch
▸ How To Design A Good API and Why it Matters by Joshua Bloch

25. CHALLENGE 3TESTING

26. TESTING IS EVEN MORE IMPORTANT FOR
LIBRARIES THAN FOR APPLICATIONS
BECAUSE YOU HAVE NO IDEA HOW YOUR
CUSTOMERS ARE GOING TO USE IT.

27. SOLUTION 1
PREFER JUNIT41
OVER JUNIT3
1
Because parametric tests will save your life!

28. SOLUTION 2
AUTOMATE ALL THE THINGS!

29. LET JENKINS BECOME YOUR
BEST FRIEND

30. USEFUL JENKINS PLUGINS
▸ Job Config History
▸ Git
▸ Android Emulator
▸ Matrix Job2
▸ Junit2
2
It comes preloaded with Jenkins

31. SOLUTION 3
WRITE SAMPLE APPS!

32. ▸ Additional integration tests
▸ Showcase how to use your library
▸ Validate your core principles

33. CHALLENGE 4JAR OR AAR?

34. DO YOU WANT TO SUPPORT ECLIPSE?
SHOW ME YOUR HANDS ONCE AGAIN

35. SOLUTION
USE AAR

36. CHALLENGE 5WHERE TO PUBLISH?

37. SOLUTION
BINTRAY

38. HOW TO PRODUCE A SOURCE JAR
task androidSourcesJar(type: Jar) {
from android.sourceSets.main.java.srcDirs
}

39. HOW TO PRODUCE A JAVADOC JAR
android.libraryVariants.all { variant ->
task("javadoc${variant.name.capitalize()}", type: Javadoc) {
description "Generates Javadoc for $variant.name."
group 'Docs'
source = variant.javaCompile.source
ext.androidJar = files(project.android.getBootClasspath())
classpath = files(variant.javaCompile.classpath.files) +
ext.androidJar
exclude '**/BuildConfig.java'
exclude '**/R.java'
}
}

40. Bintray also provides a Gradle plugin for the actual publishing
The configuration is not trivial, and in the beginning it might be easier
to just do the release manually on the binary website

41. CHALLENGE 6INTROSPECTION IS SLOW

42. Sometimes libraries need to be smart behind the scenes to save the
user from writing boilerplate code

43. SOLUTION
ANNOTATION PROCESSING

44. ! PROS:
▸ It allows you to write new Java files
▸ It happens at compilation time
! CONS
▸ It does not allow to modify existing code
▸ It's not very easy to use

45. SOME VERY cool libraries USE IT!
▸ Dagger
▸ Butter Knife
▸ AutoValue/AutoParcel
▸ Realm

46. BAD NEWS
Android does not include the package
javax.annotation.processing

47. WORKAROUND
Create two Java sub-projects:
1. annotations (used both by the library and the processor)
2. annotations processor

48. YOU NEED TO INCLUDE THE ANNOTATIONS IN THE JAVADOC
android.libraryVariants.all { variant ->
task("javadoc${variant.name.capitalize()}", type: Javadoc) {
description "Generates Javadoc for $variant.name."
group 'Docs'
source = variant.javaCompile.source
source "../annotations/src/main/java" // <-- Remember this!
ext.androidJar = files(project.android.getBootClasspath())
classpath = files(variant.javaCompile.classpath.files)
+ ext.androidJar
exclude '**/BuildConfig.java'
exclude '**/R.java'
}
}

49. CHALLENGE 7
SOMETIMES ANNOTATION
PROCESSING IS NOT
POWERFUL ENOUGH

50. ! PROS:
▸ It allows you to write new Java files
▸ It happens at compilation time
! CONS
▸ It does not allow to modify existing code
▸ It's not very easy to use

51. SOLUTION
BYTECODE WEAVING

52. ! PROS:
▸ It allows to modify existing code ❤
▸ It's easier to use compared to Annotation Processing
! CONS
▸ You really need to know what you are doing
▸ It might look weird in the debugger

53. TOOLS THAT ALLOW TO DO BYTECODE WEAVING
▸ Javassist
▸ ASM
▸ AspectJ

54. THANK YOU STEPHANE NICOLAS!
https://github.com/stephanenicolas/injects

55. CHALLENGE 8NATIVE CODE

56. THE BAD NEWS
THE ANDROID GRADLE PLUGIN
DOES NOT SUPPORT NDK
ANYMORE

57. THE GOOD NEWS
GOOGLE IS DEVELOPING AN
EXPERIMENTAL NEW PLUGIN
THAT SUPPORTS NDK

58. HOW TO START USING IT
buildscript {
repositories {
jcenter()
}
dependencies {
classpath 'com.android.tools.build:gradle-experimental:0.2.0'
}
}
apply plugin: 'com.android.model.application'

59. THE DSL IS SLIGHTLY DIFFERENT
model { // <-- This!
android {
compileSdkVersion = 22 // Now it's a property, not a method
buildToolsVersion = "22.0.1" // Same here and the rest of the example
defaultConfig.with { // Use the with method
applicationId = "com.example.user.myapplication"
minSdkVersion.apiLevel = 15 // Use the apiLevel property
targetSdkVersion.apiLevel = 22 // Same here
versionCode = 1
versionName = "1.0"
}
}
}

60. FIND OUT MORE ABOUT IT
http://tools.android.com/tech-docs/new-build-system/gradle-
experimental

61. ONE ANNOYING LIMITATION
NO SUPPORT FOR CREATING
AND DEPENDING ON STATIC
LIBRARIES

62. THANK YOU!

63. REALM 부스에
많이 방문해주세요!
오늘만 열어요