Multiplatform Library Development

Multiplatform Kotlin facilitates code-sharing by making platform-agnostic portions of the standard library available in common code that is written once but can run on any target. As Multiplatform development really starts to take off over the next year, there must also be a robust ecosystem of third party libraries available to application developers.

I’ll talk through my experiences building one of the early libraries in the mobile Multiplatform space, with lessons on how to find shared abstractions around different platform APIs, how to handle the fast-paced evolution of this environment, and what this all felt like as a first-time library developer. When we're done, you’ll be ready to leverage the growing ecosystem as well as make your own contributions.

Transcript

1. Multiplatform Library Development Russell Wolf 8/27/19
2. About Me • @RussHWolf ( or ) • Android Developer at Rocket Insights • Organizer of Kotlin Office Hours in Boston • Kotlin/Everywhere Nov 8-9 • Author of Multiplatform Settings
3. Multiplatform Kotlin • Compile common code to multiple targets • JVM, JS, Android, Desktop, iOS, Embedded, WASM • Use platform-specific code to access platform APIs
4. Kotlin/Native • Compile Kotlin to LLVM targets • No VM! • Desktop, Embedded, Mobile • Interop with C or Objective-C
5. Native != Multiplatform
6. Mobile Multiplatform • Android (JVM) / iOS (Native) • Mobile is the killer app for Multiplatform • Same use-case • Similar capabilities
7. Multiplatform Code
8. Common Code class CommonFoo {
 fun bar(list: List<String>) {
 list.forEach { println(it) }
 }
 }
9. Platform-Specific Code expect val platform: String actual val platform = "Android" actual val platform = "iOS"
10. Platform-Specific Code interface Foo { ... } class AndroidFoo : Foo { ... } class IosFoo : Foo { ... } class SwiftFoo : NsObject, Foo { ... } class MockFoo : Foo { ... }
11. Architecture
12. Architecture Common
13. Architecture Common iOS Framework Android AAR
14. Architecture Common iOS Framework Android AAR JS Java JAR
15. Architecture Common iOS Framework Android AAR Android App iOS App
16. ... ... Architecture Common iOS Framework Android AAR Android App iOS App
17. Architecture Common iOS Framework Android AAR ... ... Android App iOS App ...
18. Architecture ...
19. History
21. • https://github.com/russhwolf/ multiplatform-settings • Key-value storage based on platform APIs • Operators and Property Delegates Multiplatform Settings
22. Multiplatform Settings interface Settings {
 fun putInt(key: String, value: Int)
 } 
 class AndroidSettings(
 val delegate: SharedPreferences
 ) : Settings { 
 override fun putInt(…) = delegate.putInt(…)
 } 
 class AppleSettings(
 val delegate: NSUserDefaults
 ) : Settings {
 override fun putInt(…) = delegate.setInteger(…)
 }
23. Multiplatform Settings interface Settings {
 fun putInt(key: String, value: Int)
 }
 class MockSettings(
 val delegate: MutableMap<String, Any>
 ) : Settings { 
 override fun putInt(…) = delegate.set(…)
 }
 
 
 
 

24. Multiplatform Settings interface Settings {
 fun putInt(key: String, value: Int)
 }
 class JsSettings(
 val delegate: Storage = localStorage
 ) : Settings { 
 override fun putInt(…) = delegate.set(…)
 }
 class JvmSettings(
 val delegate: Properties
 ) : Settings {
 override fun putInt(…) = delegate.setProperty(…)
 }
25. Multiplatform Settings operator fun Settings.set( key: String, value: Int ): Unit = putInt(key, value) settings["a"] = 3 fun Settings.int( key: String? = null, defaultValue: Int = 0 ): ReadWriteProperty<Any?, Int> = ... var a by settings.int("a")
26. Lessons
27. Expect/Actual vs Interface expect class Settings {
 fun putInt(key: String, value: Int)
 }
 actual class Settings(
 val delegate: SharedPreferences
 ) { 
 actual fun putInt(…) = delegate.putInt(…)
 }
 actual class Settings(
 val delegate: NSUserDefaults
 ) {
 actual fun putInt(…) = delegate.setInteger(…)
 }
28. Expect/Actual vs Interface interface Settings {
 fun putInt(key: String, value: Int)
 } expect class PlatformSettings: Settings {
 override fun putInt(key: String, value: Int)
 }
 actual class PlatformSettings(
 val delegate: SharedPreferences
 ) : Settings { 
 actual fun putInt(…) = delegate.putInt(…)
 }
 actual class PlatformSettings(…) { … }
29. Expect/Actual vs Interface interface Settings {
 fun putInt(key: String, value: Int)
 } 
 class AndroidSettings(
 val delegate: SharedPreferences
 ) : Settings { 
 override fun putInt(…) = delegate.putInt(…)
 } 
 class AppleSettings(
 val delegate: NSUserDefaults
 ) : Settings {
 override fun putInt(…) = delegate.setInteger(…)
 }
30. Testing • Improve API coverage of tests • Fake/In-memory implementation (MockSettings) • Share tests
31. Testing settings :main
32. Testing settings :main settings :test
33. Testing settings :main settings :test mock :main mock :test
34. Testing settings :main settings :test mock :main mock :test tests :main
35. Testing settings :main settings :test mock :main mock :test tests :main
36. Listener APIs
37. Listener APIs • SharedPreferences
 .OnSharedPreferenceChangeListener • Key-specific • Might get called for repeated values • NSNotificationCenter
 NSUserDefaultsDidChangeNotification • Can't tell what changed
38. Listener APIs val prev = cache.value val current = delegate.all[key] if (prev != current) { callback.invoke() cache.value = current }
 val prev = cache.value val current = delegate.objectForKey(key) if (prev != current) { callback.invoke() cache.value = current }
39. Listener APIs val prev = cache.value val current = delegate.all[key] if (prev != current) { callback.invoke() cache.value = current }
 val prev = cache.value val current = delegate.objectForKey(key) if (prev != current) { callback.invoke() cache.value = current } ??? ???
40. Listener APIs val prev = cache.value val current = delegate.all[key] if (prev != current) { callback.invoke() cache.value = current }
 val prev = cache.value val current = delegate.objectForKey(key) if (prev != current) { callback.invoke() cache.value = current } ??? ???
41. Listener APIs interface ObservableSettings : Settings { … } class AndroidSettings(…) : ObservableSettings { … } class JsSettings(…) : Settings { … }
42. Experimental @Experimental annotation class ExperimentalListener
 
 @ExperimentalListener interface ObservableSettings : 
 Settings { … }
 
 @Experimental annotation class ExperimentalJs @ExperimentalJs class JsSettings(…) : 
 Settings { … }
43. Experimental @Experimental(level = WARNING) annotation class ExperimentalListener
 
 @ExperimentalListener interface ObservableSettings : 
 Settings { … }
 
 @Experimental(level = WARNING) annotation class ExperimentalJs @ExperimentalJs class JsSettings(…) : 
 Settings { … }
44. Experimental • @ExperimentalFoo vs @UseExperimental(ExperimentalFoo::class) • @UseExperimental will not leak to callers
45. PRs
46. PRs val a by settings.int("a")
47. PRs val a by settings.int("a") val a by settings.int()
48. PRs val a by settings.int("a") val a by settings.int() val a: Int by settings()
49. PRs
50. Gradle • Android • publishAllLibraryVariants() • iOS • exec { commandLine(
 "xcrun", "simctl", "spawn", 
 "iPhone Xʀ", testBinaryPath
 ) } • JS • browser()
51. Other Notes • Kotlin/Native has no version compatibility! Keep up-to- date to support clients • Gradle is complicated • https://kotlinlang.org/docs/reference/building-mpp- with-gradle.html • Easier than last year
52. Coming Soon • Maven Central • But at least on jcenter now... • CI Build • iOS arm32 • JVM auto-persistence • JVM, JS, macOS samples
53. What about other stuff? • Jetbrains • Coroutines • Serialization • Ktor Client • IO • ... • Community • SqlDelight • Stately • CoroutineWorker • Reaktive • ...
54. Community Libs • https://github.com/topics/kotlin-multiplatform • https://github.com/AAkira/Kotlin-Multiplatform-Libraries • https://github.com/bipinvaylu/awesome-kotlin- multiplatform
55. Yours?
56. Strategies • Wrap platform APIs • Implementation already exists! • Takes effort to equalize behavior/API
57. Strategies • Pure-Kotlin • Effort to create new implementation • All common = all platforms
58. The time is now!
59. Thanks! • Questions? • @RussHWolf ( or ) • https://github.com/russhwolf/multiplatform-settings
 • Shameless plug: Kotlin/Everywhere Boston
 https://sessionize.com/kotlin-in-boston-everywhere