What's new with Kotlin Multiplatform libraries?

How do I create libraries for Kotlin Multiplatform projects? Kotlin lets you share common code between different target platforms, but by default only a limit set of platform-agnostic APIs are available to the code. As Multiplatform development really starts to take off, there must also be a robust ecosystem of third party libraries available to application developers.

I’ll talk through what it looks like to create such a library, drawing from my experience building and maintaining one of my own starting in the early days of Kotlin/Native. We'll talk about 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. What's new with Kotlin Multiplatform libraries? Russell Wolf Oct 8, 2020 @Russhwolf ( or ) #session-chat-d1-s2-t3
2. Kotlin
3. Kotlin • Kotlin/JVM on Android • Interop with Java • Preferred language for Android development
4. Multiplatform Kotlin • Compile common code to multiple targets • JVM, Android, JS, iOS, Desktop, Embedded • Use platform-specific code to access platform APIs • Now in Alpha!
5. Kotlin Mobile Multiplatform • Android (JVM) / iOS (Native) • Mobile is the killer app for Multiplatform • Same use-case • Similar capabilities
6. Shared Code
7. Shared Code Android Common iOS
8. Shared Code Android Common iOS
9. Shared Code Android Common iOS
10. Shared Code Android Common iOS
11. Shared Code Android Common iOS
12. Multiplatform Code
13. Common Code // Common
 class Thing(val data: List<String>) { fun doSomething() { data.forEach { println(it) } } }
14. Platform-Specific Code // Common
 expect val platform: String
 // Android
 actual val platform = "Android" 
 // iOS
 actual val platform = "iOS"
15. Platform-Specific Code // Common
 interface Logger { fun log(message: String) }
 // Android
 class AndroidLogger : Logger { ... }
 // iOS - Kotlin
 class IosLogger { ... }
 // iOS - Swift class SwiftLogger : Logger { ... } // Common
 class TestLogger : Logger
16. Architecture
17. Common iOS Framework Android AAR Architecture
18. Common iOS Framework Android AAR Android App iOS App Architecture
19. ... ... Common iOS Framework Android AAR Android App iOS App Architecture
20. Common iOS Framework Android AAR ... ... Android App iOS App ... Architecture
21. ... Architecture
22. In the Beginning...
24. • Key-value storage based on platform APIs • Operators and Property Delegates • https://github.com/russhwolf/ multiplatform-settings Multiplatform Settings
25. Multiplatform Settings // Common interface Settings { fun putInt(key: String, value: Int) }
 
 // Android class AndroidSettings( val delegate: SharedPreferences ) : Settings { override fun putInt(key: String, value: Int) = delegate.edit().putInt(key, value).apply() }

26. Multiplatform Settings // Common interface Settings { fun putInt(key: String, value: Int) }
 // iOS class AppleSettings( val delegate: NSUserDefaults ) : Settings { override fun putInt(key: String, value: Int) = delegate.setInteger(value, key) }

27. Multiplatform Settings // Common interface Settings { fun putInt(key: String, value: Int) }
 
 // JS class JsSettings( val delegate: Storage = localStorage ) : Settings { override fun putInt(key: String, value: Int) = delegate[key] = value.toString() }

28. Multiplatform Settings // Common interface Settings { fun putInt(key: String, value: Int) }
 
 // JVM class JvmPreferencesSettings( val delegate: Preferences ) : Settings { override fun putInt(key: String, value: Int) = delegate.putInt(key, value) }

29. Multiplatform Settings // Common interface Settings { fun putInt(key: String, value: Int) }
 
 // Windows class WindowsSettings( val rootKeyName: String ) : Settings { override fun putInt(key: String, value: Int) = putRegistryValue(key, REG_DWORD) { 
 alloc<ULONGVar> { this.value = value.toUInt() } 
 } }
30. Multiplatform Settings // Common interface Settings { fun putInt(key: String, value: Int) }
 
 // Common (Tests) class MockSettings( val delegate: MutableMap<String, Any> ) : Settings { override fun putInt(key: String, value: Int) = delegate[key] = value }

31. Multiplatform Settings // Common
 operator fun Settings.set(
 key: String, 
 value: Int
 ) = putInt(key, value) settings["a"] = 3 fun Settings.int(
 key: String? = null, 
 defaultValue: Int = 0
 ): ReadWriteProperty<Any?, Int> = ... var a by settings.int("a")
32. Stories & Lessons
33. 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(…) }
34. 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(…) { … }
35. 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(…) }
36. Listener APIs
37. Listener APIs • SharedPreferences .OnSharedPreferenceChangeListener • Passes key to callback • Might get called for repeated values • NSNotificationCenter NSUserDefaultsDidChangeNotification • Can't tell what changed
38. Listener APIs val current = delegate.all[key] if (prev != current) { callback.invoke() prev = current } val current = delegate.objectForKey(key) if (prev != current) { callback.invoke() prev = current }
39. Listener APIs val current = delegate.all[key] if (prev != current) { callback.invoke() prev = current } val current = delegate.objectForKey(key) if (prev != current) { callback.invoke() prev = current } ???
40. Listener APIs val current = delegate.all[key] if (prev != current) { callback.invoke() prev = current } val current = delegate.objectForKey(key) if (prev != current) { callback.invoke() prev = current } ???
41. Listener APIs interface ObservableSettings : Settings { … } class AndroidSettings(…) : ObservableSettings { … } class JsSettings(…) : Settings { … }
42. Nullable Getters fun getInt(
 key: String,
 defaultValue: Int = 0
 ): Int
 
 fun getIntOrNull(
 key: String
 ): Int?
43. Testing
44. Testing settings :main
45. Testing settings :main settings :test
46. Testing settings :main settings :test mock :main mock :test
47. Testing settings :main settings :test mock :main mock :test
48. Testing settings :main settings :test mock :main mock :test
49. Testing settings :main settings :test mock :main mock :test tests :main
50. Testing settings :main settings :test mock :main mock :test tests :main
51. So what's new?
52. Gradle Metadata and Hierarchical Projects
53. Gradle Metadata and Hierarchical Projects Common iOS Android
54. Gradle Metadata and Hierarchical Projects Common iOS Android
55. Gradle Metadata and Hierarchical Projects Common iOS Arm64 Android iOS Arm32 iOS X64
56. Gradle Metadata and Hierarchical Projects Common Android iOS iOS Arm64 iOS Arm32 iOS X64
57. Gradle Metadata and Hierarchical Projects Common iOS Arm32 Android iOS Arm64 iOS X64 iOS iOS 64-bit iOS 32-bit
58. Gradle Metadata and Hierarchical Projects kotlin.mpp.enableGranularSourceSetsMetadata=true
 kotlin.native.enableDependencyPropagation=false
 kotlin.mpp.enableCompatibilityMetadataVariant=true
59. Doc updates • Updated multiplatform docs page
 https://kotlinlang.org/docs/reference/mpp-intro.html • 1.4 Migration docs
 https://kotlinlang.org/docs/reference/migrating- multiplatform-project-to-14.html • KMM Portal
 https://kotlinlang.org/lp/mobile/
60. Coming Soon • Flow and Serialization extensions • Other Platforms/Implementations • Apple Keychain • Linux • DataStore?
61. What about other stuff? • Jetbrains • Coroutines • Serialization • Ktor Client • Datetime • IO • AtomicFU
 
 
 • Community • https://github.com/ AAkira/Kotlin- Multiplatform-Libraries • https://kotlinlang.org/ lp/mobile/ecosystem/
62. Yours?
63. Strategies • Wrap platform APIs • Implementation already exists! • Takes effort to equalize behavior/API
64. Strategies • Pure-Kotlin • Effort to create new implementation • All common = all platforms
65. The time is now!
66. Thanks! • Questions? • @RussHWolf ( or ) • Multiplatform Settings
 https://github.com/russhwolf/multiplatform-settings
 • Kotlin Mobile Multiplatform
 https://kotlinlang.org/lp/mobile/ • Other community libraries
 https://github.com/AAkira/Kotlin-Multiplatform-Libraries • Kotlin 1.4 online event Oct 12-15
 https://kotlinlang.org/lp/event-14/