Bottom-Up Code-Sharing with Kotlin Multiplatform

Kotlin Multiplatform enables sharing logic and architecture across platforms while still interacting with each platform’s native APIs. We’ll talk through what this looks like by walking through a sample application. We’ll highlight helpful patterns for both production and test code, as well as available tools and dependencies. Once we’re done you’ll be ready to leverage Kotlin’s code-sharing capabilities in your own projects, and never need to write the same logic twice ever again.

Transcript

1. Bottom-up Code Sharing with Kotlin Multiplatform Russell Wolf 8/14/19
2. About Me • @RussHWolf ( or ) • Android Developer at Rocket Insights • Organizer of Kotlin Office Hours in Boston • Author of Multiplatform Settings
3. Code Sharing
4. Why Share Code? • Duplicated effort • Accidental differences • DRY - Don't Repeat Yourself
5. Why Not Share Code? • Performance issues • New language and API • Less stabile/updated
6. Multiplatform Kotlin • Compile common code to multiple targets • JVM, JS, Android, Desktop, iOS, Embedded, WASM • Use platform-specific code to access platform APIs
7. Kotlin/Native • Compile Kotlin to LLVM targets • No VM! • Desktop, Embedded, Mobile • Interop with C or Objective-C
8. Mobile Multiplatform • Android (JVM) / iOS (Native) • Mobile is the killer app for Multiplatform • Same use-case • Similar capabilities
9. Multiplatform Code
10. A Note on Colors • This presentation is color-coded by platform
 
 Common
 Android
 iOS
 JVM
 Gradle (KTS)
 Swift/SQL/Other
11. Common Code class CommonFoo {
 fun bar(list: List<String>) {
 list.forEach { println(it) }
 }
 }
12. Platform-Specific Code expect val platform: String actual val platform = "Android" actual val platform = "iOS"
13. Platform-Specific Code interface Foo { ... } class AndroidFoo : Foo { ... } class IosFoo : Foo { ... } class SwiftFoo : NsObject, Foo { ... } class MockFoo : Foo { ... }
14. Architecture
15. Architecture Common
16. Architecture Common iOS Framework Android AAR
17. Architecture Common iOS Framework Android AAR Android App iOS App
18. ... ... Architecture Common iOS Framework Android AAR Android App iOS App
19. Architecture Common iOS Framework Android AAR ... ... Android App iOS App ...
21. Soluna • https://github.com/russhwolf/ soluna • Compute rise/set times of Sun and Moon • Use-cases • Calendar • Mobile Notifications • API? • WIP
22. Soluna Core
23. Soluna Core Calendar
24. Soluna Mobile Shared Core Calendar
25. Soluna Mobile Shared Android App iOS App Core Calendar
26. Core • Rise/Set calculation • Compute position of sun/moon at noon in equatorial coordinates • Solve for time when that position is on horizon • DegreeMath • No dates or time zones • TODO - Moon calculation is buggy
27. Core fun sunTimes( year: Int, month: Int, day: Int, offset: Double, // Hours latitude: Double, // Degrees longitude: Double // Degrees ): Pair<Double?, Double?> { // Hours ... return riseTime?.value to setTime?.value }
28. Core interface AngleUnit<T : AngleUnit<T>> { val value: Double } inline class Degree( override val value: Double ) : AngleUnit<Degree> fun sin(degrees: Degree) = kotlin.math.sin(degrees.toRadians().value)
29. Calendar • java.awt drawing commands • java.time date manipulation • TODO • Architecture/Tests • CLI • TornadoFx UI
30. Calendar for (i in 1..numberOfDays) { val date = LocalDate.of(year, month, i) val (sunRiseTime, sunSetTime) = sunTimes(year, month.value, i, 0.0, latitude, longitude) val sunRiseDate: ZonedDateTime = sunRiseTime?.toDateTime(date, timeZone) val sunSetDate: ZonedDateTime = sunSetTime?.toDateTime(date, timeZone) ... }
32. Mobile • Configure alert notifications based on rise/set times • Store configuration in SqlDelight database • Manual coords or geocode via Google Maps API • Schedule and generate alerts using native APIs • Still in progress...
33. Data Models • Location • Label, Latitude, Longitude, Time Zone • Reminder • Type (rise/set), Minutes Before • Many-to-One relationship
34. Mobile Architecture • Database stores locations/ reminders • Google Maps API helps populate location data • Repository & ViewModels • Coroutine architecture Google Api Client Repository ViewModel ViewModel ViewModel Database
35. Google Api Client Repository ViewModel ViewModel ViewModel Database
36. SqlDelight • https://github.com/square/sqldelight/ • Generate model classes and DAO from Sqlite statements
37. SqlDelight CREATE TABLE location ( id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT, label TEXT NOT NULL UNIQUE, latitude REAL NOT NULL, longitude REAL NOT NULL, timeZone TEXT NOT NULL ); selectAllLocations: SELECT id, label FROM location; 

38. SqlDelight data class Location( val id: Long, val label: String, val latitude: Double, val longitude: Double, val timeZone: String )
 
 override fun selectAllLocations(): 
 Query<SelectAllLocations> = ...
39. SqlDelight • AndroidSqliteDriver and NativeSqliteDriver • Tests via in-memory database • Generated code is interfaces
40. Repository ViewModel ViewModel ViewModel Database Google Api Client
41. Ktor • https://github.com/ktorio/ktor • Multiplatform server and web client • Coroutines and Kotlinx.serialization
42. Ktor val httpClient = HttpClient(httpClientEngine) { defaultRequest { url.protocol = URLProtocol.HTTPS url.host = "maps.googleapis.com/maps/api" parameter("key", BuildKonfig.GOOGLE_API_KEY) } install(JsonFeature) { serializer = KotlinxSerializer().apply { setMapper(
 GeocodeResponse::class, 
 GeocodeResponse.serializer()) } } }
43. Ktor suspend fun getGeocode(placeId: String):
 GeocodeResponse = httpClient.get { url { encodedPath = "geocode/json" parameter("place_id", placeId) } }
44. Ktor • Separate engines per platform • MockEngine for testing
45. ViewModel ViewModel ViewModel Database Google Api Client Repository
46. Repository • Central data layer • Wraps Sql calls in suspend functions • Coordinates API calls
47. Native Threading is Hard • Data must be frozen to pass between threads • myData.freeze() • Different paradigm between Native and JVM
48. Background Tasks expect suspend fun <T> runInBackground(block: () -> T): T
 
 actual suspend fun <T> runInBackground(block: () -> T): T =
 withContext(Dispatchers.IO) { block() }
 actual suspend fun <T> runInBackground(block: () -> T): T = 
 suspendCoroutine<T> { continuation -> val future = worker.execute(TransferMode.SAFE, 
 { block.freeze() }, 
 { it() }
 ) future.consume { continuation.resume(it) } }
49. Repository suspend fun getLocations():
 List<LocationSummary> = runInBackground { database.locationQueries .selectAllLocations() .executeAsList() }
50. Repository suspend fun geocodeLocation(location: String): 
 GeocodeData? {
 val placeId = 
 googleApiClient.getPlaceAutocomplete(location) val coords = 
 googleApiClient.getGeocode(placeId)
 val timeZone = 
 googleApiClient
 .getTimeZone(coords.lat, coords.lng, epochSeconds)
 return GeocodeData(coords.lat, coords.lng, timeZone) }
51. Repository Database Google Api Client ViewModel ViewModel ViewModel
52. ViewModels • Expose state to views • Platform interop boundary • Callback API • No public suspend
53. ViewModels var state: T by Delegates.observable() { 
 _, _, newValue -> viewStateListener?.invoke(newValue) }
 
 protected fun updateAsync(
 action: suspend (T) -> T
 ): Job = coroutineScope.launch { state = action(state) }
54. ViewModels fun geocodeLocation(location: String) = 
 updateAsync { val geocodeData = 
 repository.geocodeLocation(location) state.copy(
 geocodeTrigger =
 EventTrigger.create(geocodeData)
 ) }
55. Repository Database Google Api Client ViewModel ViewModel ViewModel iOS App
56. iOS App • Swift/Xcode • Consumes ViewModels and renders UI • xcode-compat Gradle plugin • https://github.com/Kotlin/ xcode-compat
57. iOS App val repository: SelunaRepository = ...
 
 let viewModel = AddLocationViewModel(
 repository: SwiftKotlinBridgeKt.repository
 )
 
 @IBAction func onGeocodeClick(_ sender: Any) { viewModel.geocodeLocation(
 location: labelInput.text ?? ""
 ) }
58. iOS Tests tasks.create("iosTest") { dependsOn("linkDebugTestIos") doLast { val testBinaryPath = (kotlin.targets["ios"] as KotlinNativeTarget)
 .binaries
 .getTest("DEBUG")
 .outputFile.absolutePath exec { commandLine(
 "xcrun", "simctl", "spawn", "iPhone Xʀ", testBinaryPath
 ) } } }
59. Other Notes • BuildKonfig
 https://github.com/yshrsmz/BuildKonfig
 
 buildConfigField(
 FieldSpec.Type.STRING, 
 "GOOGLE_API_KEY"
 "...") • Obj-C/Swift Generics 
 https://kotlinlang.org/docs/reference/native/objc_interop.html
 
 (targets["ios"] as KotlinNativeTarget)
 .compilations["main"]
 .extraOpts
 .add("-Xobjc-generics")
60. Other Notes • Native Debuggers • AppCode, IDEA Ultimate EAP • Xcode (Touchlab) • Testing is hard • No multiplatform mocking
61. TODO • Integrate reminders in data layer • Lots more UI • Android! • Notifications • (and actually call into Core)
62. Lessons • Libraries! SqlDelight, Coroutines, Ktor, Serialization, BuildKonfig • Layered architecture to separate concerns • Share what's worth sharing
63. Thanks! • Questions? • @RussHWolf ( or ) • Project: https://github.com/russhwolf/soluna • Gradle Docs: https://kotlinlang.org/docs/reference/building- mpp-with-gradle.html • Library resources: • https://github.com/AAkira/Kotlin-Multiplatform-Libraries • https://github.com/bipinvaylu/awesome-kotlin- multiplatform • https://github.com/topics/kotlin-multiplatform