Hi, have you met Kotlin Multiplatform | DevFest Hamburg

Kotlin Multiplatform is getting more and more hype every day. It started as an experimental technology, then alpha, beta and now it's on the path of becoming stable.

We constantly read of new companies and teams that are using KMP for experiments and production projects alike. And we're left wondering: why pick KMP over any other cross-platform solution? How to approach it? And, most importantly, is it possible to start using it in existing projects?

In this talk, I'll answer these questions, clarifying all the doubts and making you ready to use and love Kotlin Multiplatform.

Transcript

1. 👨💻 Senior Android Engineer 
 Google Developer Expert for Kotlin Marco Gomiero @marcoGomier Hi, have you met Kotlin Multiplatform?
2. @marcoGomier Kotlin Multiplatform is now stable! 🥳 https://blog.jetbrains.com/kotlin/2023/11/kotlin-multiplatform-stable/
3. @marcoGomier “Classic” Cross Platform Solutions • All-in approach • Everything is shared, UI included • Different platforms have different patterns
4. @marcoGomier Kotlin Multiplatform • Incremental approach • You choose what to share (even UI, if you want)
5. @marcoGomier Common Kotlin Kotlin/JVM Kotlin/JS Kotlin/Native JVM Android Browser NodeJS iOS macOS watchOS tvOS Linux Windows Supported Platforms
6. @marcoGomier Common Kotlin Kotlin/JVM Kotlin/JS Kotlin/Native JVM Android Browser NodeJS iOS macOS watchOS tvOS Linux Windows Mobile
7. @marcoGomier Beyond Mobile https://hamburg.devfest.de/sessions/108
8. @marcoGomier Common and Platform-specific code • Write regular Kotlin code in the common source set • Platform-specific code goes into platform source sets
9. @marcoGomier Platform-specific code • Expect/Actual
10. @marcoGomier Expect/Actual https://kotlinlang.org/docs/multiplatform-connect-to-apis.html
11. @marcoGomier Expect/Actual expect fun debugLog(tag: String, message: String)
12. @marcoGomier expect fun debugLog(tag: String, message: String) import android.util.Log actual fun debugLog(tag: String, message: String) { Log.d(tag, message) } import platform.Foundation.NSLog actual fun debugLog(tag: String, message: String) { if (Platform.isDebugBinary) { NSLog("%s: %s", tag, message) } }
13. @marcoGomier Platform-specific code • Expect/Actual • Interfaces
14. @marcoGomier internal interface XmlFetcher { suspend fun fetchXml(url: String): ParserInput } Interface
15. @marcoGomier internal interface XmlFetcher { suspend fun fetchXml(url: String): ParserInput } internal class JvmXmlFetcher( private val callFactory: Call.Factory, ): XmlFetcher { override suspend fun fetchXml(url: String): ParserInput { val request = createRequest(url) return ParserInput( inputStream = callFactory.newCall(request).await() ) } } internal class IosXmlFetcher( private val nsUrlSession: NSURLSession, ): XmlFetcher { override suspend fun fetchXml(url: String): ParserInput = suspendCancellableCoroutine { continuation -> ... } }
16. @marcoGomier import org.jsoup.Jsoup internal class JvmHtmlParser : HtmlParser { override fun getTextFromHTML(html: String): String? { return try { val doc = Jsoup.parse(html) doc.text() } catch (e: Throwable) { null } } } import shared import SwiftSoup class IosHtmlParser: HtmlParser { func getTextFromHTML(html: String) -> String? { do { let doc: Document = try SwiftSoup.parse(html) return try doc.text() } catch { return nil } } } interface HtmlParser { fun getTextFromHTML(html: String): String? }
17. @marcoGomier Platform-specific code • Expect/Actual • Interfaces • Prefer interfaces, if possible
18. @marcoGomier How it works?
19. @marcoGomier shared androidApp .aar Android
20. @marcoGomier shared androidApp iosApp .aar Framework iOS Android
21. @marcoGomier XCFramework
22. @marcoGomier NEW https://devstreaming-cdn.apple.com/videos/wwdc/2019/416h8485aty341c2/416/416_binary_frameworks_in_swift.pdf
23. @marcoGomier https://devstreaming-cdn.apple.com/videos/wwdc/2019/416h8485aty341c2/416/416_binary_frameworks_in_swift.pdf NEW iOS mac OS watch OS tvOS
24. @marcoGomier Kotlin -> Obj-c -> Swift
25. @marcoGomier when (appState.newsState) { is NewsState.Loading -> { progressBar.visibility = View.VISIBLE ... } is NewsState.Error -> { errorButton.visibility = View.VISIBLE errorMessage.visibility = View.VISIBLE ... } is NewsState.Success -> { progressBar.visibility = View.GONE errorMessage.visibility = View.GONE recyclerView.visibility = View.VISIBLE ... } }
26. @marcoGomier Some gotchas • No namespaces • No default parameters • Enums are not Swift-friendly (no values) • Sealed classes are simple classes • Coroutines and Flows
27. @marcoGomier https://github.com/rickclephas/KMP-NativeCoroutines
28. @marcoGomier https://github.com/touchlab/SKIE
29. @marcoGomier Make it a team effort
30. @marcoGomier New projects
31. @marcoGomier Create a new app project
32. @marcoGomier
33. @marcoGomier
34. @marcoGomier
35. @marcoGomier shared androidApp Gradle Module Android
36. @marcoGomier // settings.gradle.kts include(":shared") // build.gradle.kts implementation(project(":shared")) Android
37. @marcoGomier shared androidApp iosApp Gradle Module Framework iOS Android
38. @marcoGomier Create a new app project
39. @marcoGomier Regular Framework
40. @marcoGomier Create a new app project
41. @marcoGomier CocoaPods integration https://kotlinlang.org/docs/reference/native/cocoapods.html Pod::Spec.new do |spec| spec.name = 'shared' spec.version = '1.0' spec.homepage = 'Link to the Shared Module homepage' spec.source = { :http=> ''} spec.authors = '' spec.license = '' spec.summary = 'Some description for the Shared Module' spec.vendored_frameworks = 'build/cocoapods/framework/shared.framework' spec.libraries = 'c++' spec.ios.deployment_target = '14.1' spec.pod_target_xcconfig = { 'KOTLIN_PROJECT_PATH' => ':shared', 'PRODUCT_MODULE_NAME' => 'shared', } spec.script_phases = [ { :name => 'Build shared', :execution_position => :before_compile, :shell_path => '/bin/sh', :script => <<-SCRIPT if [ "YES" = "$OVERRIDE_KOTLIN_BUILD_IDE_SUPPORTED" ]; then echo "Skipping Gradle build task invocation due to OVERRIDE_KOTLIN_BUILD_IDE_SUPPORTED environment exit 0 fi set -ev REPO_ROOT="$PODS_TARGET_SRCROOT" "$REPO_ROOT/../gradlew" -p "$REPO_ROOT" $KOTLIN_PROJECT_PATH:syncFramework \ -Pkotlin.native.cocoapods.platform=$PLATFORM_NAME \ -Pkotlin.native.cocoapods.archs="$ARCHS" \ -Pkotlin.native.cocoapods.configuration="$CONFIGURATION" SCRIPT } ] end
42. @marcoGomier iOSApp: Podfile https://kotlinlang.org/docs/reference/native/cocoapods.html target 'iosApp' do use_frameworks! platform :ios, '14.1' pod 'shared', :path => '../shared' end
43. @marcoGomier shared androidApp iosApp Gradle Module Framework iOS Android
44. @marcoGomier shared androidApp iosApp Framework Same Repository Gradle Module
45. @marcoGomier Existing projects
46. @marcoGomier 🙅 shared androidApp iosApp Gradle Module Framework Same Repository Existing projects
47. @marcoGomier 💡 Create a library!
48. @marcoGomier Create a new library project
49. @marcoGomier
50. @marcoGomier Common Kotlin Android App iOS App .aar XCFramework 

51. @marcoGomier Common Kotlin Android App iOS App .aar XCFramework 
 Android App Repository KMP Repository iOs App Repository
52. @marcoGomier How to publish: Android
53. @marcoGomier Setup a Maven repository to share the artifacts: build.gradle.kts plugins { //... id("maven-publish") } group = "com.prof18.hn.foundation" version = "1.0" publishing { repositories { maven { credentials { username = "username" password = "pwd" } url = URI.create("https://mymavenrepo.it") } } }
54. @marcoGomier Publish the artifacts ./gradlew publish
55. @marcoGomier Publish the artifacts ./gradlew publish ./gradlew publishToMavenLocal
56. @marcoGomier How to publish: iOS
57. @marcoGomier 🥵
58. @marcoGomier XCFramework
59. @marcoGomier Build an XCFramework https://kotlinlang.org/docs/multiplatform-build-native-binaries.html#build-xcframeworks import org.jetbrains.kotlin.gradle.plugin.mpp.apple.XCFramework val libName = "LibraryName" kotlin { val xcFramework = XCFramework(libName) ios { binaries.framework(libName) { xcFramework.add(this) } } ... }
60. @marcoGomier XCFramework: Gradle tasks assemble${libName}XCFramework assemble${libName}DebugXCFramework assemble${libName}ReleaseXCFramework
61. @marcoGomier Build an XCFramework
62. @marcoGomier https://github.com/touchlab/KMMBridge
63. @marcoGomier https://github.com/prof18/kmp-framework-bundler
64. @marcoGomier Where to start
65. @marcoGomier shared androidApp iosApp Gradle Module Framework Same Repository New projects
66. @marcoGomier 🙅 shared androidApp iosApp Gradle Module Framework Same Repository Existing projects
67. @marcoGomier 💡 Create a library!
68. @marcoGomier • Boring code to write multiple times • Code/feature that centralizes the source of truth • Code/feature that can be gradually extracted Where to start?
69. @marcoGomier • DTOs • Common Models • Utility methods • Analytics • . . . Where to start?
70. @marcoGomier Start little
71. @marcoGomier Start little Go bigger then
72. @marcoGomier Start little then go bigger • Validate the process with “little” effort • Then you can go bigger and share more “features” 

73. @marcoGomier Ecosystem
74. @marcoGomier • Networking: Ktor, Apollo • Persistence: SQLDelight, multiplatform-settings, Realm • Serialization: kotlinx.serialization • Dependency Injection: Koin, Kodein, kotlin-inject • Asynchronous: Coroutines, Reaktive • Date/Time: kotlinx-datetime • Logging: Kermit, Napier Common Libraries And much more ...
75. @marcoGomier https://github.com/AAkira/Kotlin-Multiplatform-Libraries https://github.com/terrakok/kmp-awesome
76. @marcoGomier https://github.com/touchlab/xcode-kotlin
77. @marcoGomier https://github.com/touchlab/xcode-kotlin
78. @marcoGomier https://github.com/touchlab/xcode-kotlin
79. @marcoGomier https://github.com/touchlab/CrashKiOS
80. @marcoGomier https://github.com/touchlab/CrashKiOS
81. @marcoGomier Conclusions
82. @marcoGomier • Kotlin Multiplatform ! = Cross Platform • Stable since Kotlin 1.9.20 Conclusions
83. @marcoGomier https://www.droidcon.com/2023/07/31/10-myths-about-crossplatform-mobile-development-with-kotlin/
84. @marcoGomier • Kotlin Multiplatform ! = Cross Platform • Stable since Kotlin 1.9.20 • It’s the future • It’s a joint and team approach Conclusions
85. Bibliography / Useful Links • https: // kotlinlang.org/docs/multiplatform.html • https: // kotlinlang.org/docs/native-overview.html • https: // kotlinlang.org/docs/js-overview.html • https: // kotlinlang.org/docs/multiplatform-share-on-platforms.html • https: // kotlinlang.org/docs/multiplatform-connect-to-apis.html • https: // devstreaming-cdn.apple.com/videos/wwdc/2019/416h8485aty341c2/416/416_binary_frameworks_in_swift.pdf • https: // developer.apple.com/videos/play/wwdc2019/416/ • https: // medium.com/@aoriani/list/writing-swiftfriendly-kotlin-multiplatform-apis-c51c2b317fce • https: // kotlinlang.org/docs/native-objc-interop.html • https: // speakerdeck.com/kpgalligan/kotlinconf-2023-kotlin-mobile-multiplatform-for-teams • https: // speakerdeck.com/kpgalligan/sdk-design-and-publishing-for-kotlin-multiplatform-mobile • https: // kotlinlang.org/docs/reference/native/cocoapods.html • https: // kotlinlang.org/docs/multiplatform-build-native-binaries.html#build-xcframeworks • https: // github.com/touchlab/KMMBridge • https: // github.com/prof18/kmp-framework-bundler • https: // github.com/touchlab/xcode-kotlin • https: // github.com/terrakok/kmp-awesome • https: // github.com/AAkira/Kotlin-Multiplatform-Libraries • https: // github.com/touchlab/CrashKiOS • https: // www.droidcon.com/2023/07/31/10-myths-about-crossplatform-mobile-development-with-kotlin/ • https: // blog.jetbrains.com/kotlin/2023/11/kotlin-multiplatform-stable/
86. @marcoGomier Thank you! > Twitter: @marcoGomier 
 > Github: prof18 
 > Website: marcogomiero.com 
 > Mastodon: androiddev.social/@marcogom 👨💻 Senior Android Engineer 
 Google Developer Expert for Kotlin Marco Gomiero