Hi, have you met Kotlin Multiplatform | GDG Braunschweig

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 @ TIER
   

   Google Developer Expert for Kotlin
   Marco Gomiero
   @marcoGomier
   Hi, have you met
   
   
   Kotlin
   Multiplatform?
   

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2. @marcoGomier
   “Classic” Cross Platform Solutions
   • All-in approach
   
   
   • Everything is shared, UI included
   
   
   • Different platforms have different patterns
   

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3. @marcoGomier
   Kotlin Multiplatform
   • Incremental approach
   
   
   • You choose what to share (even UI, if you want)
   

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4. @marcoGomier
   Common Kotlin
   Kotlin/JVM Kotlin/JS
   Kotlin/Native
   JVM
   
   
   Android
   Browser
   
   
   NodeJS
   iOS
   
   
   macOS
   
   
   watchOS
   
   
   tvOS
   
   
   Linux
   
   
   Windows
   Supported Platforms
   

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5. @marcoGomier
   Common Kotlin
   Kotlin/JVM Kotlin/JS
   Kotlin/Native
   JVM
   
   
   Android
   Browser
   
   
   NodeJS
   iOS
   
   
   macOS
   
   
   watchOS
   
   
   tvOS
   
   
   Linux
   
   
   Windows
   Mobile + JVM
   

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6. @marcoGomier
   Common and Platform-specific code
   • Write regular Kotlin code in the
   common source set
   
   
   • Platform-specific code goes into
   platform source sets
   

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7. @marcoGomier
   Platform-specific code
   • Expect/Actual
   
   
   

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8. @marcoGomier
   Expect/Actual
   https://kotlinlang.org/docs/multiplatform-connect-to-apis.html
   

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9. @marcoGomier
   Expect/Actual
   expect fun debugLog(tag: String, message: String)
   

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10. @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)
    
    
    }
    
    
    }
    

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11. @marcoGomier
    Platform-specific code
    • Expect/Actual
    
    
    • Interfaces
    
    
    

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12. @marcoGomier
    Interfaces
    interface HtmlParser {
    
    
    fun getTextFromHTML(html: String): String?
    
    
    }
    

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13. @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?
    
    
    }
    

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14. @marcoGomier
    Platform-specific code
    • Expect/Actual
    
    
    • Interfaces
    
    
    • Prefer interfaces, if possible
    

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15. @marcoGomier
    How it works?
    

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16. @marcoGomier
    shared
    androidApp
    .aar
    Android
    

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17. @marcoGomier
    shared
    androidApp iosApp
    .aar Framework
    iOS
    Android
    

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18. @marcoGomier
    XCFramework
    

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19. @marcoGomier
    NEW
    https://devstreaming-cdn.apple.com/videos/wwdc/2019/416h8485aty341c2/416/416_binary_frameworks_in_swift.pdf
    

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20. @marcoGomier https://devstreaming-cdn.apple.com/videos/wwdc/2019/416h8485aty341c2/416/416_binary_frameworks_in_swift.pdf
    NEW
    iOS mac
    OS
    watch
    OS
    tvOS
    

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21. @marcoGomier
    Kotlin
    ->
    Obj-c
    ->
    Swift
    

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22. @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
    
    
    ...
    
    }
    
    
    }
    
    
    

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23. @marcoGomier
    Some gotchas
    • No namespaces
    
    
    • No default parameters
    
    
    • Enums are not Swift-friendly (no values)
    
    
    • Sealed classes are simple classes
    
    
    • Coroutines and Flows
    

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24. @marcoGomier https://github.com/rickclephas/KMP-NativeCoroutines
    

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25. @marcoGomier https://github.com/touchlab/SKIE
    

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26. @marcoGomier https://github.com/icerockdev/moko-kswift
    

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27. @marcoGomier
    Make it a team effort
    

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28. @marcoGomier https://medium.com/@aoriani/list/writing-swiftfriendly-kotlin-multiplatform-apis-c51c2b317fce
    

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29. @marcoGomier
    New projects
    

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30. @marcoGomier
    Create a new app project
    

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31. @marcoGomier
    

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32. @marcoGomier
    

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33. @marcoGomier
    

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34. @marcoGomier
    shared
    androidApp
    Gradle Module
    Android
    

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35. @marcoGomier
    // settings.gradle.kts
    
    
    include(":shared")
    
    
    // build.gradle.kts
    
    
    implementation(project(":shared"))
    Android
    

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36. @marcoGomier
    shared
    androidApp iosApp
    Gradle Module Framework
    iOS
    Android
    

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37. @marcoGomier
    Regular Framework
    

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38. @marcoGomier
    Create a new app project
    

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39. @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
    
    
    

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40. @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
    

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41. @marcoGomier
    shared
    androidApp iosApp
    Gradle Module Framework
    iOS
    Android
    

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42. @marcoGomier
    shared
    androidApp iosApp
    Framework
    Same Repository
    Gradle
    Module
    

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43. @marcoGomier
    Existing projects
    

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44. @marcoGomier
    🙅
    shared
    androidApp iosApp
    Gradle
    Module
    Framework
    Same Repository
    Existing projects
    

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45. @marcoGomier
    Create a new library project
    

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46. @marcoGomier
    

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47. @marcoGomier
    Common Kotlin
    Android App iOS App
    .aar XCFramework
    

    

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48. @marcoGomier
    Common Kotlin
    Android App iOS App
    .aar XCFramework
    

    Android App Repository
    KMP Repository
    iOs App Repository
    

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49. @marcoGomier
    How to publish: Android
    

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50. @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")
    
    
    }
    
    
    }
    
    
    }
    

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51. @marcoGomier
    Publish the artifacts
    ./gradlew publish
    
    
    

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52. @marcoGomier
    Publish the artifacts
    ./gradlew publish
    
    
    ./gradlew publishToMavenLocal
    
    
    

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53. @marcoGomier
    How to publish: iOS
    

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54. @marcoGomier
    🥵
    

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55. @marcoGomier
    XCFramework
    

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56. @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)
    
    
    }
    
    
    }
    
    
    ...
    
    
    }
    

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57. @marcoGomier
    XCFramework: Gradle tasks
    assemble${libName}XCFramework
    
    
    assemble${libName}DebugXCFramework
    
    
    assemble${libName}ReleaseXCFramework
    

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58. @marcoGomier
    Build an XCFramework
    

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59. @marcoGomier https://github.com/touchlab/KMMBridge
    

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60. @marcoGomier https://github.com/prof18/kmp-framework-bundler
    

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61. @marcoGomier
    Where to start
    

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62. @marcoGomier
    shared
    androidApp iosApp
    Gradle
    Module
    Framework
    Same Repository
    New projects
    

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63. @marcoGomier
    🙅
    shared
    androidApp iosApp
    Gradle
    Module
    Framework
    Same Repository
    Existing projects
    

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64. @marcoGomier
    💡 Create a library!
    

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65. @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?
    

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66. @marcoGomier
    • DTOs
    
    
    • Common Models
    
    
    • Utility methods
    
    
    • Analytics
    
    
    •
    . . .
    Where to start?
    

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67. @marcoGomier
    Start little
    

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68. @marcoGomier
    Start little
    Go bigger
    then
    

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69. @marcoGomier
    Start little then go bigger
    • Validate the process with “little” effort
    
    
    • Then you can go bigger and share more “features”
    

    

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70. @marcoGomier
    Ecosystem
    

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71. @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
    ...
    

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72. @marcoGomier
    https://github.com/AAkira/Kotlin-Multiplatform-Libraries
    https://github.com/terrakok/kmp-awesome
    

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73. @marcoGomier https://github.com/touchlab/xcode-kotlin
    

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74. @marcoGomier https://github.com/touchlab/xcode-kotlin
    

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75. @marcoGomier https://github.com/touchlab/xcode-kotlin
    

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76. @marcoGomier https://github.com/touchlab/CrashKiOS
    

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77. @marcoGomier https://github.com/touchlab/CrashKiOS
    

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78. @marcoGomier
    Conclusions
    

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79. @marcoGomier
    • Kotlin Multiplatform
    ! =
    Cross Platform
    
    
    • Beta stage, Stable late 2023
    
    
    Conclusions
    

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80. @marcoGomier https://www.droidcon.com/2023/07/31/10-myths-about-crossplatform-mobile-development-with-kotlin/
    

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81. @marcoGomier
    • Kotlin Multiplatform
    ! =
    Cross Platform
    
    
    • Beta stage, Stable late 2023
    
    
    • It’s the future
    
    
    • It’s a joint and team approach
    Conclusions
    

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82. 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/
    

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83. @marcoGomier
    Thank you!
    > Twitter: @marcoGomier
    

    > Github: prof18
    

    > Website: marcogomiero.com
    

    > Mastodon: androiddev.social/@marcogom
    👨💻 Senior Android Engineer @ TIER
    

    Google Developer Expert for Kotlin
    Marco Gomiero
    

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