Building a Multiplatform library for iOS and Android

Sharing code between platforms is a powerful technique, but it may be hard to accomplish without rich APIs that we have in Android, JVM, or iOS platforms. Kotlin Multiplatform libraries can be used to fix that, bringing rich APIs directly in the common Kotlin code.

What an exciting frontier!

In this talk we'll go over the creation of a Kotlin Multiplatform library. You'll learn how modules for each platform work, decide what parts of the code makes sense to share and what doesn't. All based in the experiences gathered after creating a library at Mixtiles used to fetch images from different cloud providers.

Transcript

1. Building a Multiplatform Library
   for Android and iOS
   David González
   @dggonzalez
   malmstein@gmail.com
   

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5. expect / actual mechanism
   

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6. expect / actual mechanism
   internal expect fun writeLogMessage(message: String, logLevel: LogLevel)
   

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7. expect / actual mechanism
   fun logDebug(message: String) = writeLogMessage(message, LogLevel.DEBUG)
   fun logWarn(message: String) = writeLogMessage(message, LogLevel.WARN)
   fun logError(message: String) = writeLogMessage(message, LogLevel.ERROR)
   

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8. expect / actual mechanism
   internal actual fun writeLogMessage(message: String, logLevel: LogLevel) {
   println("[$logLevel]: $message")
   }
   

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9. expect / actual mechanism
   internal actual fun writeLogMessage(message: String, logLevel: LogLevel) {
   when (logLevel) {
   LogLevel.DEBUG -> console.log(message)
   LogLevel.WARN -> console.warn(message)
   LogLevel.ERROR -> console.error(message)
   }
   }
   

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13. Native vs Multiplatform
    

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14. Native vs Multiplatform
    apply plugin: ‘kotlin-native-platform'
    apply plugin: 'kotlin-multiplatform'
    

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15. // for Native only projects
    apply plugin: 'kotlin-native-platform'
    // for projects targetting multiple platforms
    apply plugin: 'kotlin-multiplatform'
    Native vs Multiplatform
    

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16. Platform-Specific declarations
    

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17. Platform-Specific declarations
    expect class AtomicRef(value: V) {
    fun get(): V
    fun set(value: V)
    fun getAndSet(value: V): V
    fun compareAndSet(expect: V, update: V): Boolean
    }
    actual typealias AtomicRef = java.util.concurrent.atomic.AtomicReference
    

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22. Root Folder
    

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23. Root Folder
    AndroidApp
    SharedCode
    iOSApp
    

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24. Root Folder
    AndroidApp
    SharedCode
    iOSApp
    build.gradle
    build
    src
    …
    

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25. Root Folder
    AndroidApp
    SharedCode
    iOSApp
    iosApp
    iosApp.xcodeproj
    iosAppTests
    

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26. Root Folder
    AndroidApp
    SharedCode
    iOSApp
    Src
    androidMain
    androidTest
    commonMain
    commonTest
    iosMain
    iosTest
    

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27. SharedCode
    Src
    androidMain
    androidTest
    commonMain
    commonTest
    iosMain
    iosTest
    

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28. kotlin {
    sourceSets {
    commonMain.dependencies {
    api "org.jetbrains.kotlin:kotlin-stdlib-common:$kotlin_version"
    implementation “org….:kotlinx-coroutines-core-common:$coroutines_version”
    implementation "io.ktor:ktor-client:$ktor_version"
    implementation "io.ktor:ktor-client-core:$ktor_version"
    implementation "io.ktor:ktor-client-json:$ktor_version"
    }
    commonTest.dependencies {
    implementation "org.jetbrains.kotlin:kotlin-test-common:$version"
    implementation "org.jetbrains.kotlin:kotlin-test-annotations:$version"
    }
    androidMain.dependencies {
    api "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:$version"
    SharedPhotos/build.gradle
    

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29. kotlin {
    sourceSets {
    commonMain.dependencies {
    api "org.jetbrains.kotlin:kotlin-stdlib-common:$kotlin_version"
    implementation “org….:kotlinx-coroutines-core-common:$coroutines_version”
    implementation "io.ktor:ktor-client:$ktor_version"
    implementation "io.ktor:ktor-client-core:$ktor_version"
    implementation "io.ktor:ktor-client-json:$ktor_version"
    }
    commonTest.dependencies {
    implementation "org.jetbrains.kotlin:kotlin-test-common:$version"
    implementation "org.jetbrains.kotlin:kotlin-test-annotations:$version"
    }
    androidMain.dependencies {
    api "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:$version"
    SharedPhotos/build.gradle
    

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30. sourceSets {
    commonMain.dependencies {
    api "org.jetbrains.kotlin:kotlin-stdlib-common:$kotlin_version"
    implementation “org….:kotlinx-coroutines-core-common:$coroutines_version”
    implementation "io.ktor:ktor-client:$ktor_version"
    implementation "io.ktor:ktor-client-core:$ktor_version"
    implementation "io.ktor:ktor-client-json:$ktor_version"
    }
    commonTest.dependencies {
    implementation "org.jetbrains.kotlin:kotlin-test-common:$version"
    implementation "org.jetbrains.kotlin:kotlin-test-annotations:$version"
    }
    androidMain.dependencies {
    api "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:$version"
    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-android:$version"
    implementation "io.ktor:ktor-client-android:$ktor_version"
    implementation "io.ktor:ktor-client-core-jvm:$ktor_version"
    implementation "io.ktor:ktor-client-json-jvm:$ktor_version"
    }
    

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31. commonTest.dependencies {
    implementation "org.jetbrains.kotlin:kotlin-test-common:$version"
    implementation "org.jetbrains.kotlin:kotlin-test-annotations:$version"
    }
    androidMain.dependencies {
    api "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:$version"
    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-android:$version"
    implementation "io.ktor:ktor-client-android:$ktor_version"
    implementation "io.ktor:ktor-client-core-jvm:$ktor_version"
    implementation "io.ktor:ktor-client-json-jvm:$ktor_version"
    }
    androidTest.dependencies {
    implementation "org.jetbrains.kotlin:kotlin-test:$kotlin_version"
    implementation "org.jetbrains.kotlin:kotlin-test-junit:$kotlin_version"
    }
    iosMain.dependencies {
    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core-native:$version"
    implementation "io.ktor:ktor-client-ios:$ktor_version"
    

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32. androidMain.dependencies {
    api "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:$version"
    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-android:$version"
    implementation "io.ktor:ktor-client-android:$ktor_version"
    implementation "io.ktor:ktor-client-core-jvm:$ktor_version"
    implementation "io.ktor:ktor-client-json-jvm:$ktor_version"
    }
    androidTest.dependencies {
    implementation "org.jetbrains.kotlin:kotlin-test:$kotlin_version"
    implementation "org.jetbrains.kotlin:kotlin-test-junit:$kotlin_version"
    }
    iosMain.dependencies {
    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core-native:$version"
    implementation "io.ktor:ktor-client-ios:$ktor_version"
    implementation "io.ktor:ktor-client-core-native:$ktor_version"
    implementation "io.ktor:ktor-client-json-native:$ktor_version"
    }
    }
    }
    

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33. implementation "io.ktor:ktor-client-core-jvm:$ktor_version"
    implementation "io.ktor:ktor-client-json-jvm:$ktor_version"
    }
    androidTest.dependencies {
    implementation "org.jetbrains.kotlin:kotlin-test:$kotlin_version"
    implementation "org.jetbrains.kotlin:kotlin-test-junit:$kotlin_version"
    }
    iosMain.dependencies {
    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core-native:$version"
    implementation "io.ktor:ktor-client-ios:$ktor_version"
    implementation "io.ktor:ktor-client-core-native:$ktor_version"
    implementation "io.ktor:ktor-client-json-native:$ktor_version"
    }
    }
    }
    

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35. apply plugin: 'com.android.application'
    apply plugin: 'kotlin-android'
    apply plugin: 'kotlin-android-extensions'
    dependencies {
    implementation project(':CloudPhotos')
    implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
    ...
    }
    androidApp/build.gradle
    

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37. targets {
    fromPreset(presets.jvm, 'android')
    def buildForDevice = project.findProperty("device")?.toBoolean() ?: false
    def iOSTarget = System.getenv('SDK_NAME')?.startsWith('iphoneos') ?
    presets.iosArm64 : presets.iosX64
    fromPreset(iOSTarget, 'ios') {
    compilations.main.outputKinds('FRAMEWORK')
    }
    }
    CloudPhotos/build.gradle
    

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38. task packForXCode(type: Sync) {
    final File frameworkDir = new File(buildDir, "xcode-frameworks")
    final String mode = project.findProperty("XCODE_CONFIGURATION")?.toUpperCase() ?: 'DEBUG'
    final def framework = kotlin.targets.iOS.binaries.getFramework(“CloudPhotos", mode)
    inputs.property "mode", mode
    dependsOn framework.linkTask
    from { framework.outputFile.parentFile }
    into frameworkDir
    doLast {
    new File(frameworkDir, 'gradlew').with {
    text = "#!/bin/bash\nexport 'JAVA_HOME=${System.getProperty("java.home")}'\ncd ‘$.
    {rootProject.rootDir}'\n./gradlew \$@\n"
    setExecutable(true)
    }
    }
    }
    tasks.build.dependsOn packForXCode
    CloudPhotos/build.gradle
    

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40. https://github.com/AlecStrong/kotlin-native-cocoapods
    

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43. androidMain
    commonMain
    iOSMain
    Multiplatform Library
    

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44. androidMain
    commonMain
    iOSMain
    Platform.kt expect fun platformName(): String
    Multiplatform Library
    

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45. androidMain
    commonMain
    iOSMain
    actual fun platformName(): String = "Android Shared Photos"
    Platform.kt
    Platform.kt expect fun platformName(): String
    Multiplatform Library
    

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46. androidMain
    commonMain
    iOSMain
    Platform.kt expect fun platformName(): String
    actual fun platformName(): String = "Android Shared Photos"
    Platform.kt
    Platform.kt actual fun platformName(): String = "iOS Shared Photos”
    Multiplatform Library
    

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47. // androidApp/...MainActivity.kt
    import com.malmstein.sharedphotos.platform.platformName
    title = platformName()
    // iosApp/… ViewController.swift
    import shared
    self.label.text = PlatformKt.platformName()
    

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48. // androidApp/...MainActivity.kt
    import com.malmstein.sharedphotos.platform.platformName
    title = platformName()
    // iosApp/… ViewController.swift
    import shared
    self.label.text = PlatformKt.platformName()
    

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52. https://github.com/Kotlin/kotlinx.serialization
    

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54. internal expect val ApplicationDispatcher: CoroutineDispatcher
    CloudPhotos/commonMain
    

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55. internal actual val ApplicationDispatcher: CoroutineDispatcher = Dispatchers.Main
    CloudPhotos/androidMain
    

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56. internal actual val ApplicationDispatcher: CoroutineDispatcher = NsQueueDispatcher()
    internal class NsQueueDispatcher: CoroutineDispatcher() {
    override fun dispatch(context: CoroutineContext, block: Runnable) {
    val queue = dispatch_get_main_queue()
    dispatch_async(queue) {
    block.run()
    }
    }
    }
    CloudPhotos/iosMain
    

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59. class CloudPhotosRepository(private val api: Api = Api()) {
    private val client: HttpClient = HttpClient {
    install(JsonFeature) {
    serializer = KotlinxSerializer(Json.nonstrict).apply {
    setMapper(InstagramResponseJson::class, InstagramResponseJson.serializer())
    }
    }
    }
    suspend fun callInstagram(token: String): InstagramResponseJson =
    client.get {
    url("https://api.instagram.com/v1/users/self/media/recent?
    access_token=$token")
    }
    fun loadInstagramAsync(callback: (List) -> Unit) {
    CloudPhotosRepository
    

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60. private val client: HttpClient = HttpClient {
    install(JsonFeature) {
    serializer = KotlinxSerializer(Json.nonstrict).apply {
    setMapper(InstagramResponseJson::class, InstagramResponseJson.serializer())
    }
    }
    }
    suspend fun callInstagram(token: String): InstagramResponseJson =
    client.get {
    url("https://api.instagram.com/v1/users/self/media/recent?
    access_token=$token")
    }
    fun loadInstagramAsync(callback: (List) -> Unit) {
    GlobalScope.apply {
    launch(ApplicationDispatcher) {
    val photos = api.instagramPhotos(apiToken)
    .data.map { InstagramPhoto.fromJson(it) }
    callback(photos)
    }
    }
    

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61. }
    }
    suspend fun callInstagram(token: String): InstagramResponseJson =
    client.get {
    url("https://api.instagram.com/v1/users/self/media/recent?
    access_token=$token")
    }
    fun loadInstagramAsync(callback: (List) -> Unit) {
    GlobalScope.apply {
    launch(ApplicationDispatcher) {
    val photos = api.instagramPhotos(apiToken)
    .data.map { InstagramPhoto.fromJson(it) }
    callback(photos)
    }
    }
    

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62. import com.malmstein.sharedphotos.data.CloudPhotosRepository
    val repository = CloudPhotosRepository()
    repository.loadInstagramAsync {
    photosAdapter.setResults(it)
    }
    AndroidApp/PhotosActivity.kt
    

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63. import shared
    let api = Api()
    let repository = CloudPhotosRepository(api: api)
    typealias PhotoClosure = ([InstagramPhoto]) -> Void
    func requestPhotos(_ closure: @escaping PhotoClosure) {
    repository.loadInstagramAsync { photos -> KotlinUnit in
    closure(gifs)
    return KotlinUnit()
    }
    }
    requestPhotos { [weak self] photos in
    iOSApp/ViewController.swift
    

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64. import shared
    let api = Api()
    let repository = CloudPhotosRepository(api: api)
    typealias PhotoClosure = ([InstagramPhoto]) -> Void
    func requestPhotos(_ closure: @escaping PhotoClosure) {
    repository.loadInstagramAsync { photos -> KotlinUnit in
    closure(gifs)
    return KotlinUnit()
    }
    }
    requestPhotos { [weak self] photos in
    iOSApp/ViewController.swift
    

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65. import shared
    let api = Api()
    let repository = CloudPhotosRepository(api: api)
    typealias PhotoClosure = ([InstagramPhoto]) -> Void
    func requestPhotos(_ closure: @escaping PhotoClosure) {
    repository.loadInstagramAsync { photos -> KotlinUnit in
    closure(gifs)
    return KotlinUnit()
    }
    }
    requestPhotos { [weak self] photos in
    guard let self = self else { return }
    self.photos = photos
    self.collectionView?.reloadData()
    }
    

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66. import shared
    let api = Api()
    let repository = CloudPhotosRepository(api: api)
    typealias PhotoClosure = ([InstagramPhoto]) -> Void
    func requestPhotos(_ closure: @escaping PhotoClosure) {
    repository.loadInstagramAsync { photos -> KotlinUnit in
    closure(gifs)
    return KotlinUnit()
    }
    }
    requestPhotos { [weak self] photos in
    guard let self = self else { return }
    self.photos = photos
    self.collectionView?.reloadData()
    }
    

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67. let api = Api()
    let repository = CloudPhotosRepository(api: api)
    typealias PhotoClosure = ([InstagramPhoto]) -> Void
    func requestPhotos(_ closure: @escaping PhotoClosure) {
    repository.loadInstagramAsync { photos -> KotlinUnit in
    closure(gifs)
    return KotlinUnit()
    }
    }
    requestPhotos { [weak self] photos in
    guard let self = self else { return }
    self.photos = photos
    self.collectionView?.reloadData()
    }
    

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70. https://github.com/square/sqldelight
    

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71. import com.squareup.sqldelight.db.SqlDriver
    expect object SharedDb {
    val transacter: CloudPhotosTransacter
    fun setupDatabase(driver: SqlDriver)
    fun clearDatabase()
    }
    // Called from Swift
    @Suppress("unused")
    import com.squareup.sqldelight.drivers.ios.NativeSqliteDriver
    SharedDb.setupDatabase(NativeSqliteDriver(DatabaseSchema, "SharedPhotosDB"))
    

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72. val transacter: CloudPhotosTransacter
    fun setupDatabase(driver: SqlDriver)
    fun clearDatabase()
    }
    // Called from Swift
    @Suppress("unused")
    import com.squareup.sqldelight.drivers.ios.NativeSqliteDriver
    SharedDb.setupDatabase(NativeSqliteDriver(DatabaseSchema, "SharedPhotosDB"))
    // Called from Android
    import com.squareup.sqldelight.android.AndroidSqliteDriver
    val androidDriver: SqlDriver
    = AndroidSqliteDriver(DatabaseSchema, context,"SharedPhotosDB")
    SharedDb.setupDatabase(androidDriver)
    

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73. // Called from Swift
    @Suppress("unused")
    import com.squareup.sqldelight.drivers.ios.NativeSqliteDriver
    SharedDb.setupDatabase(NativeSqliteDriver(DatabaseSchema, "SharedPhotosDB"))
    // Called from Android
    import com.squareup.sqldelight.android.AndroidSqliteDriver
    val androidDriver: SqlDriver
    = AndroidSqliteDriver(DatabaseSchema, context,"SharedPhotosDB")
    SharedDb.setupDatabase(androidDriver)
    

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74. object TokenRepository {
    fun getToken(): Token {
    val tokenQueries = SharedDb.transactor.tokenQueries
    val token = tokenQueries.selectInstagramToken().executeAsOne()
    return token
    }
    commonMain/data/TokenRepository
    

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76. package org.jetbrains.kotlinconf.presentation
    import kotlinx.coroutines.*
    import kotlin.coroutines.*
    // TODO: Use Dispatchers.Main instead when it will be supported on iOS
    open class CoroutinePresenter(
    private val mainContext: CoroutineContext,
    private val baseView: BaseView
    ): CoroutineScope {
    private val job = Job()
    private val exceptionHandler = CoroutineExceptionHandler { _, throwable -
    baseView.showError(throwable)
    }
    commonMain/presentation/CoroutinesPresenter
    

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77. class InstagramPhotosPresenter(val view: InstagramPhotosView)
    : CoroutinePresenter() {
    fun loadInstagramPhotos(): List {
    view.showLoading()
    ...
    val photos = repository.loadInstagramPhotos()
    view.showPhotos(photos)
    }
    commonMain/presentation/InstagramPresenter
    

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78. AndroidApp
    Multiplatform Library
    iOSApp
    

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79. AndroidApp
    Multiplatform Library
    iOSApp
    InstagramPhotosView
    

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80. AndroidApp
    Multiplatform Library
    iOSApp
    InstagramPhotosView
    InstagramActivity: InstagramPhotosView
    

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81. AndroidApp
    Multiplatform Library
    iOSApp
    InstagramPhotosView
    InstagramActivity: InstagramPhotosView
    InstagramViewController: InstagramPhotosView
    

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82. interface InstagramPhotosView {
    fun showLoading()
    fun showPhotos(photos: List)
    fun showError(message: String)
    }
    // Calling from Android
    class InstagramActivity: FragmentActivity(), InstagramPhotosView
    override fun showLoading() {
    loadingSpinner.setVisibility(View.VISIBLE)
    }
    commonMain/presentation/InstagramPhotosView.kt
    

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83. fun showPhotos(photos: List)
    fun showError(message: String)
    }
    // Calling from Android
    class InstagramActivity: FragmentActivity(), InstagramPhotosView
    override fun showLoading() {
    loadingSpinner.setVisibility(View.VISIBLE)
    }
    // Calling from iOS
    class InstagramViewController: UIViewController {
    extension PhotosViewController: InstagramPhotosView {
    func showLoading() {
    self.loadingSpinner.startAnimating()
    }
    }
    

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84. // Calling from Android
    class InstagramActivity: FragmentActivity(), InstagramPhotosView
    override fun showLoading() {
    loadingSpinner.setVisibility(View.VISIBLE)
    }
    // Calling from iOS
    class InstagramViewController: UIViewController {
    extension PhotosViewController: InstagramPhotosView {
    func showLoading() {
    self.loadingSpinner.startAnimating()
    }
    }
    

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86. class DateTest {
    @Test
    fun testParse() {
    val date = "2017-10-24T13:31:19".parseDate()
    assertEquals(2017, date.year)
    assertEquals(Month.OCTOBER, date.month)
    assertEquals(24, date.dayOfMonth)
    assertEquals(13, date.hours)
    assertEquals(31, date.minutes)
    }
    }
    

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87. commonTest works if
    there are no Coroutines
    involved
    

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88. runBlocking does not
    exist in all platforms
    

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89. // commonTest
    expect fun platformRunBlocking(
    block: suspend CoroutineScope.() -> T) : T
    // androidTest + iosTest
    actual fun platformRunBlocking(
    block: suspend CoroutineScope.() -> T): T {
    return runBlocking { block() }
    }
    https://github.com/bakkenbaeck/PorchPirateProtector
    

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90. @Test
    fun shouldShowLoadingIndicator() = platformRunBlocking {
    val view = TestInstagramPhotosView()
    val presenter = PhotosPresenter(view)
    val photos = presenter.loadInstagramPhotos()
    assertTrue(view.loadingIndicatorStarted)
    ...
    

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91. Testing androidMain
    also tests
    commonMain
    

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92. What is missing
    Concurrency
    Date / Time
    Locale
    File I / O
    Networking
    

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93. Conclusions
    Use immutable objects instead of mutable (although…)
    Keep everyone engaged, not just the Android team
    Types in Swift / Obj-C can be different in Kotlin and vice-versa
    Experience developing on iOS is not as good as on Android
    Can’t debug Kotlin from Xcode (or maybe…)
    

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95. https://github.com/touchlab/xcode-kotlin
    https://github.com/touchlab/KotlinXcodeSync
    

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96. https://github.com/touchlab/CrashKiOS
    

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97. References
    Kotlin Conf App: https://github.com/JetBrains/kotlinconf-app
    SharedPhotosLibrary: https://github.com/malmstein/cloudphotos-mpp
    Droidcon App Multiplatform: https://github.com/touchlab/DroidconKotlin
    Repositories:
    Talks:
    Kotlin Multiplatform Libraries by Kevin Galligan: Vimeo
    

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99. Thank you!
    David González
    @dggonzalez
    malmstein@gmail.com
    

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