Commit University November 2022 - KMM Survival Guide: how to tackle everyday struggles between Kotlin and Swift

Slides used during my talk at Commit University in November 2022 about how to use Kotlin Multiplatform Mobile, what issues are currently unsolved, and how to work around them!

Blog post: https://www.commitsoftware.it/kotlin-multiplatform-mobile-how-it-works/

Video: https://www.youtube.com/watch?v=SRP3nnAQ5P4

Transcript

1. KMM Survival Guide:
   how to tackle everyday
   struggles between Kotlin
   and Swift
   Emanuele Papa
   Commit University November 2022
   

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2. Working at Zest One in Chiasso, Switzerland
   Who I am?
   Emanuele Papa, Android Developer
   Find me at www.emanuelepapa.dev
   

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3. What is KMM?
   Kotlin Multiplatform
   Mobile is an SDK for iOS
   and Android app
   development. It offers all
   the combined benefits of
   creating cross-platform
   and native apps.
   

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4. Shared module: you write Kotlin code as you are used to do.
   Android module: you see no difference and just use the shared module
   code
   iOS project: using Kotlin Native, the code is compiled into an
   AppleFramework like it was written in Objective-C.
   Then you can just use it in the same way as when you import an iOS
   dependency.
   How does it work?
   

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5. Swift and Kotlin are both modern languages and have a lot of
   syntax similarities
   Swift is like Kotlin
   

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6. Everything looks cool!
   
   
   I can become an iOS developer!
   
   
   (or I can become an Android developer!)
   Expectation
   

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7. Not everything on KMM works out of the box.
   Unfortunately, most of the issues arise because the
   Kotlin code is transformed into ObjectiveC code.
   Reality
   

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8. KMM directly supports Swift interoperability
   (KT-49521)
   Write shared code in a slightly different way to better
   support iOS development (and iOS developers)
   Solutions
   

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9. Primitive types
   fun getRandomInt(): Int = Random.nextInt(until = 10)
   Shared code
   

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10. Primitive types
    val randomInt: Int = getRandomInt()
    Android
    

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11. Primitive types
    let randomInt: Int32 = RandomNumberGeneratorKt.getRandomInt()
    iOS
    

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12. Sealed classes
    sealed class KMMIntResult
    data class SuccessKMMIntResult(
    val value: Int
    ) : KMMIntResult()
    data class ErrorKMMIntResult(
    val throwable: Throwable
    ) : KMMIntResult()
    Shared code
    

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13. Sealed classes
    fun getRandomIntWrappedInIntResult(): KMMIntResult {
    val isSuccess = Random.nextBoolean()
    return if(isSuccess) {
    SuccessKMMIntResult(Random.nextInt(until = 10))
    } else {
    ErrorKMMIntResult(RuntimeException("There was an error, Int not generated"))
    }
    }
    Shared code
    

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14. Sealed classes
    val randomInt: KMMIntResult = getRandomIntWrappedInIntResult()
    val randomIntText: String = when (randomInt) {
    is KMMIntResult.ErrorKMMIntResult -> {
    "Error: ${randomInt.throwable.message}"
    }
    is KMMIntResult.SuccessKMMIntResult -> {
    "Success: ${randomInt.value}"
    }
    }
    Android
    

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15. Sealed classes
    let randomInt: KMMIntResult = RandomNumberGeneratorKt.getRandomIntWrappedInIntResult()
    let randomIntText: String
    switch randomInt {
    case let error as KMMIntResult.ErrorKMMIntResult:
    randomIntText = "Error: \(error.throwable.message ?? error.throwable.description())"
    case let success as KMMIntResult.SuccessKMMIntResult:
    randomIntText = "Success: \(success.value)"
    default:
    randomIntText = "This never happens"
    }
    iOS
    

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16. Generic sealed class
    sealed class KMMResult
    data class SuccessKMMResult(
    val value: Value
    ): KMMResult()
    data class ErrorKMMResult(
    val throwable: Throwable
    ): KMMResult()
    Shared code
    

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17. Generic sealed class
    fun getRandomIntWrappedInResult(): KMMResult {
    val isSuccess = Random.nextBoolean()
    return if(isSuccess) {
    SuccessKMMResult(Random.nextInt(until = 10))
    } else {
    ErrorKMMResult(RuntimeException("There was an error, Int not generated"))
    }
    }
    Shared code
    

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18. Generic sealed class
    Android
    val randomInt: KMMResult = getRandomIntWrappedInResult()
    val randomIntText: String = when (randomInt) {
    is KMMResult.ErrorKMMResult -> {
    "Error: ${randomInt.throwable.message}"
    }
    is KMMResult.SuccessKMMResult -> {
    "Success: ${randomInt.value}"
    }
    }
    

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19. Generic sealed class
    iOS
    let randomInt: KMMResult = RandomNumberGeneratorKt.getRandomIntWrappedInIntResult()
    let randomIntText: String
    switch randomInt {
    case let error as KMMResultErrorKMMResult:
    randomIntText = "Error: \(error.throwable.message ?? error.throwable.description())"
    case let success as KMMResultSuccessKMMResult:
    randomIntText = "Success: \(success.value)"
    default:
    randomIntText = "This never happens"
    }
    

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20. Generics sealed class
    First solution
    data class ErrorKMMResult(
    val throwable: Throwable
    ): KMMResult()
    data class ErrorKMMResult(
    val throwable: Throwable
    ): KMMResult()
    case let error as
    KMMResultErrorKMMResult:
    case let error as
    KMMResultErrorKMMResult:
    

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21. Generics sealed class
    Second solution
    enum SwiftResult {
    case error(String)
    case success(Value)
    }
    

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22. Generics sealed class
    Second solution
    func toSwiftResult(kmmResult: KMMResult) -> SwiftResult {
    if let successResult = kmmResult as? KMMResultSuccessKMMResult {
    return SwiftResult.success(successResult.value!)
    }
    if let errorResult = kmmResult as? KMMResultErrorKMMResult {
    return SwiftResult.error(errorResult.throwable.message ?? errorResult.throwable.description())
    }
    return SwiftResult.error("Unexpected error converting to SwiftResult")
    }
    

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23. Use moko-kswift
    https://github.com/icerockdev/moko-kswift
    KSwift is gradle plugin to generate Swift-friendly API for Kotlin/Native framework.
    Note: at the moment, all subclasses in a sealed class must be nested, otherwise the
    generation fails.
    Generics sealed classes
    Definitive solution
    

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24. public enum KMMResultKs {
    case successKMMResult(KMMResultSuccessKMMResult)
    case errorKMMResult(KMMResultErrorKMMResult)
    public var sealed: KMMResult {
    switch self {
    case .successKMMResult(let obj):
    return obj as shared.KMMResult
    case .errorKMMResult(let obj):
    return obj as shared.KMMResult
    }
    }
    public init(_ obj: KMMResult) {
    if let obj = obj as? shared.KMMResultSuccessKMMResult {
    self = .successKMMResult(obj)
    } else if let obj = obj as? shared.KMMResultErrorKMMResult {
    self = .errorKMMResult(obj)
    } else {
    fatalError("KMMResultKs not synchronized with KMMResult class")
    }
    }
    }
    Generics sealed classes
    

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25. Inline class
    fun formatFirstAndLastName(firstName: String, lastName: String): String =
    "$firstName $lastName"
    formatFirstAndLastName("Emanuele", "Papa")
    formatFirstAndLastName("Papa", "Emanuele")
    Shared code
    

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26. Inline class
    fun formatFirstAndLastName(firstName: FirstName, lastName: LastName): String =
    "${firstName.firstName} ${lastName.lastName}"
    value class FirstName(val firstName: String)
    value class LastName(val lastName: String)
    formatFirstAndLastName(
    FirstName("John"),
    LastName("Doe")
    )
    Shared code
    

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27. Inline class
    ProfileFormatterKt.formatFirstAndLastName(firstName: , lastName: )
    KT-32352
    iOS
    

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28. Inline class
    fun formatFirstAndLastName(
    firstName: FirstNameIos,
    lastName: LastNameIos
    ): String {
    return formatFirstAndLastName(FirstName(firstName.firstName), LastName(lastName.lastName))
    }
    data class FirstNameIos(
    val firstName: String
    )
    data class LastNameIos(
    val lastName: String
    )
    Shared iOS code
    

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29. Inline class
    ProfileFormatterKt.formatFirstAndLastName(
    firstName: FirstNameIos(firstName: "John"),
    lastName: LastNameIos(lastName: "Doe")
    )
    iOS code
    

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30. A coroutine is a concurrency design pattern that you
    can use on Android to simplify code that executes
    asynchronously.
    All of this doesn't exist in ObjectiveC
    Coroutines
    

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31. Coroutines
    class CoroutinesProfileFormatter {
    suspend fun formatFirstAndLastNameWithCoroutines(
    firstName: String,
    lastName: String
    ): String {
    delay(1000)
    return "$firstName $lastName"
    }
    }
    Shared code
    

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32. Coroutines
    fun onShowMyNameClicked() {
    viewModelScope.launch {
    mutableState.value = mutableState.value.copy(
    formattedName = coroutinesProfileFormatter.formatFirstAndLastNameWithCoroutines(
    "John",
    "Doe"
    )
    )
    }
    }
    

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33. Coroutines
    func onShowMyNameClicked() {
    coroutinesProfileFormatter.formatFirstAndLastNameWithCoroutines(
    firstName: "John",
    lastName: "Doe"
    ) { formattedName, error in
    DispatchQueue.main.async {
    self.state.formattedName = formattedName!
    }
    }
    }
    

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34. Coroutines
    https://github.com/rickclephas/KMP-NativeCoroutines
    Solution
    func onShowMyNameWithCombineClicked() {
    let formatFirstAndLastNamePublisher = createPublisher(
    for: coroutinesProfileFormatter.formatFirstAndLastNameWithCoroutinesNative(
    firstName: "Async",
    lastName: "Doe"
    ))
    formatFirstAndLastNamePublisher
    .subscribe(on: DispatchQueue.main)
    .receive(on: DispatchQueue.main)
    .sink { completion in
    print("Received completion: \(completion)")
    } receiveValue: { value in
    self.state.formattedNameWithAsync = value
    }
    }
    

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35. Coroutines
    The new memory manager is just being promoted to Beta and it's enabled by
    default from Kotlin 1.7.20.
    In the new memory manager (MM), we're lifting restrictions on object sharing:
    there's no need to freeze objects to share them between threads anymore.
    To enable it in Kotlin < 1.7.20
    kotlin.native.binary.memoryModel=experimental
    https://github.com/JetBrains/kotlin/blob/master/kotlin-native/NEW_MM.md
    

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36. Result
    fun getRandomDouble(): Result {
    return if(Random.nextBoolean()) {
    Result.success(Random.nextDouble())
    } else {
    Result.failure(Throwable("Can't generate a new double"))
    }
    }
    Shared code
    

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37. Result
    fun onShowRandomDoubleClicked() {
    val randomDouble = getRandomDouble()
    randomDouble.fold(
    onSuccess = { double ->
    mutableState.value = mutableState.value.copy(formattedRandomDouble = double.toString())
    },
    onFailure = { throwable ->
    mutableState.value = mutableState.value.copy(formattedRandomDouble = throwable.toString())
    })
    }
    Android
    

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38. Result
    func onShowRandomDoubleClicked() {
    let randomDouble = RandomDoubleGeneratorKt.getRandomDouble()
    //randomDouble has type Any?
    state.formattedRandomDouble = String(describing: randomDouble)
    }
    iOS
    

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39. This is another thing which code generation might solve...
    Default parameters
    KT-38685
    

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40. Exception handling
    Kotlin -> Only unchecked exceptions
    Swift -> Only checked errors
    Exception specified with @Throws -> Propagated as NSError
    Exception without @Throws -> iOS crash
    Always catch Exceptions in the shared code and return a Result like class
    

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41. Pure Kotlin -> ✅
    KMM library -> ✅
    Custom (expect interface, actual 1 native library for Android, 1 native
    library for iOS) -> ✅❌
    Third-party libraries
    

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42. class
    SystemInfoRetrieverImpl(
    private val systemInfoRetriever: SystemInfoRetrieverNativeWrapper
    ): SystemInfoRetriever
    interface
    SystemInfoRetrieverNativeWrapper {
    fun getSystemName(): String
    }
    class
    SystemInfoRetrieverAndroid :
    SystemInfoRetrieverNativeWrapper
    class
    SystemInfoRetrieverIOS:
    SystemInfoRetrieverNativeWrapper
    interface
    SystemInfoRetriever {
    fun getSystemName(): String
    }
    Third-party libraries
    

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43. Use this Gradle plugin to create a Swift Package Manager manifest
    and an XCFramework for iOS devs when you create a KMM library
    SPM
    https://github.com/ge-org/multiplatform-swiftpackage
    

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44. iOS devs will like:
    an Xcode plugin which allows debugging of Kotlin code
    running in an iOS application, directly from Xcode.
    https://github.com/touchlab/xcode-kotlin
    IDEs
    

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45. Android devs will like:
    an IDE similar to IntelliJ IDEA/Android Studio but for
    iOS: AppCode!
    IDEs
    

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46. Let's hope most of these issues will be
    officially fixed soon...
    but in the meanwhile, let's rock with
    KMM!
    The future
    

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47. Thank you!
    

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48. Drop a line to e.papa@zest.one
    Get in touch with me
    www.emanuelepapa.dev
    

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