Best Practices for Developing Amazing Kotlin Multiplatform Apps

Appdevcon edition

Transcript

1. Kotlin Multiplatform Apps Pamela Hill Developer Advocate - JetBrains Best Practices for Developing ✨Amazing✨
2. Introduction
3. • It’s not an introduction to Kotlin Multiplatform (that was Marco’s talk). • It’s not about all the best practices of the technology. • It’s not about processes and tooling. What this talk is not about
4. Three sets of best practices to address some common questions and mistakes. 1. Question: How do I make my Kotlin/Swift interop look more like idiomatic Swift? 2. Question: Can I / how do I consume suspend functions and Flows from Swift? 3. Mistake: Misusing expect/actual building blocks. What this talk is about
5. Improving your Kotlin/Swift interop 🤝
6. • Decision made in ~2018 for Kotlin to be interoperable with Objective-C, not Swift • Why? ◦ Can be used in all iOS projects, not just Swift ones ◦ Swift was still on the road to maturity/adoption at that stage Kotlin/Objective-C interop
7. This means: Kotlin is not directly interoperable with Swift, but indirectly via an Objective-C bridge.
8. Some Kotlin features: • Work exactly as expected • Work with a small workaround • Work better with a community solution • Don’t work (small minority) What this means for Swift developers
9. JetBrains is working on Swift export, which will solve the Kotlin-to-Swift interop challenges. 󰠁󰳕
10. The community is also helping improve the current experience with plugins and libraries. 🤝
11. • Interoperability encyclopedia • Explains how Kotlin features currently work from Swift • Lots of code samples • Gives workarounds / community solutions • Includes playground app with runnable samples Kotlin/Swift Interopedia [kotl.in/interopedia]
12. ✅ Some features work exactly as expected ✏ Some features need a slight modification 🤝 Some features need a community solution 🛑 Some features don’t work
13. • Classes • Properties • Functions & constructors • Higher-order functions • Some basic types • Some collections • Etc. Examples
14. ✅ Some features work exactly as expected ✏ Some features need a slight modification 🤝 Some features need a community solution 🛑 Some features don’t work
15. • Basic types like integers and Char • Collections with basic types • Objects • Companion objects • Some generics • Etc. Examples
16. object AnalyticsLogger { const val LOGIN_SCREEN_NAME = "login_screen" fun logLogin() { println(LOGIN_SCREEN_NAME) } } Slight modification example - Objects ✓ Property called ‘shared’ appears Used from Kotlin: AnalyticsLogger.LOGIN_SCREEN_NAME AnalyticsLogger.logLogin() Used from Swift:
17. ✅ Some features work exactly as expected ✏ Some features need a slight modification 🤝 Some features need a community solution 🛑 Some features don’t work
18. Some community rock stars 🎸 @TouchlabHQ • Created SKIE @RickClephas • Created KMP-NativeCoroutines • Contributed several Swift interop annotations
19. • Kotlin compiler plugin • Narrows gap between Kotlin and Swift • Stepping stone to official Swift interop • List of features always growing, but for now: ◦ Enumerations (enums, sealed hierarchies) ◦ Functions (default arguments etc) ◦ Reactive (coroutines and Flows) [next section] SKIE [https://skie.touchlab.co/]
20. plugins { … id("co.touchlab.skie") version ("0.6.1") } Installing the SKIE plugin build.gradle.kts
21. Enum Interop Somewhere in Kotlin enum class Colors { RED, GREEN, BLUE }
22. func colorAbbreviations(colors: Colors) -> String { switch colors { case .red: return "R" case .green: return "G" case .blue: return "B" default: return "D" } } Enum Interop - from Swift Somewhere in Swift
23. func colorAbbreviations(colors: Colors) -> String { switch colors { case .red: return "R" case .green: return "G" case .blue: return "B" default: return "D" } } Enum Interop - from Swift Somewhere in Swift Required: Switch must be exhaustive
24. 󰡸 Detective Pamela and the case of the curious Swift code.
25. __attribute__((swift_name("Colors"))) @interface ComposeAppColors : ComposeAppKotlinEnum<ComposeAppColors *> @property (class, readonly) ComposeAppColors *red __attribute__((swift_name("red"))); @property (class, readonly) ComposeAppColors *green __attribute__((swift_name("green"))); @property (class, readonly) ComposeAppColors *blue __attribute__((swift_name("blue"))); + (ComposeAppKotlinArray<ComposeAppColors *> *)values __attribute__((swift_name("values()"))); @property (class, readonly) NSArray<ComposeAppColors *> *entries __attribute__((swift_name("entries"))); @end Enum Interop - The Investigation 󰡸 Somewhere in Objective-C
26. Conclusion: Enum values become properties. 󰡸
27. Touchlab’s SKIE to the rescue! 🦸
28. public enum Colors : Swift.Hashable, Swift.CaseIterable, Swift._ObjectiveCBridgeable { case red case green case blue … } SKIE Enum Interop - The Investigation 󰡸
29. public enum Colors : Swift.Hashable, Swift.CaseIterable, Swift._ObjectiveCBridgeable { case red case green case blue … } SKIE Enum Interop - The Investigation 󰡸
30. func colorAbbreviations(colors: Colors) -> String { switch colors { case .red: return "R" case .green: return "G" case .blue: return "B" } } SKIE Enum interop Somewhere in Swift No default case needed!
31. sealed class NetworkResponse { data class Data(val body: String) : NetworkResponse() data object Error : NetworkResponse() } Sealed Classes Interop Somewhere in Kotlin
32. func processNetworkResponse(response: NetworkResponse){ switch response { case is NetworkResponse.Data: print("Data \(body)"); case is NetworkResponse.Error: print("Error"); default: print("Default") } } Sealed Classes Interop - from Swift Somewhere in Swift
33. func processNetworkResponse(response: NetworkResponse){ switch response { case is NetworkResponse.Data: print("Data \(body)"); case is NetworkResponse.Error: print("Error"); default: print("Default") } } Sealed Classes Interop - from Swift Somewhere in Swift Required: Switch must be exhaustive
34. __attribute__((swift_name("NetworkResponse"))) @interface ComposeAppNetworkResponse : ComposeAppBase @end __attribute__((swift_name("NetworkResponse.Data"))) @interface ComposeAppNetworkResponseData : ComposeAppNetworkResponse … @property (readonly) NSString *body __attribute__((swift_name("body"))); @end __attribute__((swift_name("NetworkResponse.Error"))) @interface ComposeAppNetworkResponseError : ComposeAppNetworkResponse … @end Sealed Classes Interop - The Investigation 󰡸 Somewhere in Objective-C
35. __attribute__((swift_name("NetworkResponse"))) @interface ComposeAppNetworkResponse : ComposeAppBase @end __attribute__((swift_name("NetworkResponse.Data"))) @interface ComposeAppNetworkResponseData : ComposeAppNetworkResponse … @property (readonly) NSString *body __attribute__((swift_name("body"))); @end __attribute__((swift_name("NetworkResponse.Error"))) @interface ComposeAppNetworkResponseError : ComposeAppNetworkResponse … @end Sealed Classes Interop - The Investigation 󰡸 Somewhere in Objective-C
36. __attribute__((swift_name("NetworkResponse"))) @interface ComposeAppNetworkResponse : ComposeAppBase @end __attribute__((swift_name("NetworkResponse.Data"))) @interface ComposeAppNetworkResponseData : ComposeAppNetworkResponse … @property (readonly) NSString *body __attribute__((swift_name("body"))); @end __attribute__((swift_name("NetworkResponse.Error"))) @interface ComposeAppNetworkResponseError : ComposeAppNetworkResponse … @end Sealed Classes Interop - The Investigation 󰡸 Somewhere in Objective-C
37. Conclusion: Sealed sub-classes become subtypes. 󰡸
38. SKIE saves the day… again! 󰭈
39. public extension ComposeApp.Skie.KotlinProject__composeApp.NetworkResponse { @frozen enum __Sealed : Swift.Hashable { case data(ComposeApp.NetworkResponse.Data) case error(ComposeApp.NetworkResponse.Error) } } SKIE Sealed Classes Interop - The Investigation 󰡸 Somewhere in Swift
40. public func onEnum<__Sealed : ComposeApp.NetworkResponse>(of sealed: __Sealed) -> ComposeApp.Skie.KotlinProject__composeApp.NetworkResponse.__Sealed { if let sealed = sealed as? ComposeApp.NetworkResponse.Data { return ComposeApp.Skie.KotlinProject__composeApp.NetworkResponse.__Sealed.data(seal ed) } else if let sealed = sealed as? ComposeApp.NetworkResponse.Error { return ComposeApp.Skie.KotlinProject__composeApp.NetworkResponse.__Sealed.error(sea led) } else { fatalError("Unknown subtype. This error should not happen under normal circumstances since SirClass: ComposeApp.NetworkResponse is sealed.") } } SKIE Sealed Classes Interop - The Investigation 󰡸 Somewhere in Swift
41. public func onEnum<__Sealed : ComposeApp.NetworkResponse>(of sealed: __Sealed) -> ComposeApp.Skie.KotlinProject__composeApp.NetworkResponse.__Sealed { if let sealed = sealed as? ComposeApp.NetworkResponse.Data { return ComposeApp.Skie.KotlinProject__composeApp.NetworkResponse.__Sealed.data(seal ed) } else if let sealed = sealed as? ComposeApp.NetworkResponse.Error { return ComposeApp.Skie.KotlinProject__composeApp.NetworkResponse.__Sealed.error(sea led) } else { fatalError("Unknown subtype. This error should not happen under normal circumstances since SirClass: ComposeApp.NetworkResponse is sealed.") } } SKIE Sealed Classes Interop - The Investigation 󰡸 Somewhere in Swift
42. public func onEnum<__Sealed : ComposeApp.NetworkResponse>(of sealed: __Sealed) -> ComposeApp.Skie.KotlinProject__composeApp.NetworkResponse.__Sealed { if let sealed = sealed as? ComposeApp.NetworkResponse.Data { return ComposeApp.Skie.KotlinProject__composeApp.NetworkResponse.__Sealed.data(seal ed) } else if let sealed = sealed as? ComposeApp.NetworkResponse.Error { return ComposeApp.Skie.KotlinProject__composeApp.NetworkResponse.__Sealed.error(sea led) } else { fatalError("Unknown subtype. This error should not happen under normal circumstances since SirClass: ComposeApp.NetworkResponse is sealed.") } } SKIE Sealed Classes Interop - The Investigation 󰡸 Somewhere in Swift
43. public func onEnum<__Sealed : ComposeApp.NetworkResponse>(of sealed: __Sealed) -> ComposeApp.Skie.KotlinProject__composeApp.NetworkResponse.__Sealed { if let sealed = sealed as? ComposeApp.NetworkResponse.Data { return ComposeApp.Skie.KotlinProject__composeApp.NetworkResponse.__Sealed.data(seal ed) } else if let sealed = sealed as? ComposeApp.NetworkResponse.Error { return ComposeApp.Skie.KotlinProject__composeApp.NetworkResponse.__Sealed.error(sea led) } else { fatalError("Unknown subtype. This error should not happen under normal circumstances since SirClass: ComposeApp.NetworkResponse is sealed.") } } SKIE Sealed Classes Interop - The Investigation 󰡸 Somewhere in Swift
44. func processNetworkResponse(response: NetworkResponse){ switch onEnum(of: response) { case .data: print("Data \(body)"); case .error: print("Error"); } } SKIE Sealed Classes interop Somewhere in Swift No default case needed!
45. fun connect(retries: Int = 2) { … } Default Args Interop Somewhere in Kotlin
46. connect() Default Args Interop - from Swift Somewhere in Swift Missing argument for parameter ‘retries’ in call
47. Conclusion: Default arguments are not supported in Objective-C. 󰡸
48. SKIE - you rock star! 󰭉
49. @DefaultArgumentInterop.Enabled fun connect(retries: Int = 2) { … } SKIE Default Args Interop - The Investigation 󰡸 Somewhere in Kotlin
50. public func connect() { return ComposeApp.PlatformKt.connect() } public func connect(retries: Swift.Int32) { return ComposeApp.PlatformKt.connect(retries: retries) } SKIE Default Args Interop - The Investigation 󰡸 Somewhere in Swift
51. connect() SKIE Default Args Interop Somewhere in Swift 👏
52. ✅ Some features work exactly as expected ✏ Some features need a slight modification 🤝 Some features need a community solution 🛑 Some features don’t work
53. • Annotations • Inline classes • Some generics Examples
54. Calling coroutines and flows from Swift 🤙
55. Android iOS Demo
56. Android with Compose Use LaunchedEffect var rocketLaunches by remember { mutableStateOf(emptyList<RocketLaunch>()) } LaunchedEffect(Unit) { rocketLaunches = sdk.getLaunches() } PullToRefreshRocketLaunches( … listRocketLaunch = rocketLaunches, … )
57. Android with Compose Use LaunchedEffect var rocketLaunches by remember { mutableStateOf(emptyList<RocketLaunch>()) } LaunchedEffect(Unit) { rocketLaunches = sdk.getLaunches() } PullToRefreshRocketLaunches( … listRocketLaunch = rocketLaunches, … )
58. Android with Compose Use LaunchedEffect var rocketLaunches by remember { mutableStateOf(emptyList<RocketLaunch>()) } LaunchedEffect(Unit) { rocketLaunches = sdk.getLaunches() } PullToRefreshRocketLaunches( … listRocketLaunch = rocketLaunches, … )
59. iOS with SwiftUI Approach 1: Completion Handlers sdk.getLaunches(completionHandler: { launches, error in if let launches = launches { self.launches = .result(launches) } else { self.launches = .error(error?.localizedDescription ?? "error") } })
60. iOS with SwiftUI Approach 1: Completion Handlers sdk.getLaunches(completionHandler: { launches, error in if let launches = launches { self.launches = .result(launches) } else { self.launches = .error(error?.localizedDescription ?? "error") } })
61. Oh no! 🫣 That’s not very readable!
62. Oh no #2! 🫣 The operation is also not cancellable!
63. iOS with SwiftUI Approach 2: async/await @MainActor class ViewModel: ObservableObject { … Task { do { let launches = try await sdk.getLaunches() self.launches = .result(launches) } catch { self.launches = .error(error.localizedDescription) } } }
64. iOS with SwiftUI Approach 2: async/await @MainActor class ViewModel: ObservableObject { … Task { do { let launches = try await sdk.getLaunches() self.launches = .result(launches) } catch { self.launches = .error(error.localizedDescription) } } }
65. iOS with SwiftUI Approach 2: async/await @MainActor class ViewModel: ObservableObject { … Task { do { let launches = try await sdk.getLaunches() self.launches = .result(launches) } catch { self.launches = .error(error.localizedDescription) } } }
66. iOS with SwiftUI Approach 2: async/await @MainActor class ViewModel: ObservableObject { … Task { do { let launches = try await sdk.getLaunches() self.launches = .result(launches) } catch { self.launches = .error(error.localizedDescription) } } }
67. Oh no! 🫣 The operation is still not cancellable!
68. This approach is still experimental! 🧪
69. iOS with SwiftUI Approach 3: KMP-NativeCoroutines class SpaceXSDK () { private val api = SpaceXApi() @NativeCoroutines @Throws(Exception::class) suspend fun getLaunches(): List<RocketLaunch> { return api.getAllLaunches() } }
70. Community rock stars to the rescue! 󰭉
71. iOS with SwiftUI Approach 3: KMP-NativeCoroutines @MainActor class ViewModel: ObservableObject { … let task = Task { do { let launches = try await asyncFunction(for: sdk.getLaunches()) self.launches = .result(launches) } catch { self.launches = .error(error.localizedDescription) } } task.cancel() } }
72. iOS with SwiftUI Approach 3: KMP-NativeCoroutines @MainActor class ViewModel: ObservableObject { … let task = Task { do { let launches = try await asyncFunction(for: sdk.getLaunches()) self.launches = .result(launches) } catch { self.launches = .error(error.localizedDescription) } } task.cancel() } }
73. iOS with SwiftUI Approach 4: SKIE class SpaceXSDK () { private val api = SpaceXApi() @Throws(Exception::class) suspend fun getLaunches(): List<RocketLaunch> { return api.getAllLaunches() } }
74. iOS with SwiftUI Approach 4: SKIE @MainActor class ViewModel: ObservableObject { … let task = Task { do { let launches = try await sdk.getLaunches() self.launches = .result(launches) } catch { self.launches = .error(error.localizedDescription) } } task.cancel() } }
75. Demo
76. The problem with Flows on iOS • Objective-C does not support generics on protocols • So, flow interfaces lose their generic value type • Still need cancellation support and main thread safety
77. Community rock stars save the day… again! 󰭈
78. iOS with SwiftUI Solution 1: KMP-NativeCoroutines @MainActor class ViewModel: ObservableObject { @Published var launches = LoadableLaunches.loading … func startObserving(){ self.launches = .loading Task { do { let sequence = asyncSequence(for: self.sdk.getLaunches()) for try await data in sequence { self.launches = .result(data) } } catch { print("Failed with error: \(error)") self.launches = .error("Error") } }
79. iOS with SwiftUI Solution 1: KMP-NativeCoroutines @MainActor class ViewModel: ObservableObject { @Published var launches = LoadableLaunches.loading … func startObserving(){ self.launches = .loading Task { do { let sequence = asyncSequence(for: self.sdk.getLaunches()) for try await data in sequence { self.launches = .result(data) } } catch { print("Failed with error: \(error)") self.launches = .error("Error") } }
80. class ViewModel: ObservableObject { @Published var launches = LoadableLaunches.loading … func startObserving(){ self.launches = .loading Task { let sequence = self.sdk.getLaunches() for await data in sequence { self.launches = .result(data) } } } } iOS with SwiftUI Solution 2: SKIE Becomes Swift AsyncSequence Doesn’t need @MainActor annotation
81. KMP-NativeCoroutines • Generates needed wrappers • Requires annotation, function wrappers • Supports async/await, Combine, and RxSwift • Available for longer than SKIE, and may encounter fewer edge-cases
82. SKIE • Augments the Objective-C API produced by the Kotlin compiler • Requires no additional work after setup • Supports async/await, while Combine, RxSwift require adapters • Offers other features to produce a Swift-friendly API from Kotlin
83. Using the expect/actual mechanism correctly ✅
84. 🔥Hot🔥 tip #1: Check for a suitable Kotlin Multiplatform library first
85. github.com/terrakok/kmp-awesome
87. But if you must…
88. iosMain Kotlin/Native androidMain Kotlin/JVM commonMain Common Kotlin import java.util.* actual fun randomUUID() = UUID.randomUUID().toString() import platform.Foundation.NSUUID actual fun randomUUID(): String = NSUUID().UUIDString() expect fun randomUUID(): String Expect/actual functions
89. 🔥Hot🔥 tip #2: Use expect/actual functions for simple cases
90. 🔥Hot🔥 tip #3: Use interfaces for more complex cases
91. class AndroidPlatform: Platform class iOSPlatform: Platform interface Platform { val name: String } Interfaces iosMain Kotlin/Native androidMain Kotlin/JVM commonMain Common Kotlin
92. But how do you provide the implementation?
93. class AndroidPlatform: Platform … actual fun getPlatform() = AndroidPlatform() class iOSPlatform: Platform … actual fun getPlatform() = iOSPlatform() interface Platform { val name: String } expect fun getPlatform(): Platform iosMain Kotlin/Native androidMain Kotlin/JVM commonMain Common Kotlin Factory functions
94. class AndroidPlatform: Platform … class MyApp : Application() { override fun onCreate() { super.onCreate() app(AndroidPlatform()) } } class iOSPlatform: Platform … @main struct iOSApp : App { init() { app(IOSPlatform()) } } interface Platform { val name: String } fun app(p: Platform) { // app logic } Early Entry Points iosMain Kotlin/Native androidMain Kotlin/JVM commonMain Common Kotlin
95. 🔥Hot🔥 tip #4: Use Dependency Injection frameworks to provide interface implementations
96. class AndroidPlatform: Platform … actual val platformModule: Module = module { single<Platform> { AndroidPlatform() } } class iOSPlatform: Platform … actual val platformModule = module { single<Platform> { IOSPlatform() } } interface Platform { val name: String } expect val platformModule: Module iosMain Kotlin/Native androidMain Kotlin/JVM commonMain Common Kotlin Dependency Injection with Koin
97. ✅ Koin ✅ Kodein ✅ kotlin-inject ❌ Dagger ❌ Hilt
98. Questions? 󰢨󰢧 Stay in touch 𝕏 @pamelaahill 󰟲 pamelaahill.com