Lessons learned after building apps with Kotlin Multiplatform

With Kotlin Multiplatform and Compose Multiplatform, you can share code and UI across multiple platforms, from Android and iOS to macOS, Windows, and Linux. But building apps on all these platforms brings unexpected challenges you don’t anticipate at the beginning.

In this talk, I’ll share what I’ve learned (often the hard way) while developing apps with Kotlin Multiplatform. You’ll hear why sharing less code can sometimes save more time, how dealing with the iOS Keychain from background services turned into a debugging odyssey, and why placing your database in the wrong Windows folder can lead to silent data loss during app updates. We’ll also cover sandboxing on macOS, the trade-offs between using interfaces and `expect/actual`, and the complexities of distributing apps across platforms.

Whether you’re just getting started with Kotlin Multiplatform or already deep into it, this talk might just save you from a few future debugging headaches.

Transcript

1. LESSON LEARNED AFTER BUILDING APPS WITH KOTLIN MULTIPLATFORM MARCO GOMIERO Senior Android Engineer @ Airalo Android GDE
7. airalo.com reader fl ow.app feed fl ow.dev uniwhere.com
8. Platform-speci fi c APIs
9. expect/actual
10. Prefer interfaces
11. Android Common iOS
12. Android Common iOS interface XmlFetcher { suspend fun fetchXml(url: String): ParserInput }
13. Android Common iOS internal class AndroidXmlFetcher( private val callFactory: Call.Factory, ) : XmlFetcher { override suspend fun fetchXml(url: String): ParserInput { val request = createRequest(url) return ParserInput( inputStream = callFactory.newCall(request).await() ) } }
14. Android Common iOS internal class IosXmlFetcher( private val nsUrlSession: NSURLSession, ) : XmlFetcher { override suspend fun fetchXml(url: String): ParserInput = suspendCancellableCoroutine { continuation -> ... continuation.resume(ParserInput(nsData = data)) } }
15. With interfaces you can create fakes
16. Android Android Common iOS
17. Common iOS Android Test Android class FakeAndroidXmlFetcher: XmlFetcher { override suspend fun fetchXml(url: String): ParserInput { val file = File("test.xml") return ParserInput( inputStream = FileInputStream(file), ) } }
18. With interfaces you can also inject stuff from Swift
19. Android Common iOS App interface HtmlParser { fun getTextFromHTML(html: String): String? }
20. iOS App Android class IosHtmlParser: HtmlParser { func getTextFromHTML(html: String) -> String? { let doc: Document? = try? SwiftSoup.parse(html) return try? doc?.text() } } Common
21. Use expect/actual for abstracting the same concept
22. Android Common iOS expect class ParserInput
23. Android Common iOS actual data class ParserInput( val inputStream: InputStream, )
24. Android Common iOS actual data class ParserInput( val data: NSData, )
25. You don’t need to share everything
26. Android Common iOS App interface HtmlParser { fun getTextFromHTML(html: String): String? }
27. Android Common iOS App internal class AndroidHtmlParser : HtmlParser { override fun getTextFromHTML(html: String): String? { val doc = Jsoup.parse(html) return doc.text() } }
28. Android Common iOS App class IosHtmlParser: HtmlParser { func getTextFromHTML(html: String) -> String? { let doc: Document? = try? SwiftSoup.parse(html) return try? doc?.text() } }
29. What to share?
30. What to share? It depends 🤷
31. What to share? Start from “boring” code It depends 🤷
32. Code that centralizes the source of truth What to share? It depends 🤷 Start from “boring” code
33. Code that can be gradually extracted What to share? It depends 🤷 Start from “boring” code Code that centralizes the source of truth
34. You need infrastructure
35. You need infrastructure 📦 Monorepo
36. KMP module XCFramework Android App iOS App Same Repository
37. You need infrastructure 📦 📦 Multiple Repos 📦 Monorepo
38. KMP module XCFramework Android App iOS App .aar Maven Repo Swift Package KMP library repository Android App Repository iOS App Repository
39. KMP module Android App .aar Maven Repo Maven Repository GitHub Packages Repsy Self hosted
40. KMP module XCFramework iOS App Swift Package Swift Package GitHub Releases Custom artifacts storage (S3, etc)
41. Don’t keep binaries in a repo
42. https://x.com/kpgalligan/status/1723364386046173493?s=20
43. // swift-tools-version:5.3 import PackageDescription let package = Package( name: "MyLibrary", platforms: [ .macOS(.v10_14), .iOS(.v13), .tvOS(.v13) ], products: [ .library( name: "MyLibrary", targets: ["SomeRemoteBinaryPackage"]) ], dependencies: [ // Dependencies declare other packages that this package depends on. ], targets: [ .binaryTarget( name: "SomeRemoteBinaryPackage", url: "https://url/to/some/remote/xcframework.zip", checksum: "The checksum of the ZIP archive that contains the XCFramework." ) ] ) https://developer.apple.com/documentation/xcode/distributing-binary-frameworks-as-swift-packages
44. https://developer.apple.com/documentation/xcode/distributing-binary-frameworks-as-swift-packages // swift-tools-version:5.3 import PackageDescription let package = Package( name: "MyLibrary", platforms: [ .macOS(.v10_14), .iOS(.v13), .tvOS(.v13) ], products: [ .library( name: "MyLibrary", targets: ["SomeRemoteBinaryPackage"]) ], dependencies: [ // Dependencies declare other packages that this package depends on. ], targets: [ .binaryTarget( name: "SomeRemoteBinaryPackage", url: "https://url/to/some/remote/xcframework.zip", checksum: "The checksum of the ZIP archive that contains the XCFramework." ) ] )
45. touchlab.co/kmmbridge/
46. Starting from scratch
47. Starting from scratch Easier to go with monorepo
48. Starting in an existing project
49. Starting in an existing project Easier to go with multiple Repos and Library model
50. Starting in an existing project Easier to go with multiple Repos and Library model When scaling up to full feature dev, maybe switch to sharing sources
51. There’s no silver bullet 🪄
52. touchlab.co/kmp-teams-use-source
53. Local setup
54. Local setup Gradle Composite Builds
55. includeBuild("../RSS-Parser") { dependencySubstitution { substitute(module("com.prof18.rssparser:rssparser")).using(project(":rssparser")) } } docs.gradle.org/current/userguide/composite_builds.html
56. includeBuild("../RSS-Parser") { dependencySubstitution { substitute(module("com.prof18.rssparser:rssparser")).using(project(":rssparser")) } } docs.gradle.org/current/userguide/composite_builds.html
57. Local setup Gradle Composite Builds
58. Local setup KMM Bridge Local Dev Flow Gradle Composite Builds
59. ./gradlew spmDevBuild touchlab.co/kmmbridge/spm/ioslocaldevspm
60. touchlab.co/kmmbridge/spm/ioslocaldevspm
61. You need to care about APIs
62. Kotlin → Obj-c → Swift
63. Use SKIE 🌌
64. skie.touchlab.co/
65. Swift interop is coming
66. kotlinlang.org/docs/native-swift-export.html
67. Make it a team effort
68. Debugging 🍎 🐛
69. touchlab.co/xcodekotlin
71. crashkios.touchlab.co/
73. Releasing on multiple platforms can be challenging
74. Understand how other platforms work
75. Understand how other platforms work Signing and provisioning pro fi les on Apple
76. Understand how other platforms work Signing and provisioning pro fi les on Apple Sandboxing on macOS
77. Understand how other platforms work Signing and provisioning pro fi les on Apple Sandboxing on macOS New fi le format for the Windows Store
78. Some random f*ckups learnings
79. Put the database in the right spot from the beginning!
80. developer.apple.com/documentation/xcode/con fi guring-app-groups
81. App Widget Share Extension
82. internal fun iosDirPath(folder:String):String{ val paths = NSSearchPathForDirectoriesInDomains(NSApplicationSupportDirectory, NSUserDomainMask, true) val documentsDirectory = paths[0] as String val databaseDirectory = "$documentsDirectory/$folder" val fileManager = NSFileManager.defaultManager() if (!fileManager.fileExistsAtPath(databaseDirectory)) fileManager.createDirectoryAtPath(databaseDirectory, true, null, null); //Create folder return databaseDirectory } github.com/touchlab/SQLiter/
83. internal fun iosDirPath(folder:String):String{ val paths = NSSearchPathForDirectoriesInDomains(NSApplicationSupportDirectory, NSUserDomainMask, true) val documentsDirectory = paths[0] as String val databaseDirectory = "$documentsDirectory/$folder" val fileManager = NSFileManager.defaultManager() if (!fileManager.fileExistsAtPath(databaseDirectory)) fileManager.createDirectoryAtPath(databaseDirectory, true, null, null); //Create folder return databaseDirectory } github.com/touchlab/SQLiter/
84. internal fun iosDirPath(folder:String):String{ val paths = NSSearchPathForDirectoriesInDomains(NSApplicationSupportDirectory, NSUserDomainMask, true) val documentsDirectory = paths[0] as String val databaseDirectory = "$documentsDirectory/$folder" val fileManager = NSFileManager.defaultManager() if (!fileManager.fileExistsAtPath(databaseDirectory)) fileManager.createDirectoryAtPath(databaseDirectory, true, null, null); //Create folder return databaseDirectory } github.com/touchlab/SQLiter/ folder == “database”
85. fun getAppGroupDatabasePath(): String { val fileManager = NSFileManager.defaultManager() val containerURL = fileManager.containerURLForSecurityApplicationGroupIdentifier("group.com.mycompany.myapp") check(containerURL != null) { "Could not access App Group container" } val containerPath = containerURL.path val directoryPath = "$containerPath/databases" if (!fileManager.fileExistsAtPath(directoryPath)) { fileManager.createDirectoryAtPath( directoryPath, withIntermediateDirectories = true, attributes = null, error = null, ) } return directoryPath }
86. fun getAppGroupDatabasePath(): String { val fileManager = NSFileManager.defaultManager() val containerURL = fileManager.containerURLForSecurityApplicationGroupIdentifier("group.com.mycompany.myapp") check(containerURL != null) { "Could not access App Group container" } val containerPath = containerURL.path val directoryPath = "$containerPath/databases" if (!fileManager.fileExistsAtPath(directoryPath)) { fileManager.createDirectoryAtPath( directoryPath, withIntermediateDirectories = true, attributes = null, error = null, ) } return directoryPath }
87. fun getAppGroupDatabasePath(): String { val fileManager = NSFileManager.defaultManager() val containerURL = fileManager.containerURLForSecurityApplicationGroupIdentifier("group.com.mycompany.myapp") check(containerURL != null) { "Could not access App Group container" } val containerPath = containerURL.path val directoryPath = "$containerPath/databases" if (!fileManager.fileExistsAtPath(directoryPath)) { fileManager.createDirectoryAtPath( directoryPath, withIntermediateDirectories = true, attributes = null, error = null, ) } return directoryPath } Now it’s hardcoded :)
88. fun createDatabaseDriver(): SqlDriver = NativeSqliteDriver( schema = MyDatabase.Schema, onConfiguration = { conf -> conf.copy( extendedConfig = conf.extendedConfig.copy( basePath = getAppGroupDatabasePath(), ), ) }, name = "MyDatabase.db", ) sqldelight.github.io/sqldelight/2.0.2/native_sqlite/
89. fun createDatabaseDriver(): SqlDriver = NativeSqliteDriver( schema = MyDatabase.Schema, onConfiguration = { conf -> conf.copy( extendedConfig = conf.extendedConfig.copy( basePath = getAppGroupDatabasePath(), ), ) }, name = "MyDatabase.db", ) sqldelight.github.io/sqldelight/2.0.2/native_sqlite/
90. By default, you can’t access the Keychain in background
91. developer.apple.com/documentation/uikit/using-background-tasks-to-update-your-app
92. public class KeychainSettings : Settings { private val cleaner: Cleaner? public constructor(service: String) { val cfService = CFBridgingRetain(service) defaultProperties = mapOf(kSecClass to kSecClassGenericPassword, kSecAttrService to cfService) cleaner = createCleaner(cfService) { CFBridgingRelease(it) } } } github.com/russhwolf/multiplatform-settings/
93. public class KeychainSettings : Settings { private val cleaner: Cleaner? public constructor(service: String) { val cfService = CFBridgingRetain(service) defaultProperties = mapOf(kSecClass to kSecClassGenericPassword, kSecAttrService to cfService) cleaner = createCleaner(cfService) { CFBridgingRelease(it) } } } github.com/russhwolf/multiplatform-settings/
94. public class KeychainSettings : Settings { private val cleaner: Cleaner? public constructor(service: String) { val cfService = CFBridgingRetain(service) defaultProperties = mapOf(kSecClass to kSecClassGenericPassword, kSecAttrService to cfService) cleaner = createCleaner(cfService) { CFBridgingRelease(it) } } public constructor(vararg defaultProperties: Pair<CFStringRef?, CFTypeRef?>) { this.defaultProperties = mapOf(kSecClass to kSecClassGenericPassword, *defaultProperties) cleaner = null } } github.com/russhwolf/multiplatform-settings/
95. public class KeychainSettings : Settings { private val cleaner: Cleaner? public constructor(service: String) { val cfService = CFBridgingRetain(service) defaultProperties = mapOf(kSecClass to kSecClassGenericPassword, kSecAttrService to cfService) cleaner = createCleaner(cfService) { CFBridgingRelease(it) } } public constructor(vararg defaultProperties: Pair<CFStringRef?, CFTypeRef?>) { this.defaultProperties = mapOf(kSecClass to kSecClassGenericPassword, *defaultProperties) cleaner = null } } github.com/russhwolf/multiplatform-settings/
96. object KeychainSettingsWrapper { private val cfService = CFBridgingRetain("FeedFlow2") val settings = KeychainSettings( kSecAttrService to cfService, kSecAttrAccessible to kSecAttrAccessibleAfterFirstUnlock, ) private val cleaner = createCleaner(cfService) { CFBridgingRelease(it) } }
97. object KeychainSettingsWrapper { private val cfService = CFBridgingRetain("FeedFlow2") val settings = KeychainSettings( kSecAttrService to cfService, kSecAttrAccessible to kSecAttrAccessibleAfterFirstUnlock, ) private val cleaner = createCleaner(cfService) { CFBridgingRelease(it) } }
98. Put the database in the right spot from the beginning. AGAIN! 😅
99. "${System.getProperty("user.home")}\\AppData\\Local\\FeedFlow" windowsreport.com/local-locallow-roaming-difference/ Gets wiped after an app update
100. "${System.getProperty("user.home")}\\AppData\\Local\\FeedFlow" windowsreport.com/local-locallow-roaming-difference/ "${System.getProperty("user.home")}\\AppData\\Roaming\\FeedFlow"
101. Conclusions
102. Conclusions Introduce changes incrementally
103. Conclusions Introduce changes incrementally Don’t force yourself to share everything
104. Conclusions Introduce changes incrementally Don’t force yourself to share everything Establish infrastructure and strategy
105. Conclusions Introduce changes incrementally Don’t force yourself to share everything Establish infrastructure and strategy It’s a team effort
106. Conclusions Introduce changes incrementally Don’t force yourself to share everything Establish infrastructure and strategy It’s a team effort Understand how other platforms work
107. THANK YOU! marcoGomier marcogomiero.com @marcogomiero.com @marcogom@androiddev.social