From Kotlin to Native and back: accessing native macOS API in Compose Multiplatform | droidcon Berlin

Compose Multiplatform makes it easy to build cross-platform desktop apps with Kotlin and Compose, but what about native APIs, like iCloud on macOS? Accessing such APIs isn't possible through the regular Compose Multiplatform toolchain. However, with a bit of “magic,” we can turn dreams (or feature requests) into reality.

In this talk, we'll explore how to combine Kotlin/Native and the JNI (Java Native Interface) to bridge the gap between a JVM-based UI and native system features. We'll write Kotlin code, compile it into a native library, and call it back from Kotlin.

You'll learn how to build Kotlin/Native code into a native macOS dynamic library and integrate it into a Compose Multiplatform desktop app, unlocking access to iCloud and enabling features like backup and restore for your app’s data.

Transcript

1. FROM KOTLIN TO NATIVE AND BACK: accessing native macOS API in Compose Multiplatform. MARCO GOMIERO Senior Android Engineer @ Airalo Android GDE
2. Compose Multiplatform
3. Compose Multiplatform
4. feed fl ow.dev
5. Hi, I came across this rss app and I fi nd it very interesting. I downloaded it on all my devices and I would like to have them synchronised. While I managed to do so via my iCloud account on iPhone and iPad, it seems that on my MacBook I can only do this via a dropbox account, not iCloud. So my Mac will not be in sync with my news as for the other devices. Is there a way to do this? Thank you on my MacBook I can only do this via a dropbox account, not iCloud.
6. Hi, I came across this rss app and I fi nd it very interesting. I downloaded it on all my devices and I would like to have them synchronised. While I managed to do so via my iCloud account on iPhone and iPad, it seems that on my MacBook I can only do this via a dropbox account, not iCloud. So my Mac will not be in sync with my news as for the other devices. Is there a way to do this? Thank you on my MacBook I can only do this via a dropbox account, not iCloud.
7. 🤔
9. Kotlin Native 🤝 JNI tl;dr;
10. Kotlin Native & JNI
11. 📦 Kotlin Native & JNI Compose Multiplatform dylib
12. Why?
13. Compose Multiplatform
14. Compose Multiplatform JVM
15. Compose Multiplatform JVM macOS
16. Compose Multiplatform JVM macOS
17. Compose Multiplatform JVM macOS
18. Compose Multiplatform JVM macOS
19. How?
20. How? JNI
21. How? JNI (Java Native Interface) https://docs.oracle.com/javase/8/docs/technotes/guides/jni/
22. How? JNI (Java Native Interface) JNA
23. How? JNI (Java Native Interface) JNA (Java Native Access) https://github.com/java-native-access/jna
24. How? JNI (Java Native Interface) JNA (Java Native Access) Project Panama https://openjdk.org/projects/panama/
25. How? JNI (Java Native Interface) JNA (Java Native Access) Project Panama https://openjdk.org/projects/panama/ 🆕
26. How? JNI JNA Project Panama ✅
29. https://bsky.app/pro fi le/kpgalligan.bsky.social/post/3lcgoc32zf22c
30. 💡
31. 💡 Kotlin Native
32. Kotlin Native
33. Kotlin Native • Apple • Linux • Android NDK • Windows
34. Kotlin Native
35. Kotlin Native 🔗 C & Objective-C
36. Kotlin Native 🔗 C & Objective-C cinterop
37. import platform.Foundation.NSFileManager
38. import platform.Foundation.NSFileManager https://developer.apple.com/documentation/foundation/ fi lemanager?language=objc
39. import platform.Foundation.NSFileManager val documentsDirectory: NSURL? = NSFileManager.defaultManager.URLsForDirectory( directory = NSDocumentDirectory, inDomains = NSUserDomainMask, ).firstOrNull() as? NSURL?
40. import platform.Foundation.NSFileManager val documentsDirectory: NSURL? = NSFileManager.defaultManager.URLsForDirectory( directory = NSDocumentDirectory, inDomains = NSUserDomainMask, ).firstOrNull() as? NSURL? https://developer.apple.com/documentation/foundation/ fi lemanager/urls(for:in:)?language=objc
41. import platform.Foundation.NSFileManager val documentsDirectory: NSURL? = NSFileManager.defaultManager.URLsForDirectory( directory = NSDocumentDirectory, inDomains = NSUserDomainMask, ).firstOrNull() as? NSURL?
42. import platform.Foundation.NSFileManager val documentsDirectory: NSURL? = NSFileManager.defaultManager.URLsForDirectory( directory = NSDocumentDirectory, inDomains = NSUserDomainMask, ).firstOrNull() as? NSURL?
43. val documentsDirectory: NSURL? = NSFileManager.defaultManager.URLsForDirectory( directory = NSDocumentDirectory, inDomains = NSUserDomainMask, ) .firstOrNull() as? NSURL? @kotlinx.cinterop.ExternalObjCClass public open class NSFileManager : platform.darwin.NSObject { public companion object : platform.Foundation.NSFileManagerMeta, kotlinx.cinterop.ObjCClassOf<platform.Foundation.NSFileManager> { } @kotlinx.cinterop.ObjCConstructor public constructor() { /* compiled code */ } @kotlinx.cinterop.ObjCMethod public open external fun URLsForDirectory( directory: kotlin.ULong /* from: platform.Foundation.NSSearchPathDirectory */, inDomains: kotlin.ULong, /* from: platform.Foundation.NSSearchPathDomainMask */ ): kotlin.collections.List<*> { /* compiled code */ } // Other methods }
44. val documentsDirectory: NSURL? = NSFileManager.defaultManager.URLsForDirectory( directory = NSDocumentDirectory, inDomains = NSUserDomainMask, ) .firstOrNull() as? NSURL? @kotlinx.cinterop.ExternalObjCClass public open class NSFileManager : platform.darwin.NSObject { public companion object : platform.Foundation.NSFileManagerMeta, kotlinx.cinterop.ObjCClassOf<platform.Foundation.NSFileManager> { } @kotlinx.cinterop.ObjCConstructor public constructor() { /* compiled code */ } @kotlinx.cinterop.ObjCMethod public open external fun URLsForDirectory( directory: kotlin.ULong /* from: platform.Foundation.NSSearchPathDirectory */, inDomains: kotlin.ULong, /* from: platform.Foundation.NSSearchPathDomainMask */ ): kotlin.collections.List<*> { /* compiled code */ } // Other methods }
45. val documentsDirectory: NSURL? = NSFileManager.defaultManager.URLsForDirectory( directory = NSDocumentDirectory, inDomains = NSUserDomainMask, ) .firstOrNull() as? NSURL? @kotlinx.cinterop.ExternalObjCClass public open class NSFileManager : platform.darwin.NSObject { public companion object : platform.Foundation.NSFileManagerMeta, kotlinx.cinterop.ObjCClassOf<platform.Foundation.NSFileManager> { } @kotlinx.cinterop.ObjCConstructor public constructor() { /* compiled code */ } @kotlinx.cinterop.ObjCMethod public open external fun URLsForDirectory( directory: kotlin.ULong /* from: platform.Foundation.NSSearchPathDirectory */, inDomains: kotlin.ULong, /* from: platform.Foundation.NSSearchPathDomainMask */ ): kotlin.collections.List<*> { /* compiled code */ } // Other methods }
46. Generate JNI bindings
47. 📝 .def fi le
48. 📝 .def fi le headers = jni.h
49. 📝 .def fi le headers = jni.h https://kotlinlang.org/docs/native-de fi nition- fi le.html
50. kotlin { }
51. kotlin { macosArm64("ikloud") { compilations.getByName("main") { cinterops { val jni by creating { packageName = “com.prof18.jni" val javaHome = File(System.getProperty("java.home")) includeDirs( Callable { File(javaHome, "include") }, Callable { File(javaHome, "include/darwin") }, ) } } } binaries { sharedLib { baseName = "ikloud" } } } }
52. kotlin { macosArm64("ikloud") { compilations.getByName("main") { cinterops { } } } }
53. kotlin { macosArm64("ikloud") { compilations.getByName("main") { cinterops { val jni by creating { packageName = “com.prof18.jni" val javaHome = File(System.getProperty("java.home")) includeDirs( Callable { File(javaHome, "include") }, Callable { File(javaHome, "include/darwin") }, ) } } } } }
54. kotlin { macosArm64("ikloud") { compilations.getByName("main") { cinterops { val jni by creating { packageName = “com.prof18.jni" val javaHome = File(System.getProperty("java.home")) includeDirs( Callable { File(javaHome, "include") }, Callable { File(javaHome, "include/darwin") }, ) } } } } }
55. kotlin { macosArm64("ikloud") { compilations.getByName("main") { cinterops { val jni by creating { packageName = “com.prof18.jni" val javaHome = File(System.getProperty("java.home")) includeDirs( Callable { File(javaHome, "include") }, Callable { File(javaHome, "include/darwin") }, ) } } } } }
58. kotlin { macosArm64("ikloud") { compilations.getByName("main") { cinterops { val jni by creating { packageName = “com.prof18.jni" val javaHome = File(System.getProperty("java.home")) includeDirs( Callable { File(javaHome, "include") }, Callable { File(javaHome, "include/darwin") }, ) } } } } }
59. kotlin { macosArm64("ikloud") { } }
60. kotlin { macosArm64("ikloud") { binaries { sharedLib { baseName = "ikloud" } } } }
61. Coding time ☁
62. fun uploadToICloud( ) { }
63. fun uploadToICloud(env: CPointer<JNIEnvVar>, clazz: jclass) { }
64. fun uploadToICloud(env: CPointer<JNIEnvVar>, clazz: jclass) { val databasePath = NSURL.fileURLWithPath(path = "myfile.txt") }
65. fun uploadToICloud(env: CPointer<JNIEnvVar>, clazz: jclass) { val databasePath = NSURL.fileURLWithPath(path = "myfile.txt") val iCloudUrl: NSURL? = NSFileManager.defaultManager .URLForUbiquityContainerIdentifier(containerIdentifier = "icloud.com.myapp.identifier") ?.URLByAppendingPathComponent(pathComponent = "Documents") ?.URLByAppendingPathComponent(pathComponent = "myfile.txt") }
66. fun uploadToICloud(env: CPointer<JNIEnvVar>, clazz: jclass) { val databasePath = NSURL.fileURLWithPath(path = "myfile.txt") val iCloudUrl: NSURL? = NSFileManager.defaultManager .URLForUbiquityContainerIdentifier(containerIdentifier = "icloud.com.myapp.identifier") ?.URLByAppendingPathComponent(pathComponent = "Documents") ?.URLByAppendingPathComponent(pathComponent = "myfile.txt") if (iCloudUrl == null) { // handle error return } }
67. fun uploadToICloud(env: CPointer<JNIEnvVar>, clazz: jclass) { val databasePath = NSURL.fileURLWithPath(path = "myfile.txt") val iCloudUrl: NSURL? = NSFileManager.defaultManager .URLForUbiquityContainerIdentifier(containerIdentifier = "icloud.com.myapp.identifier") ?.URLByAppendingPathComponent(pathComponent = "Documents") ?.URLByAppendingPathComponent(pathComponent = "myfile.txt") if (iCloudUrl == null) { // handle error return } // Copy doesn't override the item NSFileManager.defaultManager.removeItemAtURL( iCloudUrl, error = null, ) }
68. fun uploadToICloud(env: CPointer<JNIEnvVar>, clazz: jclass) { val databasePath = NSURL.fileURLWithPath(path = "myfile.txt") val iCloudUrl: NSURL? = NSFileManager.defaultManager .URLForUbiquityContainerIdentifier(containerIdentifier = "icloud.com.myapp.identifier") ?.URLByAppendingPathComponent(pathComponent = "Documents") ?.URLByAppendingPathComponent(pathComponent = "myfile.txt") if (iCloudUrl == null) { // handle error return } // Copy doesn't override the item NSFileManager.defaultManager.removeItemAtURL( iCloudUrl, error = null, ) memScoped { } }
69. fun uploadToICloud(env: CPointer<JNIEnvVar>, clazz: jclass) { val databasePath = NSURL.fileURLWithPath(path = "myfile.txt") val iCloudUrl: NSURL? = NSFileManager.defaultManager .URLForUbiquityContainerIdentifier(containerIdentifier = "icloud.com.myapp.identifier") ?.URLByAppendingPathComponent(pathComponent = "Documents") ?.URLByAppendingPathComponent(pathComponent = "myfile.txt") if (iCloudUrl == null) { // handle error return } // Copy doesn't override the item NSFileManager.defaultManager.removeItemAtURL( iCloudUrl, error = null, ) memScoped { val errorPtr: ObjCObjectVar<NSError?> = alloc() } }
70. fun uploadToICloud(env: CPointer<JNIEnvVar>, clazz: jclass) { val databasePath = NSURL.fileURLWithPath(path = "myfile.txt") val iCloudUrl: NSURL? = NSFileManager.defaultManager .URLForUbiquityContainerIdentifier(containerIdentifier = "icloud.com.myapp.identifier") ?.URLByAppendingPathComponent(pathComponent = "Documents") ?.URLByAppendingPathComponent(pathComponent = "myfile.txt") if (iCloudUrl == null) { // handle error return } // Copy doesn't override the item NSFileManager.defaultManager.removeItemAtURL( iCloudUrl, error = null, ) memScoped { val errorPtr: ObjCObjectVar<NSError?> = alloc() NSFileManager.defaultManager.copyItemAtURL( srcURL = databasePath, toURL = iCloudUrl, error = errorPtr.ptr, ) } }
71. fun uploadToICloud(env: CPointer<JNIEnvVar>, clazz: jclass) { val databasePath = NSURL.fileURLWithPath(path = "myfile.txt") val iCloudUrl: NSURL? = NSFileManager.defaultManager .URLForUbiquityContainerIdentifier(containerIdentifier = "icloud.com.myapp.identifier") ?.URLByAppendingPathComponent(pathComponent = "Documents") ?.URLByAppendingPathComponent(pathComponent = "myfile.txt") if (iCloudUrl == null) { // handle error return } // Copy doesn't override the item NSFileManager.defaultManager.removeItemAtURL( iCloudUrl, error = null, ) memScoped { val errorPtr: ObjCObjectVar<NSError?> = alloc() NSFileManager.defaultManager.copyItemAtURL( srcURL = databasePath, toURL = iCloudUrl, error = errorPtr.ptr, ) if (errorPtr.value != null) { // handle error } } }
72. fun uploadToICloud(env: CPointer<JNIEnvVar>, clazz: jclass) { } @CName(“Java_com_my_package_ICloudNativeBridge_uploadToICloud”)
73. @CName(“Java_com_my_package_ICloudNativeBridge_uploadToICloud”) a pre fi x Java_ a mangled fully-quali fi ed class name com_my_package_ICloudNativeBridge an underscore separator _ a mangled method name uploadToICloud
74. package com.mypackage class ICloudNativeBridge { external fun uploadToICloud() }
75. Building
76. ./gradlew :linkReleaseSharedIkloud
77. ./gradlew :linkReleaseSharedIkloud
78. ./gradlew :linkReleaseSharedIkloud
79. Bundling 📦
80. https://www.jetbrains.com/help/kotlin-multiplatform-dev/compose-native-distribution.html#adding- fi les-to-packaged-application
81. https://www.jetbrains.com/help/kotlin-multiplatform-dev/compose-native-distribution.html#adding- fi les-to-packaged-application <RESOURCES_ROOT_DIR>/<OS_NAME>-<ARCH_NAME>
82. https://www.jetbrains.com/help/kotlin-multiplatform-dev/compose-native-distribution.html#adding- fi les-to-packaged-application <RESOURCES_ROOT_DIR>/<OS_NAME>-<ARCH_NAME> <RESOURCES_ROOT_DIR>/<OS_NAME>
83. https://www.jetbrains.com/help/kotlin-multiplatform-dev/compose-native-distribution.html#adding- fi les-to-packaged-application <RESOURCES_ROOT_DIR>/<OS_NAME>-<ARCH_NAME> <RESOURCES_ROOT_DIR>/<OS_NAME> <RESOURCES_ROOT_DIR>/common
84. compose.desktop.application.nativeDistributions { appResourcesRootDir.set(project.layout.projectDirectory.dir("resources")) }
85. Loading ⌛
86. val resourcesDir = System.getProperty("compose.application.resources.dir")
87. val resourcesDir = System.getProperty("compose.application.resources.dir") val libraryPath = resourcesDir + File.separator + System.mapLibraryName("ikloud")
88. val resourcesDir = System.getProperty("compose.application.resources.dir") val libraryPath = resourcesDir + File.separator + System.mapLibraryName("ikloud") System.load(libraryPath)
89. try { val resourcesDir = System.getProperty("compose.application.resources.dir") val libraryPath = resourcesDir + File.separator + System.mapLibraryName("ikloud") System.load(libraryPath) } catch (_: UnsatisfiedLinkError) { // handle error }
90. Pro fi t 💰
91. ICloudNativeBridge().uploadToICloud()
92. Conclusions
93. Conclusions Some additional complexity
94. Conclusions Some additional complexity Some learning curve
95. Conclusions Some additional complexity Some learning curve Worth it to avoid Objective-C
96. Conclusions Some additional complexity Some learning curve Worth it to avoid Objective-C Keep the API surface as simple as possible
97. https://www.marcogomiero.com/posts/2025/compose-desktop-macos-api-jni/
98. THANK YOU! marcoGomier marcogomiero.com @marcogomiero.com @marcogom@androiddev.social