From Android to Multiplatform and beyond | DevFest Berlin

With Kotlin Multiplatform getting increasingly established, many Android libraries became multiplatform.

But how to make an existing Android library multiplatform?

In this talk, we will cover the common challenges faced while migrating Android libraries to Kotlin Multiplatform, like handling platform-specific dependencies, re-organizing the project structure without losing the contributor's history, testing on multiple platforms, and publishing the library.

Transcript

1. @marcoGomier Marco Gomiero Senior Android Developer @ Airalo Google Developer Expert for Kotlin From Android to Multiplatform and beyond
2. @marcoGomier • Started in 2016 as an Android library RSSParser • Time to make it Multiplatform!
3. @marcoGomier Project Structure
4. @marcoGomier Android Library Source Set
5. @marcoGomier Kotlin Multiplatform Source Sets Android Library Source Set
6. @marcoGomier How to migrate to Multiplaform source sets?
7. @marcoGomier * without losing git and contributors history How to migrate to Multiplaform source sets? *
8. @marcoGomier Create a new library project
9. @marcoGomier Create a new library project https://terrakok.github.io/kmp-web-wizard/
10. @marcoGomier Create a new library project
11. @marcoGomier • Move the new source sets inside the existing library project • Rename the old source set • Duplicate and keep the old source set in the repo (for reference) • Move the existing code to the androidMain source set • Make the Android part work as before without sharing The Recipe
12. @marcoGomier • Move the new source sets inside the existing library project • Rename the old source set • Duplicate and keep the old source set in the repo (for reference) • Move the existing code to the androidMain source set • Make the Android part work as before without sharing The Recipe
13. @marcoGomier • Move the new source sets inside the existing library project • Rename the old source set • Duplicate and keep the old source set in the repo (for reference) • Move the existing code to the androidMain source set • Make the Android part work as before without sharing The Recipe
14. @marcoGomier • Move the new source sets inside the existing library project • Rename the old source set • Duplicate and keep the old source set in the repo (for reference) • Move the existing code to the androidMain source set • Make the Android part work as before without sharing The Recipe
15. @marcoGomier Platform-specific APIs
16. @marcoGomier RSSParser pre-multiplatform class Parser( private var callFactory: Call.Factory, private val charset: Charset? = null, ) { suspend fun getChannel(url: String): Channel = withContext(coroutineContext) { // If the charset is null, then "null" is saved in the database. // It's easier for retrieving data afterwards val charsetString = charset.toString() val cachedFeed = cacheManager?.getCachedFeed(url, charsetString) if (cachedFeed != null) { Log.d(TAG, "Returning object from cache") return@withContext cachedFeed } else { Log.d(TAG, "Returning data from network") val xml = CoroutineEngine.fetchXML(url, callFactory) val channel = CoroutineEngine.parseXML(xml, charset) cacheManager?.cacheFeed( url = url, channel = channel, charset = charsetString, ) return@withContext channel } } }
17. @marcoGomier suspend fun getChannel(url: String): Channel = withContext(coroutineContext) { // If the charset is null, then "null" is saved in the database. // It's easier for retrieving data afterwards val charsetString = charset.toString() val cachedFeed = cacheManager?.getCachedFeed(url, charsetString) if (cachedFeed != null) { Log.d(TAG, "Returning object from cache") return@withContext cachedFeed } else { Log.d(TAG, "Returning data from network") val xml = CoroutineEngine.fetchXML(url, callFactory) val channel = CoroutineEngine.parseXML(xml, charset) cacheManager?.cacheFeed( url = url, channel = channel, charset = charsetString, ) return@withContext channel } } }
18. @marcoGomier suspend fun getChannel(url: String): Channel = withContext(coroutineContext) { // If the charset is null, then "null" is saved in the database. // It's easier for retrieving data afterwards val charsetString = charset.toString() val cachedFeed = cacheManager?.getCachedFeed(url, charsetString) if (cachedFeed != null) { Log.d(TAG, "Returning object from cache") return@withContext cachedFeed } else { Log.d(TAG, "Returning data from network") val xml = CoroutineEngine.fetchXML(url, callFactory) val channel = CoroutineEngine.parseXML(xml, charset) cacheManager?.cacheFeed( url = url, channel = channel, charset = charsetString, ) return@withContext channel } } }
19. @marcoGomier class Parser private constructor( private var callFactory: Call.Factory, private val charset: Charset? = null, ) { suspend fun getChannel(url: String): Channel = withContext(coroutineContext) { // If the charset is null, then "null" is saved in the database. // It's easier for retrieving data afterwards val charsetString = charset.toString() val cachedFeed = cacheManager?.getCachedFeed(url, charsetString) if (cachedFeed != null) { Log.d(TAG, "Returning object from cache") return@withContext cachedFeed } else { Log.d(TAG, "Returning data from network") val xml = CoroutineEngine.fetchXML(url, callFactory) cacheManager?.cacheFeed( url = url, channel = channel, charset = charsetString, ) return@withContext channel } } } import okhttp3.Call import okhttp3.Callback import okhttp3.Request import okhttp3.Response val channel = CoroutineEngine.parseXML(xml, charset) cacheManager?.cacheFeed( url = url, channel = channel, import org.xmlpull.v1.XmlPullParser import org.xmlpull.v1.XmlPullParserFactory
20. @marcoGomier 🤔
21. @marcoGomier Expect/Actual https://kotlinlang.org/docs/multiplatform-connect-to-apis.html
22. @marcoGomier internal interface XmlFetcher { suspend fun fetchXml(url: String): ParserInput } Interface
23. @marcoGomier internal interface XmlFetcher { suspend fun fetchXml(url: String): ParserInput } internal class JvmXmlFetcher( private val callFactory: Call.Factory, ): XmlFetcher { override suspend fun fetchXml(url: String): ParserInput { val request = createRequest(url) return ParserInput( inputStream = callFactory.newCall(request).await() ) } } internal class IosXmlFetcher( private val nsUrlSession: NSURLSession, ): XmlFetcher { override suspend fun fetchXml(url: String): ParserInput = suspendCancellableCoroutine { continuation -> ... } }
24. @marcoGomier internal interface XmlParser { suspend fun parseXML(input: ParserInput): RssChannel } Interface
25. @marcoGomier internal class AndroidXmlParser( private val dispatcher: CoroutineDispatcher, ) : XmlParser { override suspend fun parseXML(input: ParserInput): RssChannel = withContext(dispatcher) { val factory = XmlPullParserFactory.newInstance() ... } } internal interface XmlParser { suspend fun parseXML(input: ParserInput): RssChannel } internal class IosXmlParser( private val dispatcher: CoroutineDispatcher, ) : XmlParser { override suspend fun parseXML(input: ParserInput): RssChannel = withContext(dispatcher) { suspendCancellableCoroutine { continuation -> ... } } } internal class JvmXmlParser( private val dispatcher: CoroutineDispatcher, ) : XmlParser { override suspend fun parseXML(input: ParserInput): RssChannel = withContext(dispatcher) { val parser = SAXParserFactory.newInstance().newSAXParser() ... } }
26. @marcoGomier class RssParser internal constructor( private val xmlFetcher: XmlFetcher, private val xmlParser: XmlParser, ) { suspend fun getRssChannel(url: String): RssChannel = withContext(coroutineContext) { val parserInput = xmlFetcher.fetchXml(url) return@withContext xmlParser.parseXML(parserInput) } } @marcoGomier
27. @marcoGomier Prefer interfaces, if possible class RssParser internal constructor( private val xmlFetcher: XmlFetcher, private val xmlParser: XmlParser, ) { suspend fun getRssChannel(url: String): RssChannel = withContext(coroutineContext) { val parserInput = xmlFetcher.fetchXml(url) return@withContext xmlParser.parseXML(parserInput) } }
28. @marcoGomier Expect/Actual internal expect class ParserInput internal actual data class ParserInput( val inputStream: InputStream ) internal actual data class ParserInput( val data: NSData )
29. @marcoGomier Networking
30. @marcoGomier Ktor? OkHttp? Platform-specific APIs?
31. @marcoGomier • Keep using OKhttp on Android and JVM • Use NSURLSession on iOS • Backward compatibility and popularity internal class JvmXmlFetcher( private val callFactory: Call.Factory, ): XmlFetcher { override suspend fun fetchXml(url: String): ParserInput { val request = createRequest(url) return ParserInput( inputStream = callFactory.newCall(request).await() ) } } internal class IosXmlFetcher( private val nsUrlSession: NSURLSession, ): XmlFetcher { override suspend fun fetchXml(url: String): ParserInput = suspendCancellableCoroutine { continuation -> ... } }
32. @marcoGomier Constructors
33. @marcoGomier Creating an RssParser Instance • Create an instance with platform-specific dependencies (OkHttp, NSURLSession) • Create an instance with default values • Create an instance from a KMP, Android, or JVM project
34. @marcoGomier Create an instance with platform-specific dependencies class RssParser internal constructor( private val xmlFetcher: XmlFetcher, private val xmlParser: XmlParser, ) { internal interface Builder { fun build(): RssParser } }
35. @marcoGomier class RssParserBuilder( private val callFactory: Call.Factory = OkHttpClient(), private val charset: Charset? = null, ): RssParser.Builder { override fun build(): RssParser { return RssParser( xmlFetcher = JvmXmlFetcher( callFactory = callFactory, ), xmlParser = AndroidXmlParser( charset = charset, dispatcher = Dispatchers.IO, ), ) } }
36. @marcoGomier class RssParserBuilder( private val callFactory: Call.Factory = OkHttpClient(), private val charset: Charset? = null, ): RssParser.Builder { override fun build(): RssParser { return RssParser( xmlFetcher = JvmXmlFetcher( callFactory = callFactory, ), xmlParser = AndroidXmlParser( charset = charset, dispatcher = Dispatchers.IO, ), ) } }
37. @marcoGomier class RssParserBuilder( private val callFactory: Call.Factory = OkHttpClient(), private val charset: Charset? = null, ): RssParser.Builder { override fun build(): RssParser { return RssParser( xmlFetcher = JvmXmlFetcher( callFactory = callFactory, ), xmlParser = AndroidXmlParser( charset = charset, dispatcher = Dispatchers.IO, ), ) } }
38. @marcoGomier class RssParserBuilder( private val callFactory: Call.Factory = OkHttpClient(), private val charset: Charset? = null, ): RssParser.Builder { override fun build(): RssParser { return RssParser( xmlFetcher = JvmXmlFetcher( callFactory = callFactory, ), xmlParser = JvmXmlParser( charset = charset, dispatcher = Dispatchers.IO, ), ) } }
39. @marcoGomier class RssParserBuilder( private val nsUrlSession: NSURLSession = NSURLSession.sharedSession, ): RssParser.Builder { override fun build(): RssParser { return RssParser( xmlFetcher = IosXmlFetcher( nsUrlSession = nsUrlSession, ), xmlParser = IosXmlParser( Dispatchers.IO ), ) } }
40. @marcoGomier class RssParserBuilder( private val nsUrlSession: NSURLSession = NSURLSession.sharedSession, ): RssParser.Builder { override fun build(): RssParser { return RssParser( xmlFetcher = IosXmlFetcher( nsUrlSession = nsUrlSession, ), xmlParser = IosXmlParser( Dispatchers.IO ), ) } }
41. @marcoGomier class RssParserBuilder( private val nsUrlSession: NSURLSession = NSURLSession.sharedSession, ): RssParser.Builder { override fun build(): RssParser { return RssParser( xmlFetcher = IosXmlFetcher( nsUrlSession = nsUrlSession, ), xmlParser = IosXmlParser( Dispatchers.IO ), ) } }
42. @marcoGomier Create an instance with default values expect fun RssParser(): RssParser
43. @marcoGomier expect fun RssParser(): RssParser actual fun RssParser(): RssParser = RssParserBuilder().build()
44. @marcoGomier Testing
45. @marcoGomier How to write one test for multiple platforms?
46. @marcoGomier Create an instance with default values internal expect object XmlParserFactory { fun createXmlParser(): XmlParser }
47. @marcoGomier internal expect object XmlParserFactory { fun createXmlParser(): XmlParser } internal actual object XmlParserFactory { actual fun createXmlParser(): XmlParser = JvmXmlParser(dispatcher = UnconfinedTestDispatcher()) } internal actual object XmlParserFactory { actual fun createXmlParser(): XmlParser = AndroidXmlParser(dispatcher = UnconfinedTestDispatcher()) } internal actual object XmlParserFactory { actual fun createXmlParser(): XmlParser = IosXmlParser(dispatcher = UnconfinedTestDispatcher()) }
48. @marcoGomier Test the Parser class XmlParserTest { @Test fun channelTitle_isCorrect() = runTest { val parser = XmlParserFactory.createXmlParser() val input = readFileFromResources("test-feed.xml") val channel = parser.parseXML(input) assertEquals("channel-title", channel.title) } }
49. @marcoGomier Test the Parser class XmlParserTest { @Test fun channelTitle_isCorrect() = runTest { val parser = XmlParserFactory.createXmlParser() val input = readFileFromResources("test-feed.xml") val channel = parser.parseXML(input) assertEquals("channel-title", channel.title) } }
50. @marcoGomier Test the Parser
51. @marcoGomier Test the Parser • Run tests on all the platforms • Run tests on a specific platform
52. @marcoGomier Test the Parser • Run tests on all the platforms • Run tests on a specific platform
53. @marcoGomier Test the Parser • Run tests on all the platforms • Run tests on a specific platform
54. @marcoGomier Test the Parser • Run tests on all the platforms • Run tests on a specific platform
55. @marcoGomier Test the Parser • Run tests on all the platforms • Run tests on a specific platform
56. @marcoGomier Test the Parser • Run tests on all the platforms • Run tests on a specific platform
57. @marcoGomier Test the Parser • Run tests on all the platforms • Run tests on a specific platform
58. @marcoGomier Test the Parser class XmlParserTest { @Test fun channelTitle_isCorrect() = runTest { val parser = XmlParserFactory.createXmlParser() val input = readFileFromResources("test-feed.xml") val channel = parser.parseXML(input) assertEquals("channel-title", channel.title) } }
59. @marcoGomier Test the Parser class XmlParserTest { @Test fun channelTitle_isCorrect() = runTest { val parser = XmlParserFactory.createXmlParser() val input = readFileFromResources("test-feed.xml") val channel = parser.parseXML(input) assertEquals("channel-title", channel.title) } }
60. @marcoGomier Gradle sets the test process's working directory to the module's directory Read files in tests but ... • No java.io.File on Kotlin/Native • On iOS, the working directory is unrelated to the project directory
61. @marcoGomier Use Environmental Variables!💡
62. @marcoGomier val rootDir = "${rootProject.rootDir.path}/rssparser/src/commonTest/resources"
63. @marcoGomier val rootDir = "${rootProject.rootDir.path}/rssparser/src/commonTest/resources" tasks.withType<Test>().configureEach { environment("TEST_RESOURCES_ROOT", rootDir) }
64. @marcoGomier val rootDir = "${rootProject.rootDir.path}/rssparser/src/commonTest/resources" tasks.withType<Test>().configureEach { environment("TEST_RESOURCES_ROOT", rootDir) } tasks.withType<KotlinNativeTest>().configureEach { environment("TEST_RESOURCES_ROOT", rootDir) }
65. @marcoGomier val rootDir = "${rootProject.rootDir.path}/rssparser/src/commonTest/resources" tasks.withType<Test>().configureEach { environment("TEST_RESOURCES_ROOT", rootDir) } tasks.withType<KotlinNativeTest>().configureEach { environment("TEST_RESOURCES_ROOT", rootDir) environment("SIMCTL_CHILD_TEST_RESOURCES_ROOT", rootDir) } https://stackover fl ow.com/a/53604237/5264056
66. @marcoGomier Read files in tests internal expect fun readFileFromResources( resourceName: String ): ParserInput
67. @marcoGomier internal actual fun readFileFromResources( resourceName: String, ): ParserInput { val path = System.getenv("TEST_RESOURCES_ROOT") val file = File("$path/$resourceName") return ParserInput( inputStream = FileInputStream(file) ) } internal expect fun readFileFromResources( resourceName: String ): ParserInput
68. @marcoGomier internal expect fun readBinaryResource( resourceName: String, ): ParserInput internal actual fun readFileFromResources( resourceName: String ): ParserInput { val s = getenv("TEST_RESOURCES_ROOT")?.toKString() val path = "$s/${resourceName}" val data = NSData.dataWithContentsOfFile(path) return ParserInput(requireNotNull(data)) } internal actual fun readFileFromResources( resourceName: String, ): ParserInput { val path = System.getenv("TEST_RESOURCES_ROOT") val file = File("$path/$resourceName") return ParserInput( inputStream = FileInputStream(file) ) }
69. @marcoGomier Publishing
70. @marcoGomier • Nothing to do! \o/ Publishing https://github.com/vanniktech/gradle-maven-publish-plugin
71. @marcoGomier Desktop (JVM), Android, iOS
72. @marcoGomier Desktop (JVM), Android, iOS, macOS, tvOS, watchOS*
73. @marcoGomier Maybe in the future •JS •WebAssembly •Linux
74. @marcoGomier Conclusions
75. @marcoGomier Conclusions • Adapting to different platforms requires time and thinking • Code organisation can be challenging, especially for maintaining git history • Prefer Interfaces over expect/actual, where possible
76. @marcoGomier https://github.com/prof18/RSS-Parser
77. @marcoGomier https://thebakery.dev/52/
78. @marcoGomier feed fl ow.dev/
79. @marcoGomier Thank you! Marco Gomiero Senior Android Developer @ Airalo Google Developer Expert for Kotlin > Twitter: @marcoGomier > Github: prof18 > Website: marcogomiero.com > Mastodon: androiddev.social/@marcogom > BlueSky: @marcogomiero.com
80. Bibliography / Useful Links • https: // kotlinlang.org/docs/multiplatform-mobile-understand-project- structure.html#source-sets • https: // kotlinlang.org/docs/multiplatform-connect-to-apis.html • https: // square.github.io/okhttp/ • https: // ktor.io/ • https: // developer.apple.com/documentation/foundation/nsurlsession • https: // kotlinlang.org/docs/multiplatform-run-tests.html • https: // publicobject.com/2023/04/16/read-a-project-file-in-a-kotlin-multiplatform-test/ • https: // github.com/vanniktech/gradle-maven-publish-plugin • https: // github.com/prof18/RSS-Parser • https: // thebakery.dev/52/ • https: // www.feedflow.dev/