Beyond one platform: migrating an Android library to Kotlin Multiplatform | Berlindroid August 2023

Back in 2016, out of my raising passion for Android development, I started working on RSSParser, a library for parsing RSS feeds on Android. Nowadays, out of my passion for Kotlin Multiplatform, I decided to make RSSParser Multiplatform... How hard can it be?

In this talk, I will share the journey of expanding an Android library into the iOS and JVM world. We will cover the challenges faced during the process, including how to handle platform-specific dependencies, code organization, and testing strategies.

By the end of this talk, you'll have a better understanding of the benefits and challenges of creating a Kotlin Multiplatform library, and you'll be equipped with the knowledge and tools you need to conquer the Multiplatform world!

Transcript

1. 👨💻 Senior Android Engineer @ TIER 
 Google Developer Expert for Kotlin Marco Gomiero @marcoGomier Beyond one platform: migrating an Android library to Kotlin Multiplatform
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
10. @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
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 Platform-specific APIs
15. @marcoGomier RSSParser pre-multiplatform 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) val channel = CoroutineEngine.parseXML(xml, charset) cacheManager?.cacheFeed( url = url, channel = channel, charset = charsetString, ) return@withContext channel } } }
16. @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 } } }
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 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
19. @marcoGomier 🤔
20. @marcoGomier Expect/Actual https://kotlinlang.org/docs/multiplatform-connect-to-apis.html
21. @marcoGomier internal interface XmlFetcher { suspend fun fetchXml(url: String): ParserInput } Interface
22. @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 -> ... } }
23. @marcoGomier internal interface XmlParser { suspend fun parseXML(input: ParserInput): RssChannel } Interface
24. @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() ... } }
25. @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
26. @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) } }
27. @marcoGomier Expect/Actual internal expect class ParserInput internal actual data class ParserInput( val inputStream: InputStream ) internal actual data class ParserInput( val data: NSData )
28. @marcoGomier Networking
29. @marcoGomier Ktor? OkHttp? Platform-specific APIs?
30. @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 -> ... } }
31. @marcoGomier Constructors
32. @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
33. @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 } }
34. @marcoGomier class RssParserBuilder( private val callFactory: Call.Factory? = null, private val charset: Charset? = null, ): RssParser.Builder { override fun build(): RssParser { val client = callFactory ?: OkHttpClient() return RssParser( xmlFetcher = JvmXmlFetcher( callFactory = client, charset = charset, ), xmlParser = AndroidXmlParser( charset = charset, dispatcher = Dispatchers.IO, ), ) } }
35. @marcoGomier class RssParserBuilder( private val callFactory: Call.Factory? = null, private val charset: Charset? = null, ): RssParser.Builder { override fun build(): RssParser { val client = callFactory ?: OkHttpClient() return RssParser( xmlFetcher = JvmXmlFetcher( callFactory = client, charset = charset, ), xmlParser = AndroidXmlParser( charset = charset, dispatcher = Dispatchers.IO, ), ) } }
36. @marcoGomier class RssParserBuilder( private val callFactory: Call.Factory? = null, private val charset: Charset? = null, ): RssParser.Builder { override fun build(): RssParser { val client = callFactory ?: OkHttpClient() return RssParser( xmlFetcher = JvmXmlFetcher( callFactory = client, charset = charset, ), xmlParser = AndroidXmlParser( charset = charset, dispatcher = Dispatchers.IO, ), ) } }
37. @marcoGomier class RssParserBuilder( private val callFactory: Call.Factory? = null, private val charset: Charset? = null, ): RssParser.Builder { override fun build(): RssParser { val client = callFactory ?: OkHttpClient() return RssParser( xmlFetcher = JvmXmlFetcher( callFactory = client, charset = charset, ), xmlParser = JvmXmlParser( charset = charset, dispatcher = Dispatchers.IO, ), ) } }
38. @marcoGomier class RssParserBuilder( private val nsUrlSession: NSURLSession? = null, ): RssParser.Builder { override fun build(): RssParser { return RssParser( xmlFetcher = IosXmlFetcher( nsUrlSession = nsUrlSession ?: NSURLSession.sharedSession, ), xmlParser = IosXmlParser( Dispatchers.Default ), ) } }
39. @marcoGomier class RssParserBuilder( private val nsUrlSession: NSURLSession? = null, ): RssParser.Builder { override fun build(): RssParser { return RssParser( xmlFetcher = IosXmlFetcher( nsUrlSession = nsUrlSession ?: NSURLSession.sharedSession, ), xmlParser = IosXmlParser( Dispatchers.Default ), ) } }
40. @marcoGomier Create an instance with default values expect fun RssParser(): RssParser
41. @marcoGomier expect fun RssParser(): RssParser actual fun RssParser(): RssParser = RssParserBuilder().build()
42. @marcoGomier Testing
43. @marcoGomier How to write one test 
 for multiple platforms?
44. @marcoGomier Create an instance with default values internal expect object XmlParserFactory { fun createXmlParser(): XmlParser }
45. @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()) }
46. @marcoGomier Test the Parser class XmlParserTest { private lateinit var parser: XmlParser @BeforeTest fun setUp() { parser = XmlParserFactory.createXmlParser() } @Test fun channelTitle_isCorrect() = runTest { val input = readBinaryResource(feedPath) val channel = parser.parseXML(input) assertEquals("channelTitle", channel.title) } }
47. @marcoGomier Test the Parser class XmlParserTest { private lateinit var parser: XmlParser @BeforeTest fun setUp() { parser = XmlParserFactory.createXmlParser() } @Test fun channelTitle_isCorrect() = runTest { val input = readBinaryResource(feedPath) val channel = parser.parseXML(input) assertEquals("channelTitle", channel.title) } }
48. @marcoGomier Test the Parser
49. @marcoGomier Test the Parser • Run tests on all the platforms • Run tests on iOS • Run test on JVM • Run test on Android
50. @marcoGomier Test the Parser • Run tests on all the platforms • Run tests on iOS • Run test on JVM • Run test on Android
51. @marcoGomier Test the Parser • Run tests on all the platforms • Run tests on iOS • Run test on JVM • Run test on Android
52. @marcoGomier Test the Parser • Run tests on all the platforms • Run tests on iOS • Run test on JVM • Run test on Android
53. @marcoGomier Test the Parser class XmlParserTest { private lateinit var parser: XmlParser @BeforeTest fun setUp() { parser = XmlParserFactory.createXmlParser() } @Test fun channelTitle_isCorrect() = runTest { val input = readBinaryResource(feedPath) val channel = parser.parseXML(input) assertEquals("channelTitle", channel.title) } }
54. @marcoGomier Test the Parser class XmlParserTest { private lateinit var parser: XmlParser @BeforeTest fun setUp() { parser = XmlParserFactory.createXmlParser() } @Test fun channelTitle_isCorrect() = runTest { val input = readBinaryResource(feedPath) val channel = parser.parseXML(input) assertEquals("channelTitle", channel.title) } }
55. @marcoGomier Read files in tests val file = File("./src/commonTest/resources/$resourceName") val inputStream = FileInputStream(file)
56. @marcoGomier Read files in tests internal expect fun readBinaryResource( resourceName: String, ): ParserInput
57. @marcoGomier internal expect fun readBinaryResource( resourceName: String, ): ParserInput internal actual fun readBinaryResource( resourceName: String, ): ParserInput { val file = File( "./src/commonTest/resources/$resourceName", ) return ParserInput( inputStream = FileInputStream(file) ) }
58. @marcoGomier internal expect fun readBinaryResource( resourceName: String, ): ParserInput internal actual fun readBinaryResource( resourceName: String, ): ParserInput { val file = File( "./src/commonTest/resources/$resourceName", ) return ParserInput( inputStream = FileInputStream(file) ) } internal actual fun readBinaryResource( resourceName: String ): ParserInput { val pathParts = resourceName.split("[.|/]".toRegex()) val path = NSBundle.mainBundle .pathForResource("resources/${pathParts[0]}", pathParts[1]) val data = NSData.dataWithContentsOfFile(path!!) return ParserInput(data!!) }
59. @marcoGomier internal expect fun readBinaryResource( resourceName: String, ): ParserInput internal actual fun readBinaryResource( resourceName: String, ): ParserInput { val file = File( "./src/commonTest/resources/$resourceName", ) return ParserInput( inputStream = FileInputStream(file) ) } internal actual fun readBinaryResource( resourceName: String ): ParserInput { val pathParts = resourceName.split("[.|/]".toRegex()) val path = NSBundle.mainBundle .pathForResource("resources/${pathParts[0]}", pathParts[1]) val data = NSData.dataWithContentsOfFile(path!!) return ParserInput(data!!) }
60. @marcoGomier internal expect fun readBinaryResource( resourceName: String, ): ParserInput internal actual fun readBinaryResource( resourceName: String, ): ParserInput { val file = File( "./src/commonTest/resources/$resourceName", ) return ParserInput( inputStream = FileInputStream(file) ) } internal actual fun readBinaryResource( resourceName: String ): ParserInput { val pathParts = resourceName.split("[.|/]".toRegex()) val path = NSBundle.mainBundle .pathForResource("resources/${pathParts[0]}", pathParts[1]) val data = NSData.dataWithContentsOfFile(path!!) return ParserInput(data!!) }
61. @marcoGomier Copy files to iOS binary tasks.register<Copy>("copyIosTestResourcesArm64") { from("src/commonTest/resources") into("build/bin/iosSimulatorArm64/debugTest/resources") } tasks.register<Copy>("copyIosTestResourcesX64") { from("src/commonTest/resources") into("build/bin/iosX64/debugTest/resources") } tasks.findByName("iosX64Test")?.dependsOn("copyIosTestResourcesX64") tasks.findByName("iosSimulatorArm64Test")?.dependsOn("copyIosTestResourcesArm64")
62. @marcoGomier https://publicobject.com/2023/04/16/read-a-project- fi le-in-a-kotlin-multiplatform-test/
63. @marcoGomier Publishing
64. @marcoGomier • Nothing to do! \o/ Publishing https://github.com/vanniktech/gradle-maven-publish-plugin
65. @marcoGomier Conclusions
66. @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
67. @marcoGomier https://github.com/prof18/RSS-Parser
68. @marcoGomier https://thebakery.dev/52/
69. @marcoGomier https://www.feed fl ow.dev/
70. 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/
71. @marcoGomier Thank you! > Twitter: @marcoGomier 
 > Github: prof18 
 > Website: marcogomiero.com 
 > Mastodon: androiddev.social/@marcogom 👨💻 Senior Android Engineer @ TIER 
 Google Developer Expert for Kotlin Marco Gomiero