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

👨💻 Senior Android Engineer @ TIER
 
Google Developer Expert for Kotlin
Marco Gomiero
@marcoGomier
Beyond one platform:
migrating an

Android library to

Kotlin Multiplatform

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@marcoGomier
• Started in 2016 as an Android library
RSSParser
• Time to make it Multiplatform!

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@marcoGomier
Project Structure

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@marcoGomier
Android Library Source Set

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@marcoGomier
Kotlin Multiplatform Source Sets
Android Library Source Set

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@marcoGomier
How to migrate to
Multiplaform source sets?

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@marcoGomier
* without losing git and contributors history
How to migrate to
Multiplaform source sets? *

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@marcoGomier
Create a new library project

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@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

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@marcoGomier
Platform-specific APIs

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@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

}

}

}

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@marcoGomier









} else {





url = url,

channel = channel,


)


}

}

}

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@marcoGomier
class Parser private constructor(

private var callFactory: Call.Factory,


) {









} else {




url = url,

channel = channel,


)


}

}

}
import okhttp3.Call

import okhttp3.Callback

import okhttp3.Request

import okhttp3.Response


url = url,

channel = channel,

import org.xmlpull.v1.XmlPullParser

import org.xmlpull.v1.XmlPullParserFactory

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@marcoGomier
🤔

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@marcoGomier
Expect/Actual
https://kotlinlang.org/docs/multiplatform-connect-to-apis.html

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@marcoGomier
internal interface XmlFetcher {

suspend fun fetchXml(url: String): ParserInput

}
Interface

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@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 ->

...

}

}

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@marcoGomier
internal interface XmlParser {

suspend fun parseXML(input: ParserInput): RssChannel

}
Interface

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@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(


) : XmlParser {




...

}

}

}

internal class JvmXmlParser(


) : XmlParser {



val parser = SAXParserFactory.newInstance().newSAXParser()

...

}

}

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@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

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@marcoGomier
Prefer interfaces, if possible



) {

suspend fun getRssChannel(url: String): RssChannel = withContext(coroutineContext) {

val parserInput = xmlFetcher.fetchXml(url)

return@withContext xmlParser.parseXML(parserInput)

}

}

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@marcoGomier
Expect/Actual
internal expect class ParserInput
internal actual data class ParserInput(

val inputStream: InputStream

)
internal actual data class ParserInput(

val data: NSData

)

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@marcoGomier
Networking

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@marcoGomier
Ktor? OkHttp?

Platform-specific APIs?

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@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 =


...

}

}

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@marcoGomier
Constructors

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@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

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@marcoGomier
Create an instance with platform-specific dependencies



) {

internal interface Builder {

fun build(): RssParser

}

}

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@marcoGomier
class RssParserBuilder(

private val callFactory: Call.Factory? = 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,

),

)

}

}

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@marcoGomier

private val callFactory: Call.Factory? = null,




val client = callFactory ?: OkHttpClient()

return RssParser(

xmlFetcher = JvmXmlFetcher(

callFactory = client,

charset = charset,

),

xmlParser = JvmXmlParser(

charset = charset,

dispatcher = Dispatchers.IO,

),

)

}

}

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@marcoGomier

private val nsUrlSession: NSURLSession? = null,



return RssParser(

xmlFetcher = IosXmlFetcher(

nsUrlSession = nsUrlSession ?: NSURLSession.sharedSession,

),

xmlParser = IosXmlParser(

Dispatchers.Default

),

)

}

}

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@marcoGomier
Create an instance with default values
expect fun RssParser(): RssParser

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@marcoGomier
expect fun RssParser(): RssParser
actual fun RssParser(): RssParser = RssParserBuilder().build()

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@marcoGomier
Testing

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@marcoGomier
How to write one test
 
for multiple platforms?

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@marcoGomier
Create an instance with default values
internal expect object XmlParserFactory {

fun createXmlParser(): XmlParser

}

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@marcoGomier
internal expect object XmlParserFactory {

fun createXmlParser(): XmlParser

}
internal actual object XmlParserFactory {

actual fun createXmlParser(): XmlParser = JvmXmlParser(dispatcher = UnconfinedTestDispatcher())

}

actual fun createXmlParser(): XmlParser = AndroidXmlParser(dispatcher = UnconfinedTestDispatcher())

}

actual fun createXmlParser(): XmlParser = IosXmlParser(dispatcher = UnconfinedTestDispatcher())

}

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@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)

}

}

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@marcoGomier
Test the Parser

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@marcoGomier
Test the Parser
• Run tests on all the platforms

• Run tests on iOS

• Run test on JVM

• Run test on Android

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@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)

}

}

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@marcoGomier
Read files in tests
val file = File("./src/commonTest/resources/$resourceName")

val inputStream = FileInputStream(file)

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@marcoGomier
Read files in tests
internal expect fun readBinaryResource(

resourceName: String,

): ParserInput

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@marcoGomier


): ParserInput
internal actual fun readBinaryResource(


): ParserInput {

val file = File(

"./src/commonTest/resources/$resourceName",

)

return ParserInput(

inputStream = FileInputStream(file)

)

}

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@marcoGomier


): ParserInput


): ParserInput {

val file = File(

"./src/commonTest/resources/$resourceName",

)

return ParserInput(

inputStream = FileInputStream(file)

)

}

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!!)

}

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@marcoGomier
Copy files

to iOS binary
tasks.register("copyIosTestResourcesArm64") {

from("src/commonTest/resources")

into("build/bin/iosSimulatorArm64/debugTest/resources")

}

tasks.register("copyIosTestResourcesX64") {

from("src/commonTest/resources")

into("build/bin/iosX64/debugTest/resources")

}

tasks.findByName("iosX64Test")?.dependsOn("copyIosTestResourcesX64")

tasks.findByName("iosSimulatorArm64Test")?.dependsOn("copyIosTestResourcesArm64")

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@marcoGomier https://publicobject.com/2023/04/16/read-a-project-
fi
le-in-a-kotlin-multiplatform-test/

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@marcoGomier
Publishing

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@marcoGomier
• Nothing to do! \o/
Publishing
https://github.com/vanniktech/gradle-maven-publish-plugin

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@marcoGomier
Conclusions

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@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

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@marcoGomier https://github.com/prof18/RSS-Parser

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@marcoGomier https://thebakery.dev/52/

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@marcoGomier https://www.feed
fl
ow.dev/

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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/

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@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

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

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

Marco Gomiero

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