Ok Multiplatform! (Droidcon NYC 2018)

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Ok Multiplatform! @jessewilson @egorand

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What’s Okio?

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• Complements java.io and java.nio • Better API • Better performance • Started as a part of OkHttp • Basis for many Square OSS libraries What’s Okio?

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Okio OkHttp Moshi Wire Retroﬁt

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Why Kotlin?

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Kotlinizing Okio

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Attempt #1: Koio

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• Build a library like Okio but for Kotlin • Multiplatform by design • JDK6, JDK7, JDK8 and JS support FAILED Attempt #1: Koio

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What went wrong?

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• Too ambitious! • Missing primitives (UTF-8 encode/decode, synchronization, etc.) • Early days of multiplatform Kotlin

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Attempt #2: okio-ktx

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• New okio-kotlin module • Extension functions for frequently used factory methods NEVER SHIPPED Attempt #2: okio-ktx

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What went wrong?

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• No path to multiplatform • Redundant APIs for Kotlin callers • We didn’t get to use Kotlin features in Okio!

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Attempt #3: Okio 2

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Goals

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Multiplatform • Okio 2 will start as a JVM-only library • Okio 2 will be written in 100% Kotlin • Okio 2 will gradually get support for JS and Native

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JVM options for I/O java.io straightforward blocking API good performance max ~2000 threads doing I/O java.nio clumsy non-blocking API (callbacks!) good performance no thread count ceiling

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JVM options for I/O java.io straightforward blocking API good performance max ~2000 threads doing I/O java.nio clumsy non-blocking API (callbacks!) good performance no thread count ceiling kotlin.coroutines straightforward blocking API good performance no thread count ceiling

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Idiomatic Kotlin API • We want good experience for both Kotlin and Java users • e.g. extension functions that can be used as static methods on Java

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Okio 1.x val source = Okio.buffer(Okio.source(File("README.md"))) Okio 2.x val source = File("README.md").source().buffer()

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Okio 1.x public byte getByte(int pos) val bytes = ByteString.decodeHex(”cafebabe”) val byte = bytes.getByte(0) Okio 2.x @JvmName(“getByte") operator fun get(index: Int): Byte val bytes = "cafebabe".decodeHex() val byte = bytes[0]

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public int getSize() val size = str.getSize() Okio 1.x val bytes = ByteString.decodeHex(”cafebabe”) val size: Int val size = str.size Okio 2.x val bytes = "cafebabe".decodeHex()

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Costs

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Costs • Everyone who’s using Okio (or OkHttp, or Retroﬁt) now has a dependency on Kotlin standard library… • …which is 939 KiB… • …but it shrinks to 7 KiB with R8 or ProGuard

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The Journey to Okio 2

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Step #1: Gradle

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Project Structure

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dependencies { classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:${versions.kotlin}" classpath "org.jetbrains.kotlin:kotlin-native-gradle-plugin:${versions.kotlinNative}" } Project level

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apply plugin: 'org.jetbrains.kotlin.platform.common' apply plugin: 'org.jetbrains.kotlin.platform.jvm' apply plugin: 'org.jetbrains.kotlin.platform.js' apply plugin: 'org.jetbrains.kotlin.platform.native'

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'kotlin': [ 'gradlePlugin': "org.jetbrains.kotlin:kotlin-gradle-plugin:${versions.kotlin}", 'stdLib': [ 'common': "org.jetbrains.kotlin:kotlin-stdlib-common", 'jdk8': "org.jetbrains.kotlin:kotlin-stdlib-jdk8", 'jdk7': "org.jetbrains.kotlin:kotlin-stdlib-jdk7", 'jdk6': "org.jetbrains.kotlin:kotlin-stdlib", 'js': "org.jetbrains.kotlin:kotlin-stdlib-js", ], 'test': [ 'common': "org.jetbrains.kotlin:kotlin-test-common", 'annotations': "org.jetbrains.kotlin:kotlin-test-annotations-common", 'jdk': "org.jetbrains.kotlin:kotlin-test-junit", 'js': "org.jetbrains.kotlin:kotlin-test-js", ], 'native': [ 'gradlePlugin': "org.jetbrains.kotlin:kotlin-native-gradle-plugin:${versions.kotlinNative}", ] ], Dependencies

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Publishing

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Milestone #1 • We’ve got a fully working Kotlin multiplatform project! • Albeit with zero Kotlin ﬁles and JVM-only support

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Step #2: ⌥⇧⌘K

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• Convert Java ﬁles to Kotlin one-by-one • Rely on existing Java unit tests! Step #2: ⌥⇧⌘K

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Milestone #2 • 100% Kotlin! • Still JVM-only

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Step #3: Backﬁll

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okio/jvm/src class PureKotlin { ... } Kotlin-only ﬁles okio/src class PureKotlin { ... }

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Java → Kotlin override fun available() = Math.min(size, Integer.MAX_VALUE).toInt() override fun available() = minOf(size, Int.MAX_VALUE).toInt()

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Platform-speciﬁc helpers okio/jvm/src System.arraycopy(src, srcPos, dest, destPos, length) okio/src expect fun arraycopy(src, srcPos, dest, destPos, length) okio/jvm/src actual fun arraycopy(src, srcPos, dest, destPos, length) { System.arraycopy(src, srcPos, dest, destPos, length) }

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Java only /** Writes the contents of this byte string to `out`. */ @Throws(IOException::class) open fun write(out: OutputStream) { out.write(data) }

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Step #4: JS/Native

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• Create actual classes • Implement platform-speciﬁc helpers Step #4: JS/Native

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Milestone #3 • 100% Kotlin! • JS/Native proof of concept

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Compatibility

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Binary NoSuchMethodError app.jar MyActivity.java library-v1.jar app.jar library-v2.jar javac java

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Source

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// Okio.java public Sink sink(File file) { return new OutputStreamSink(new FileOutputStream(file)); } Okio 1.x public static void main(String[] args) { Sink sink = Okio.sink(new File("README.md")); } fun main(args: Array) { val sink = Okio.sink(File("README.md")) } Kotlin Java

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Okio 2.x public static void main(String[] args) { Sink sink = Okio.sink(new File("README.md")); } fun main(args: Array) { val sink = File(“README.md”).sink() } Kotlin Java // Okio.kt fun File.sink(): Sink = FileOutputStream(this).sink()

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Okio 2.x public static void main(String[] args) { Sink sink = Okio.sink(new File("README.md")); } fun main(args: Array) { val sink = File(“README.md”).sink() } Kotlin Java // Okio.kt @file:JvmName(“Okio") fun File.sink(): Sink = FileOutputStream(this).sink()

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Migrating

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Okio 2 • Binary-compatible with Okio 1.x • Source-compatible with Okio 1.x for Java users • Source-incompatible with Okio 1.x for Kotlin users • But better! Kotlin-friendly API

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JApicmp

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// @JvmOverloads open fun substring(beginIndex: Int = 0, endIndex: Int = size): ByteString ./gradlew :okio:jvm:japicmp > Task :okio:jvm:japicmp FAILED FAILURE: Build failed with an exception. * What went wrong: Execution failed for task ':okio:jvm:japicmp'. > A failure occurred while executing Comparing [jvm-2.0.0-SNAPSHOT.jar] with [okio-1.14.1.jar] > Detected binary changes between jvm-2.0.0-SNAPSHOT.jar and okio-1.14.1.jar. See failure report at file:// /okio/okio/jvm/build/reports/japi.txt

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Quirks to watch out for • Internal classes are still public in bytecode • Classes and methods are ﬁnal by default

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Testing

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• Keep your Java tests! • They’re calling your API from a Java user’s perspective • Add Kotlin tests on top • They’re testing Kotlin API • Try to promote your Kotlin tests to common Testing

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ByteString.kt

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expect class ByteString // Trusted internal constructor doesn't clone data. internal constructor(data: ByteArray) : Comparable { internal val data: ByteArray internal var hashCode: Int internal var utf8: String? /** Constructs a new `String` by decoding the bytes as `UTF-8`. */ fun utf8(): String /** * Returns this byte string encoded as [Base64](http://www.ietf.org/rfc/rfc2045.txt). In violation * of the RFC, the returned string does not wrap lines at 76 columns. */ fun base64(): String /** Returns this byte string encoded as [URL-safe Base64](http://www.ietf.org/rfc/rfc4648.txt). */ fun base64Url(): String /** Returns this byte string encoded in hexadecimal. */ fun hex(): String ... } okio/src/main/okio/ByteString.kt

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actual open class ByteString internal actual constructor( internal actual val data: ByteArray ) : Serializable, Comparable { /** Writes the contents of this byte string to `out`. */ @Throws(IOException::class) open fun write(out: OutputStream) { out.write(data) } ... } okio/jvm/src/main/okio/ByteString.kt

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Roadblocks

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“Allow expect declarations with a default implementation” https://youtrack.jetbrains.com/issue/KT-20427

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“Annotate relevant standard library annotations with @OptionalExpectation” https://youtrack.jetbrains.com/issue/KT-24478 (@OptionalExpectation has been introduced in 1.2.60)

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“Class delegation should retain checked exceptions in JVM bytecode” https://youtrack.jetbrains.com/issue/KT-23935

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“expect function with default parameters can't be annotated with JvmOverloads” https://youtrack.jetbrains.com/issue/KT-24357 FIXED

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“Multiplatform: docs don't contain KDoc attached to expect members” https://github.com/Kotlin/dokka/issues/307

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Benchmarks

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1.x 2.x

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1.x 2.x

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1.x 2.x

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Results

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OKIO2 OKIO2 OKIO2 OKIO2 Released August 27, 2018

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Placeholder for the blog post screenshot

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Future Plans

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• Continue backﬁlling interfaces into common • Coroutines! • Kotlin Native! Future Plans

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• OkHttp - someday • Retroﬁt - someday • Moshi - someday soon • Wire - someday really soon

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Recommendations

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• Start with the JVM target ﬁrst • Rely on unit tests • Use JApicmp • Backﬁll • Report Kotlin multiplatform issues! Recommendations

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Thanks! @jessewilson @egorand