Refactoring Legacy Code with Kotlin and Coroutines

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Refactoring Legacy Code with Kotlin & Coroutines Kotlin Everywhere: Hamburg @askashdavies

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@askashdavies

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Java @askashdavies

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{ } @askashdavies

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Streams ↝ @askashdavies

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@askashdavies

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Kotlin @askashdavies

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Null @askashdavies

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if (foo == null) { bar(); } @askashdavies

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O"ensive Code < > @askashdavies

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Urgency @askashdavies

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Kotlin? @askashdavies

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Kotlin @askashdavies

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@askashdavies

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Google IO 2018 @askashdavies

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! " @askashdavies

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Google IO 2019 @askashdavies

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Kotlin @askashdavies

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Libraries @askashdavies

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@askashdavies

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@askashdavies

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@askashdavies

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@askashdavies

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@askashdavies

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@askashdavies

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Kotlin @askashdavies

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Idiomacy @askashdavies

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@askashdavies

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Idiomatic Code @askashdavies

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Idiomatic Code • Consistent, easier to read • Less cognitive load • Less ambiguity • Function > Style @askashdavies

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Code Style • kotlinlang.org/docs/reference/coding-conventions.html • android.github.io/kotlin-guides/style.html @askashdavies

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Refactoring Legacy Code @askashdavies

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⌃⌥⇧K @askashdavies

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@askashdavies

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@askashdavies

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public class BadJavaActivity extends Activity { @Inject Dependency dependency; } @askashdavies

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General Assumptions @askashdavies

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class BadKotlinActivity : Activity() { @Inject var dependency: Dependency? = null } @askashdavies

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class SlightlyLessBadKotlinActivity : Activity() { @Inject internal lateinit var dependency: Dependency } @askashdavies

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UninitializedPrope.yAccessException! @askashdavies

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lateinit var file: File if (::file.isInitialized) { /* ... */ } @askashdavies

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Data Classes @askashdavies

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public class User { private String firstName; private String lastName; public User(String firstName, String lastName) { this.firstName = firstName; this.lastName = lastName; } public String getFirstName() { return firstName; } public void setFirstName(String firstName) { this.firstName = firstName; } public String getLastName() { return lastName; } public void setLastName(String lastName) { this.lastName = lastName; } @Override public boolean equals(Object o) { if (this == o) { return true; } if (o == null || getClass() != o.getClass()) { return false; } User user = (User) o; return Objects.equals(firstName, user.firstName) && Objects.equals(lastName, user.lastName); } @Override public int hashCode() { return Objects.hash(firstName, lastName); } } @askashdavies

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class User(var firstName: String?, var lastName: String?) { override fun equals(o: Any?): Boolean { if (this === o) { return true } if (o == null || javaClass != o.javaClass) { return false } val user = o as User? return firstName == user!!.firstName && lastName == user.lastName } override fun hashCode(): Int { return Objects.hash(firstName, lastName) } } @askashdavies

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data class User(val firstName: String, val lastName: String) @askashdavies

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data class User(val firstName: String, val lastName: String? = null) @askashdavies

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@NotNull public final User copy(@Nullable String firstName, @Nullable String lastName) { return new User(firstName, lastName); } @askashdavies

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Kotlin • Singleton objects • String interpolation • Elvis operator ! • Destructuring • Extension functions • Scoping functions @askashdavies

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Maintaining History @askashdavies

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Maintaining History • Change extension .java -> .kt • First commit • Apply Kotlin conversion • Second commit @askashdavies

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Maintaining History Refactoring @askashdavies

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Refactoring @askashdavies

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Refactoring SOLID • Single-responsibility • Open-closed • Liskov substitution • Interface segregation • Dependency inversion @askashdavies

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Refactoring Single Responsibility @askashdavies

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Perspective @askashdavies

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Asynchronicity @askashdavies

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Concurrency is Hard @askashdavies

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Thread new Thread(() -> { foo(); }).start(); @askashdavies

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CompletableFuture CompletableFuture.supplyAsync(this::findSomeData) .thenApply(this:: intReturningMethod) .thenAccept(this::notify); @askashdavies

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RxJava Observable .fromIterable(resourceDraft.getResources()) .flatMap(resourceServiceApiClient::createUploadContainer) .zipWith(Observable.fromIterable(resourceDraft.getResources()), Pair::create) .flatMap(uploadResources()) .toList() .toObservable() .flatMapMaybe(resourceCache.getResourceCachedItem()) .defaultIfEmpty(Resource.getDefaultItem()) .flatMap(postResource(resourceId, resourceDraft.getText(), currentUser, resourceDraft.getIntent())) .observeOn(AndroidSchedulers.mainThread()) .subscribeOn(Schedulers.io()) .subscribe( resource -> repository.setResource(resourceId, resource, provisionalResourceId), resourceUploadError(resourceId, resourceDraft, provisionalResourceId) ); @askashdavies

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@askashdavies

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What If? @askashdavies

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Coroutines @askashdavies

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fun main() { GlobalScope.launch { delay(1000L) println("World!") } println("Hello,") Thread.sleep(2000L) } // Hello, // World! @askashdavies

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fun main() { GlobalScope.launch { delay(1000L) println("World!") } println("Hello,") Thread.sleep(2000L) } // Hello, // World! @askashdavies

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fun main() { GlobalScope.launch { delay(1000L) println("World!") } println("Hello,") Thread.sleep(2000L) } // Hello, // World! @askashdavies

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⚖ Stability @askashdavies

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@askashdavies

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@Annotations ( ! Here be dragons) @askashdavies

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Annotations @ExperimentalCoroutinesApi // ⚠ @askashdavies

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Annotations @ExperimentalCoroutinesApi // ⚠ @FlowPreview // ⚠ @askashdavies

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Annotations @ExperimentalCoroutinesApi // ⚠ @FlowPreview // ⚠ @ObsoleteCoroutinesApi // ⚠ @askashdavies

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Annotations @ExperimentalCoroutinesApi // ⚠ @FlowPreview // ⚠ @ObsoleteCoroutinesApi // ⚠ @InternalCoroutinesApi // ☠ @askashdavies

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Annotations @Experimental @askashdavies

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! Coroutines @askashdavies

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! Native (rst-pa-y library @askashdavies

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E"cient @askashdavies

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! Easy-to-use @askashdavies

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! suspend fun @askashdavies

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! Suspend fun main() { GlobalScope.launch { delay(1000L) println("World!") } println("Hello,") Thread.sleep(2000L) } // Hello, // World! @askashdavies

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! Suspend fun main() { GlobalScope.launch { doWorld() println("Hello,") Thread.sleep(2000L) } } suspend fun doWorld() { delay(1000L) println("World!") } // Hello, // World! @askashdavies

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! Suspend fun main() { GlobalScope.launch { doWorld() println("Hello,") Thread.sleep(2000L) } } suspend fun doWorld() { withContext(Dispatchers.IO) { delay(1000L) println("World!") } } // Hello, // World! @askashdavies

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Dispatchers @askashdavies

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Dispatchers • Default • IO • Main • Android (Main Thread Dispatcher) • JavaFx (Application Thread Dispatcher) • Swing (Event Dispatcher Thread) • Unconﬁned @askashdavies

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Testing @Test fun testFoo() = runBlockingTest { val actual = foo() // ... } suspend fun foo() { delay(1_000) // ... } @askashdavies

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Fu#her Reading • Google Codelab: Refactoring to Kotlin https://codelabs.developers.google.com/codelabs/java-to-kotlin/ • KotlinX Coroutine Test https://github.com/Kotlin/kotlinx.coroutines/tree/master/kotlinx-coroutines-test • Sean McQuillan: Coroutines + Testing = https://www.droidcon.com/media-detail?video=352671106 • Ash Davies: RxJava & Coroutines: A Practical Analysis v3 https://speakerdeck.com/ashdavies/rxjava-and-coroutines-a-practical-analysis- v3 @askashdavies

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Thanks! @askashdavies