Kotlinify Your Project!

1. KOTLINIFY YOUR PROJECT! OR NOYMAN, CITI

5. AGENDA KOTLIN ENABLES FUNCTION ORIENTED PROGRAMMING STYLE ▸ High order functions ▸ Functions as first-class citizens ▸ Type inference ▸ Extension functions ▸ Operator overloading ▸ Collection API (map, flatMap, reduce) ▸ Algebric data types (sealed classes)

6. AGENDA WHY FUNCTIONAL? ▸ Referential transparency ▸ Purity ▸ Deferred computation (lazy evaluation) ▸ Higher-Level abstractions

7. RECURSIONS THE ITERATIVE WAY fun Context?.findActivity(): Activity? { var context = this while (context is ContextWrapper) { if (context is Activity) { return context } context = context.baseContext } return null }

8. RECURSIONS THE RECURSIVE WAY fun Context?.findActivity(): Activity? = when (this) { is Activity -> this is ContextWrapper -> this.baseContext.findActivity() else -> null }

9. RECURSIONS THE (TAIL)RECURSIVE WAY tailrec fun Context?.findActivity(): Activity? = when (this) { is Activity -> this is ContextWrapper -> this.baseContext.findActivity() else -> null }

10. RECURSIONS THE (TAIL)RECURSIVE WAY @Nullable public static final Activity findActivity(@Nullable Context $receiver) { while(true) { Activity var10000; if($receiver instanceof Activity) { var10000 = (Activity)$receiver; } else { if($receiver instanceof ContextWrapper) { $receiver = ((ContextWrapper)$receiver).getBaseContext(); continue; } var10000 = null; } return var10000; } }

11. RECURSIONS FALL-THROUGH-STRUCTURE PHASE1 PHASE2 PHASE3 PHASE4 VERIFY1 VERIFY2 VERIFY3 VERIFY4

12. RECURSIONS FALL-THROUGH-SWITCH enum Verification { NO_VERIFICATION, VERIFY_PHASE_1, VERIFY_PHASE_2, VERIFY_PHASE_3, VERIFY_PHASE_4 } PHASE1 PHASE2 PHASE3 PHASE4 VERIFY1 VERIFY2 VERIFY3 VERIFY4

13. RECURSIONS FALL-THROUGH-SWITCH void verify(TestDriver driver) { switch (verification) { case VERIFY_PHASE_4: verifyPhase4(driver); case VERIFY_PHASE_3: verifyPhase3(driver); case VERIFY_PHASE_2: verifyPhase2(driver); case VERIFY_PHASE_1: verifyPhase1(driver); case NO_VERIFICATION: } } PHASE1 PHASE2 PHASE3 PHASE4 VERIFY1 VERIFY2 VERIFY3 VERIFY4

14. RECURSIONS FALL-THROUGH-WHEN? fun verify(driver: TestDriver) = when (verification) { NO_VERIFICATION -> Unit VERIFY_PHASE_1 -> verifyPhase1(driver) VERIFY_PHASE_2 -> { verifyPhase1(driver) verifyPhase2(driver) } VERIFY_PHASE_3 -> { verifyPhase1(driver) verifyPhase2(driver) verifyPhase3(driver) } VERIFY_PHASE_4 -> { verifyPhase1(driver) verifyPhase2(driver) verifyPhase3(driver) verifyPhase4(driver) } } PHASE1 PHASE2 PHASE3 PHASE4 VERIFY1 VERIFY2 VERIFY3 VERIFY4

15. RECURSIONS FALL-THROUGH-LIKE-WHEN inline fun <reified T : Enum<T>> T.prev(): T { val values = enumValues<T>() val prevOrdinal = (ordinal - 1) % values.size return values[prevOrdinal] } enum class Verification { NO_VERIFICATION, VERIFY_PHASE_1, VERIFY_PHASE_2, VERIFY_PHASE_3, VERIFY_PHASE_4 } PHASE1 PHASE2 PHASE3 PHASE4 VERIFY1 VERIFY2 VERIFY3 VERIFY4

18. RECURSIONS FALL-THROUGH-WHEN? fun verify(driver: TestDriver) = when (verification) { NO_VERIFICATION -> Unit VERIFY_PHASE_1 -> verifyPhase1(driver) VERIFY_PHASE_2 -> { verifyPhase1(driver) verifyPhase2(driver) } VERIFY_PHASE_3 -> { verifyPhase1(driver) verifyPhase2(driver) verifyPhase3(driver) } VERIFY_PHASE_4 -> { verifyPhase1(driver) verifyPhase2(driver) verifyPhase3(driver) verifyPhase4(driver) } } PHASE1 PHASE2 PHASE3 PHASE4 VERIFY1 VERIFY2 VERIFY3 VERIFY4

19. RECURSIONS FALL-THROUGH-LIKE-WHEN fun verify(driver: TestDriver) { tailrec fun verify(verification: Verification) { when (verification) { NO_VERIFICATION -> return VERIFY_PHASE_1 -> verifyPhase1(driver) VERIFY_PHASE_2 -> verifyPhase2(driver) VERIFY_PHASE_3 -> verifyPhase3(driver) VERIFY_PHASE_4 -> verifyPhase4(driver) } verify(verification.prev()) } verify(verification) } PHASE1 PHASE2 PHASE3 PHASE4 VERIFY1 VERIFY2 VERIFY3 VERIFY4

24. RECURSIONS DEFERRED RECURSION (RECURSIVE FLATMAP) ▸ see: https://goo.gl/tuDwFn

27. MAKE RX CLEARER COMPLEX ASYNC FLOW ▸ Parallelise as much as possible ▸ Respect dependencies ASYNC1 ASYNC2 ASYNC3 ASYNC4 ASYNC5 ASYNC6 ASYNC7

28. MAKE RX CLEARER COMPLEX ASYNC FLOW fun async1(): Completable { ... } fun async2(): Completable { ... } fun async3(): Completable { ... } fun async4(): Completable { ... } fun async5(): Completable { ... } fun async6(): Completable { ... } fun async7(): Completable { ... } ASYNC1 ASYNC2 ASYNC3 ASYNC4 ASYNC5 ASYNC6 ASYNC7

29. MAKE RX CLEARER COMPLEX ASYNC FLOW private fun prefetchData(): Completable = Completable.mergeArray( async1().concatWith(async2()).concatWith(async3().mergeWith(async4())), async1().concatWith(async5()), async6().concatWith(async7())) ASYNC1 ASYNC2 ASYNC3 ASYNC4 ASYNC5 ASYNC6 ASYNC7

30. MAKE RX CLEARER COMPLEX ASYNC FLOW private fun prefetchData(): Completable = Completable.mergeArray( async1().concatWith(async2()).concatWith(async3().mergeWith(async4())), async1().concatWith(async5()), async6().concatWith(async7())) ASYNC1 ASYNC6

31. MAKE RX CLEARER COMPLEX ASYNC FLOW private fun prefetchData(): Completable = Completable.mergeArray( async1().concatWith(async2()).concatWith(async3().mergeWith(async4())), async1().concatWith(async5()), async6().concatWith(async7())) ASYNC1 ASYNC2 ASYNC6

32. MAKE RX CLEARER COMPLEX ASYNC FLOW private fun prefetchData(): Completable = Completable.mergeArray( async1().concatWith(async2()).concatWith(async3().mergeWith(async4())), async1().concatWith(async5()), async6().concatWith(async7())) ASYNC1 ASYNC2 ASYNC6 ASYNC3 ASYNC4

33. MAKE RX CLEARER COMPLEX ASYNC FLOW private fun prefetchData(): Completable = Completable.mergeArray( async1().concatWith(async2()).concatWith(async3().mergeWith(async4())), async1().concatWith(async5()), async6().concatWith(async7())) ASYNC1 ASYNC2 ASYNC3 ASYNC4 ASYNC6

34. MAKE RX CLEARER COMPLEX ASYNC FLOW private fun prefetchData(): Completable = Completable.mergeArray( async1().concatWith(async2()).concatWith(async3().mergeWith(async4())), async1().concatWith(async5()), async6().concatWith(async7())) ASYNC1 ASYNC2 ASYNC3 ASYNC4 ASYNC5 ASYNC6

37. MAKE RX CLEARER COMPLEX ASYNC FLOW - CLEANER? fun inParallel(vararg sources: CompletableSource): Completable = Completable.mergeArray(*sources) infix fun Completable.andThen(next: CompletableSource): Completable = this.concatWith(next) infix fun Completable.alongside(other: CompletableSource): Completable = this.mergeWith(other) ASYNC1 ASYNC2 ASYNC3 ASYNC4 ASYNC5 ASYNC6 ASYNC7

39. MAKE RX CLEARER COMPLEX ASYNC FLOW - CLEANER! private fun prefetchData(): Completable = inParallel( async1() andThen async2() andThen (async3() alongside async4()), async1() andThen async5(), async6() andThen async7()) ASYNC1 ASYNC2 ASYNC3 ASYNC4 ASYNC5 ASYNC6 ASYNC7

40. YOU MIGHT NOT THINK THAT PROGRAMMERS ARE ARTISTS, BUT PROGRAMMING IS AN EXTREMELY CREATIVE PROFESSION. IT'S LOGIC-BASED CREATIVITY. John Romero MAKE RX CLEARER

41. ENABLE FUNCTIONAL ARCHITECTURE IMMUTABLE VALUE TYPES ▸ have no identity ▸ defined by the set of values they hold ▸ overrides equals, hashCode and toString ▸ builder pattern (?)

42. ENABLE FUNCTIONAL ARCHITECTURE IMMUTABLE VALUE TYPES ▸ Java has no built-in support :( ▸ Possible through AutoValue :) ▸ AutoValue is no fun :(

43. ENABLE FUNCTIONAL ARCHITECTURE IMMUTABLE VALUE TYPES - AUTO VALUE @AutoValue abstract class RepositoryResponse { @NonNull abstract long id(); @NonNull abstract String name(); @NonNull abstract String fullName(); @NonNull abstract UserResponse owner(); @NonNull abstract String htmlUri(); @NonNull abstract String description(); @NonNull abstract String uri(); public static Builder builder() { return new AutoValue_RepositoryResponse.Builder(); } @AutoValue.Builder abstract static class Builder { abstract Builder id(long id); abstract Builder name(String name); abstract Builder fullName(String fullName); abstract Builder owner(UserResponse owner); abstract Builder description(String description); abstract Builder uri(String uri); abstract RepositoryResponse build(); } }

44. ENABLE FUNCTIONAL ARCHITECTURE IMMUTABLE VALUE TYPES - DATA CLASS data class RepositoryResponse(val id: Long, val name: String, val fullName: String, val owner: UserResponse, val htmlUri: String, val description: String, val uri: String)

45. ENABLE FUNCTIONAL ARCHITECTURE IMMUTABLE VALUE TYPES - DATA CLASS data class RepositoryResponse(val id: Long, val name: String, val fullName: String, val owner: UserResponse, val htmlUri: String, val description: String, val uri: String) val owner: UserResponse = TODO("stub") val response = RepositoryResponse(id = 33768, name = "restor1", fullName = "Or Noyman", owner = owner, htmlUri = "...", description = "...", uri = "...")

46. ENABLE FUNCTIONAL ARCHITECTURE SEALED CLASS HIERARCHIES ▸ model all possible outcomes as data types ▸ represent through restricted class hierarchies ▸ leverage ‘when’ expressions

47. ENABLE FUNCTIONAL ARCHITECTURE SEALED CLASS HIERARCHIES fun fetchRepositories(username: String): Observable<?>

48. ENABLE FUNCTIONAL ARCHITECTURE SEALED CLASS HIERARCHIES fun fetchRepositories(username: String): Observable<FetchRepositoriesResult> sealed class FetchRepositoriesResult

49. ENABLE FUNCTIONAL ARCHITECTURE SEALED CLASS HIERARCHIES fun fetchRepositories(username: String): Observable<FetchRepositoriesResult> sealed class FetchRepositoriesResult object FetchRepositoriesLoading : FetchRepositoriesResult() object FetchRepositoriesError : FetchRepositoriesResult() data class FetchRepositoriesOk(val repo: List<RepositoryResponse>) : FetchRepositoriesResult()

50. ENABLE FUNCTIONAL ARCHITECTURE SEALED CLASS HIERARCHIES fun fetchRepositories(username: String): Observable<FetchRepositoriesResult> = githubApiService.getUsersRepositories(username) .map<FetchRepositoriesResult>(::FetchRepositoriesOk) .onErrorReturnItem(FetchRepositoriesError) .startWith(FetchRepositoriesLoading) sealed class FetchRepositoriesResult object FetchRepositoriesLoading : FetchRepositoriesResult() object FetchRepositoriesError : FetchRepositoriesResult() data class FetchRepositoriesOk(val repo: List<RepositoryResponse>) : FetchRepositoriesResult()

56. ENABLE FUNCTIONAL ARCHITECTURE WHEN fun renderFetchRepositories() { fetchRepositories("JakeWharton") .observeOn(AndroidSchedulers.mainThread()) .subscribe { response -> when (response) { is FetchRepositoriesLoading -> showShimmer() is FetchRepositoriesError -> showError() is FetchRepositoriesOk -> renderRepositories(response.repo) } } }

61. ENABLE FUNCTIONAL ARCHITECTURE VIEW STATE ▸ immutable value type ▸ represents entire screen state

62. ENABLE FUNCTIONAL ARCHITECTURE VIEW STATE REDUCER CURRENT VIEW STATE EVENT NEW VIEW STATE REDUCER

63. ENABLE FUNCTIONAL ARCHITECTURE VIEW STATE REDUCER sealed class FetchRepositoriesResult object FetchRepositoriesLoading : FetchRepositoriesResult() object FetchRepositoriesError : FetchRepositoriesResult() data class FetchRepositoriesOk(val repo: List<RepositoryResponse>) : FetchRepositoriesResult()

64. ENABLE FUNCTIONAL ARCHITECTURE VIEW STATE REDUCER sealed class Event sealed class FetchRepositoriesResult: Event() object FetchRepositoriesLoading : FetchRepositoriesResult() object FetchRepositoriesError : FetchRepositoriesResult() data class FetchRepositoriesOk(val repo: List<RepositoryResponse>) : FetchRepositoriesResult() sealed class FetchUserResult: Event() object FetchUserLoading : FetchUserResult() object FetchUserError : FetchUserResult() data class FetchUserOk(val user: UserResponse) : FetchUserResult()

65. ENABLE FUNCTIONAL ARCHITECTURE VIEW STATE REDUCER sealed class Event sealed class FetchRepositoriesResult: Event() object FetchRepositoriesLoading : FetchRepositoriesResult() object FetchRepositoriesError : FetchRepositoriesResult() data class FetchRepositoriesOk(val repo: List<RepositoryResponse>) : FetchRepositoriesResult() sealed class FetchUserResult: Event() object FetchUserLoading : FetchUserResult() object FetchUserError : FetchUserResult() data class FetchUserOk(val user: UserResponse) : FetchUserResult()

66. ENABLE FUNCTIONAL ARCHITECTURE VIEW STATE REDUCER data class ViewState(val isLoadingUser: Boolean, val isErrorUser: Boolean, val user: UserResponse?, val isLoadingRepos: Boolean, val isErrorRepos: Boolean, val repos: List<RepositoryResponse>?)

67. ENABLE FUNCTIONAL ARCHITECTURE VIEW STATE REDUCER fun ViewState.reduce(event: Event): ViewState = when (event) { FetchRepositoriesLoading -> this.copy(isLoadingRepos = true, isErrorRepos = false) FetchRepositoriesError -> this.copy(isLoadingRepos = false, isErrorRepos = true) is FetchRepositoriesOk -> this.copy(repos = event.repo, isLoadingRepos = false) FetchUserLoading -> this.copy(isLoadingUser = true, isErrorUser = false) FetchUserError -> this.copy(isLoadingUser = false, isErrorUser = true) is FetchUserOk -> this.copy(user = event.user, isLoadingUser = false) }

73. MORE ABOUT THE DEMISE OF OOP ▸ https://medium.com/@cscalfani/goodbye-object-oriented- programming-a59cda4c0e53

74. MORE MODEL-VIEW-INTENT-PATTERN ▸ Jake Wharton - Managing State with RX ▸ http://hannesdorfmann.com/android/mosby3-mvi-1

75. MORE ABOUT FUNCTIONAL KOTLIN ▸ http://kategory.io ▸ http://kategory.io/docs/quickstart/blogs/

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