Kotlin/Native at Careem: Experience Report

At Careem we've been using Kotlin/Native to share business logic between iOS and Android apps, it's time to share our experience.

Transcript

1. CocoaHeads Berlin November 2019 Kotlin/Native at Careem: Experience Report 1 Nikolay Kasyanov nikolay.kasyanov@careem.com @corristo
2. What is Careem? • Ride-hailing • Operating in MENA region • First unicorn of the region • Food & package delivery • Payments 2
3. • Ride management • Ride metering • Complex domain Captain App 3
4. How it all Started • Android-only app • Codebase was massive and monolithic • Hard to add features and fix bugs • Heavy reliance on manual testing 4
5. How it all Started • A business logic rewrite started • You guessed it: in Kotlin 5
6. 6 • Business need for iOS app for captains • Translating Kotlin to Swift didn’t go well • Kotlin/Native started to become stable enough at the time • Management was excited and skeptical at the same time How it all Started
7. Kotlin • Kotlin is a cross-platform, statically typed, general-purpose programming language with type inference (Wikipedia) • JVM language originally • Open source including tooling • https://github.com/jetbrains/kotlin • Lots of similarities with Swift 7
8. Kotlin/Native • LLVM-based Kotlin compiler backend & runtime • Reference counting based garbage collection • Open source: • https://github.com/jetbrains/kotlin-native • Imposes no UI paradigm nor architecture 8
9. • Business logic is written in Kotlin and shared • Android app uses Kotlin code as-is • iOS app uses a prebuilt framework • It just works!* 9 How it Works
10. Integration • From Xcode’s point of view it’s just a framework with Objective-C header • Add it to your project however you want • There’s a way to integrate Gradle build in your Xcode build 10
11. data class Pair<out A, out B>(val first: A, val second: B) __attribute __((objc_subclassing_restricted)) __attribute __((swift_name("KotlinPair"))) @interface CMEKotlinPair< __covariant A, __covariant B> : KotlinBase - (instancetype)initWithFirst:(A _Nullable)first second:(B _Nullable)second __attribute __((swift_name("init(first:second:)"))) __attribute __((objc_designated_initializer)); - (A _Nullable)component1 __attribute __((swift_name("component1()"))); - (B _Nullable)component2 __attribute __((swift_name(“component2()"))); - (CMEKotlinPair<A, B> *)doCopyFirst:(A _Nullable)first second:(B _Nullable)second __attribute __((swift_name("doCopy(first:second:)"))); - (BOOL)equalsOther:(id _Nullable)other __attribute __((swift_name("equals(other:)"))); - (int32_t)hashCode __attribute __((swift_name("hashCode()"))); - (NSString *)toString __attribute __((swift_name("toString()"))); @property (readonly) A _Nullable first __attribute __((swift_name("first"))); @property (readonly) B _Nullable second __attribute __((swift_name("second"))); @end; 11 Objective-C
12. Language Interop • Most types are bridged to Objective-C types • Objective-C as the “lowest common denominator” • limited generics • no default parameters • Some Kotlin is lost in translation • Works for us, YMMV 12 interface BookingStateStoreListener : StoreListener<BookingStoreState>
13. Day to Day Development • Change Kotlin code • Ensure it’s working by unit and integration tests • Build & publish klibs to local Maven repository • Rebuild iOS framework, test the app 13
14. How it Works – Building • A Gradle module in project working tree • Binary is rebuilt on demand & committed 14
15. • expect/actual • it’s all Kotlin ➡ Platform Access 15 expect class Date() { val current: Long } // iOS source set import platform.Foundation.NSDate actual class Date actual constructor () { actual val current: Long get() = NSDate().timeIntervalSince1970().toMillis() } // Android source set actual class Date actual constructor() { actual val current: Long = System.currentTimeMillis() }
16. Concurrency • Kotlin Native memory model and concurrency are very special • Object can’t be shared between threads without being frozen • Coroutine support is very limited 16
17. • actual/expect • Dependency Injection • define an interface in Kotlin • implement the protocol in Swift/Objective-C Platform Access 17
18. Platform Access 18 interface SyncBookingsApi { fun syncBookings(completionHandler: (SyncBookingsModel?, Error?) -> Unit) } class BookingApi { let urlSession: URLSession func syncBookings(completionHandler: @escaping (CME.SyncBookingsModel?, KotlinError?) -> Void) { let endpoint = SyncBookingsEndpoint() urlSession.callEndpoint(endpoint) { result in switch result { case .success(let model): completionHandler(model, nil) case .failure(error): completionHandler(nil, error.toKotlinError()) } } } }
19. Concurrency - Options • Run Kotlin & access Kotlin objects in one thread • delegate async processing to host platform • optionally freeze what you can & pass around • Wait a little more, proper coroutines are coming soon 19
20. Takeaways • Kotlin/Native is not experimental technology anymore • Consider language interop & multithreading implications on API design & implementation • Accessing platform services isn’t an issue • Be conscious what to share 20
21. • Getting iOS folks on board to use and contribute to shared codebase • Avoiding silos • Sharing ownership • Evolving shared code 21 Challenges
22. What Helped • RFC process for big changes • Discussing smaller changes in advance • At least one pull request approval per platform • Maintaining changelog 22
23. Conclusion Kotlin/Native helped us: • Support millions of rides monthly • Deal with complex business domain by only having to translate it into the code once • Have a better shared understanding of the business • Deliver iOS app faster It may work for you as well! 23
24. WE ARE HIRING!
25. Thank you! Questions?