Kotlin multiplatform + Domain models = ❤️

In most of the applications we write, the domain model with all the crucial business logic only lives in the backend to ensure consistent behavior of a system. But with Kotlin multi-platform, that same domain model can suddenly also be used in your user-facing applications.

In this talk, we will look at what that means for the applications you build. What does your code look like when you share your domain model? How do you ensure that that domain model is re-usable? What kind of trouble can you run into and what techniques can you use to avoid that trouble? And how does the ability to use the full domain model benefit the user experience? And last but not least, how can your organizational team structure make this way of working a huge success or a road towards disaster?!

Transcript

1. + Domain models = ❤ Kotlin multiplatform Nico Krijnen
2. Nico Krijnen @nicokrijnen nkrijnen nicokrijnen
3. Moving fast & Focus on innovation It's all about
4. What do we mean with "a domain"? The thing the software is about.
5. What do we mean with "a domain"? An area of knowledge. The thing the software is about.
6. What do we mean with "a domain"? The problem space you are working in. The thing the software is about. An area of knowledge.
7. What do we mean with "a domain"? The thing the software is about. An area of knowledge. The problem space you are working in. A sphere of knowledge, influence, or activity. The subject area 
 to which the user applies a program is the domain of the software. —Eric Evans
8. What do we mean with "a domain"? The thing the software is about. An area of knowledge. The problem space you are working in. The domain is the real-world context in which you're attempting to solve a problem using software. Each domain comes with expertise, vocabulary and tools that are part of that domain. – Robert Harvey, stackexchange " A sphere of knowledge, influence, or activity. The subject area 
 to which the user applies a program is the domain of the software. —Eric Evans
9. What do we mean with "a domain"? Shipping cargo Airline booking Medical research Education Insurance
10. What is a domain model? Need to look at business logic Decisions made by code that are specific to the domain of the app
13. UI HTML / CSS Application Layer Data Access Layer Navigation Offline Storage
14. Routes Services Data Access
15. Where is the business logic? Routes Services Data Access UI HTML / CSS Application Layer Data Access Layer Navigation Offline Storage
16. Where is the business logic? Routes Services Data Access UI HTML / CSS Application Layer Data Access Layer Navigation Offline Storage
17. Where is the business logic? class UserService(private val userPersistence: UserDAO) { suspend fun registerNewUser(request: NewUserRequest) { require(request.email.isNotBlank() && value.isValidEmail()) { "Please enter an email address" } require(request.password.isNotBlank() && request.password.length >= 8) { "Password must be at least 8 characters long" } userPersistence.insertUser(request) } }
18. Where is the business logic? class UserService(private val userPersistence: UserDAO) { suspend fun registerNewUser(request: NewUserRequest) { require(request.email.isNotBlank() && value.isValidEmail()) { "Please enter an email address" } require(request.password.isNotBlank() && request.password.length >= 8) { "Password must be at least 8 characters long" } userPersistence.insertUser(request) } }
19. class UserService(private val userPersistence: UserPersistence) { suspend fun registerNewUser(command: NewUserCommand) { userPersistence.insertUser(command.email, command.password) } } @JvmInline value class Email(val value: String) { init { require(value.isNotBlank() && value.isValidEmail()) { "Please enter an email address" } } } @JvmInline value class Password(val value: String) { init { require(value.isNotBlank() && value.length >= 8) { "Password must be at least 8 characters long" } } } Where is the business logic?
20. Domain model? Routes Services Data Access UI HTML / CSS Application Layer Data Access Layer Navigation Offline Storage Code that makes your app unique and deals with your domain
21. Domain model? Routes Services Data Access UI HTML / CSS Application Layer Data Access Layer Navigation Offline Storage Domain model = only 20% of your code
22. Domain Model Stub Stub Stub Domain Model Domain Model Domain Model
23. Domain Model Domain Model Domain Model Domain Model Stub Stub Stub
24. Kotlin Multiplatform!
25. Kotlin Multiplatform! Android iOS JVM JavaScript Kotlin Compiler
26. Kotlin Multiplatform! Kotlin Compiler JVM JavaScript Android TypeScript (preview) Android NDK iOS watchOS tvOS macOS Windows Linux WebAssembly
27. Domain Model Domain Model Domain Model Domain Model Domain Model
28. Ingredients of succes
29. Domain Model = Encapsulated
30. Hexagonal / Clean Architecture
31. Dependencies only point inward
32. ✅ Pure Kotlin + Stdlib ❌ JVM apis or libraries ❌ JS apis or libraries ❌ Native apis or libraries Domain model = Free of dependencies
33. Hexagonal / Clean architecture ✅ Isolated, pure-code domain model ✅ No framework code mixed in ✅ Easy to read and understand ✅ Easy to test ✅ Always-valid Immutable
34. In action
35. Loads and updates data HTTP, REST
36. @Serializable data class Registration( val email: String, val name: String, val registrationTime: Long ) common / model
37. @Serializable data class AddRegistrationCommand(val email: String, val name: String) @Serializable data class UpdateRegistrationEmailCommand(val email: String, val newEmail: String) @Serializable data class UpdateRegistrationNameCommand(val email: String, val newName: String) common / model
38. interface RegistrationService { suspend fun list(): List<Registration> suspend fun handle(command: AddRegistrationCommand): Registration suspend fun handle(command: UpdateRegistrationEmailCommand): Registration suspend fun handle(command: UpdateRegistrationNameCommand): Registration } jvm / domain
39. fun Application.registrationAPIRoutes(service: RegistrationService) { routing { get("/registration/list") { call.respond(RegistrationList(service.list())) } post("/registration") { call.receive<AddRegistrationCommand>().let { command !" service.handle(command).let { registration !" call.respond(registration) } } } put("/registration/email") { call.receive<UpdateRegistrationEmailCommand>().let { command !" service.handle(command).let { registration !" call.respond(registration) } } } !!" } } jvm / infra
40. fun listRegistrations(resultHandler: (RegistrationList) !# Unit) { fetch("/registration/list", resultHandler) } fun AddRegistrationCommand.send(resultHandler: (Registration) !# Unit) { fetch( "POST", "/registration", Json.encodeToString(this), resultHandler.asResponseHandler() ) } fun UpdateRegistrationEmailCommand.send(resultHandler: ((Registration) !# Unit)? = null) { fetch( "PUT", "/registration/email", Json.encodeToString(this), resultHandler!$asResponseHandler() ) } !!" js / api
41. fun main() { addAppToBody() registerGlobalErrorHandler() } private fun addAppToBody() { document.addEventListener("DOMContentLoaded", { listRegistrations { registrations !" document.body!$appendChild(AppComponent(registrations.items).fragment) } }) } js / app
42. class AppComponent(registrations: List<Registration>) { val fragment = document.createDocumentFragment().apply { appendChild(HeaderComponent().element) appendChild(RegistrationListComponent(registrations).element.apply { appendChild(AddRegistrationRowComponent { addedRegistration !" insertBefore(RegistrationRowComponent(addedRegistration).element, lastChild) }.element) }) } } js / component
43. class RegistrationListComponent(registrations: List<Registration>) { val element = document.createElement("div") { className = "reg-list" registrations.forEach { appendChild(RegistrationRowComponent(it).element) } } } class RegistrationRowComponent(registration: Registration) { val element = document.createElement("div") { className = "reg-list!%item" appendChild(inputWithValue("email", registration.email) { newValue !" UpdateRegistrationEmailCommand(registration.email, newValue).send() }) appendChild(inputWithValue("name", registration.name) { newValue !" UpdateRegistrationNameCommand(registration.email, newValue).send() }) appendChild(divWithText(Date(registration.registrationTime).toDateString())) } } js / component
44. Where is the business logic?
45. Let's do some live coding!
46. Can we re-use more?
47. Kotlin Multiplatform Mobile is in Beta!
48. as reached Beta! o becoming Stable now! Single codebase for the application NativeCode SharedCode Businesslogic andcore View iOS specificAPIs Android specificAPIs View Learn more about beta → Case studies Support Get started Kotlin Multiplatform Mobile is an SDK for iOS and Android app development. It offers all the combined benefits of creating cross-platform and native apps. It is trusted in production by many of the world’s leading companies, including Philips, Netflix, Leroy Merlin, and VMWare. Singl logic Maintain a other logic Implement devices. Busines andcor iOS spec We loved the “shared business, native UI” idea that Kotlin Multiplatform promoted, and it m our teams did not have to give up using their preferred toolchains.” “ Get started Case studies Kotlin Multiplatform Mobile
49. Organize for success!
50. Organize for success!
52. %
53. % % % % % %
56. Alternatives?
57. Server-driven UI
58. Server-driven UI Domain Model Domain Model Domain Model Domain Model Only view logic Return desired UI Only view logic Only view logic
59. REST + HATEOAS https://youtu.be/k5S3qcCFDiQ – REST next level: Crafting business-oriented web APIs by Julien Topçu
60. Kotlin Isolated ❤ ❤ ❤ Kotlin multiplatform & Not for everyone '( Team organization matters Domain models Key Takeaways
61. + = Nico Krijnen @nicokrijnen nkrijnen nicokrijnen ❤ Domain models Kotlin multiplatform