Writing Kotlin Multiplatform libraries that your iOS teammates are gonna love

Writing Kotlin Multiplatform
libraries that your iOS
teammates are gonna love
André Oriani

Disclaimer
My presentation, comments and
opinions are provided in my personal
capacity and not as a representative
of Walmart. They do not reflect the
views of Walmart and are not
endorsed by Walmart.

TWITTER.COM/AORIANI
ANDROIDDEV.SOCIAL/@AORIANI
MEDIUM.COM/@AORIANI
GITHUB.COM/AORIANI
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• Italian🇮🇹-Brazilian🇧🇷
• Working professionally with Android since its
first public releases.
• Worked on the development of the Homescreen and
the Instant Messaging app for the very first
Motorola Android phones, including the DROID
series.
• Currently tech lead at Walmart in the Silicon
Valley
André Oriani

Kotlin Multiplatform is awesome!

In order to accomplish our secret goal of complete
domination of the world of mobile development…

Kotlin and Swift are so similar…

Presentation Format
//KOTLIN API
data class Person(
val name: String,
val age: Int
)
//EXPORTED OBJ-C HEADER
__attribute__((objc_subclassing_restricted))
__attribute__((swift_name("Person")))
@interface SharedPerson : SharedBase
- (instancetype)initWithName:(NSString *)name age:(int32_t)age
__attribute__((swift_name("init(name:age:)")))
__attribute__((objc_designated_initializer));
- (SharedPerson *)doCopyName:(NSString *)name age:(int32_t)age
__attribute__((swift_name("doCopy(name:age:)")));
- (BOOL)isEqual:(id _Nullable)other
__attribute__((swift_name("isEqual(_:)")));
- (NSUInteger)hash __attribute__((swift_name("hash()")));
- (NSString *)description
__attribute__((swift_name("description()")));
@property (readonly) int32_t age
__attribute__((swift_name("age")));
@property (readonly) NSString *name
__attribute__((swift_name("name")));
@end
// SWIFT CLIENT CODE
let keanuReeves = Person(name: "Keanu Charles Reeves", age: 59)
[OBJ-C]

Presentation Format
//KOTLIN API
data class Person(
val name: String,
val age: Int
)
//HEADER “TRANSLATED” TO SWIFT
public class Person : KotlinBase {
public init(name: String, age: Int32)
open func doCopy(name: String, age:
Int32) -> Person
open func isEqual(_ other: Any?) ->
Bool
open func hash() -> UInt
open func description() -> String
open var age: Int32 { get }
open var name: String { get }
}
// Which one is which?
let keanuReeves = Person(name: "Keanu Charles Reeves", age: 59)

Name Clashing
THE
PROBLEM
• Kotlin
• Multiple levels of namespace due to packages;
• Method signature is the method name plus parameter types.
• Objective-C
• No support to namespaces;
• Method signature is the method name plus parameter names.
• Swift
• One level of namespace: the exported framework;
• Method signature is the method name plus parameter names.

Classes with same name but different packages
package io.aoriani.network
class Item
...
package io.aoriani.models
class Item
public class Item : KotlinBase {
public init()
}
public class Item_ : KotlinBase {
public init()
}
// Which one is which?
let item1 = Item()
let item2 = Item_()
THE
PROBLEM

Classes with same name but different packages
package io.aoriani.network
@ObjCName("ItemResponse")
class Item
...
package io.aoriani.models
@ObjCName("ItemModel")
class Item
public class ItemResponse :
KotlinBase {
public init()
}
public class ItemModel : KotlinBase {
public init()
}
let itemDomainModel = ItemModel()
let itemNetworkResponse = ItemResponse()
THE
SOLUTION

Overloading with params named the same
fun sort(data: List<Int>){}
fun sort(data: Map<String, Int>){}
public class ExampleKt : KotlinBase {
open class func sort(data: [KotlinInt])
open class func sort(data_ data:
[String : KotlinInt])
}
ExampleKt.sort(data: [1, 2, 3])
ExampleKt.sort(data_: ["A" : 1, "B": 2])
THE
PROBLEM

Overloading with params named the same
fun sort(listOfInts: List<Int>){}
fun sort(mapOfStringToInt: Map<String,
Int>){}
open class func sort(listOfInts:
[KotlinInt])
open class func sort(mapOfStringToInt:
[String : KotlinInt])
}
ExampleKt.sort(listOfInts: [1, 2, 3])
ExampleKt.sort(mapOfStringToInt: ["A" : 1, "B": 2])
THE
SOLUTION

Interface fields, same name, different types
interface Person {
val id: String
}
interface Robot {
val id: Long
}
public protocol Person {
var id: String { get }
}
public protocol Robot {
var id_: Int64 { get }
}
// Only possible because Konan renamed field for Robot
class Android: Person, Robot {
let id: String = ""
let id_: Int64 = 0
}
THE
PROBLEM

Collection with types conforming to protocol
interface Product
class Consumer {
fun consume(products: Set<Product>) {}
}
public protocol Product {}
public class Consumer : KotlinBase {
public init()
open func consume(products:
Set<AnyHashable>)
}
// Compiles fine in Swift
let consumer = Consumer()
let fakeProducts: Set<Int> = [1, 2, 3]
consumer.consume(products: fakeProducts)
THE
PROBLEM

THE
PROBLEM
Set<Product> NSSet<id<Product>>
Set<T>
where T: Hashable
[OBJ-C]
MAPS TO BRIDGES TO
• Swift Set requires elements conforming to protocol Hashable;
• Kotlin Interfaces are exported as Objetive-C protocols;
• Objective-C protocols cannot conform to other protocols, let alone Swift Protocols, thus the
argument type is erased to AnyHashable;
• All KMP Classes inherit from NSObject which conforms to Hashable;

abstract class Product
class Consumer {
fun consume(products: Set<Product>) {}
}
open class Product : KotlinBase {
public init()
}
public class Consumer : KotlinBase {
public init()
open func consume(products:
Set<Product>)
}
let consumer = Consumer()
let fakeProducts: Set<Int> = [1, 2, 3]
// Error: Cannot convert value of type 'Set<Int>' to expected
argument type 'Set<Product>'
//consumer.consume(products: fakeProducts)
let realProducts: Set<Product> = [Product(), Product()]
consumer.consume(products: realProducts)
THE
SOLUTION

Values classes
@JvmInline
value class PostalCode(private val code:
String)
class Letter(val zipcode: PostalCode)
public class Letter : KotlinBase {
public init(zipcode: Any)
open var zipcode: Any { get }
}
//This compiles fine
let letter = Letter(postalCode: UIViewController())
THE
PROBLEM

Interface extensions
interface Person {
val firstName: String
val lastName: String
}
fun Person.fullName() = "$firstName
$lastName"
public protocol Person {
var firstName: String { get }
var lastName: String { get }
}
open class func fullName(_ receiver:
Person) -> String
}
class SwiftPerson: Person {…}
let swiftPerson = SwiftPerson(firstName: "Bruce", lastName: "Wayne")
// Java déjà vu?
print(ExampleKt.fullName(swiftPerson))
THE
PROBLEM

Class extensions
abstract class Person(
val firstName: String,
val lastName: String
)
fun Person.fullName() = "$firstName
$lastName"
open class Person : KotlinBase {
public init(firstName: String,
lastName: String)
open var firstName: String { get }
open var lastName: String { get }
}
extension Person {
open func fullName() -> String
}
class SwiftPerson: Person {…}
let swiftPerson = SwiftPerson(firstName: "Bruce", lastName: "Wayne")
print(swiftPerson.fullName())
THE
SOLUTION

Filling the gaps between Kotlin and Swift
class MyRange {
fun isInRange(value: Int, range: IntRange) =
value in range
}
fun test(){
MyRange().isInRange(1, -7..6)
}
public class MyRange : KotlinBase {
public init()
open func isInRange(value: Int32,
range: KotlinIntRange) -> Bool
}
let myRange = MyRange()
// This is a bit clumsy
myRange.isInRange(value: 1, range: KotlinIntRange(start: -1,
endInclusive: 2))
THE
PROBLEM

Filling the gaps between Kotlin and Swift
class MyRange {
@ShouldRefineInSwift
fun isInRange(value: Int, range: IntRange) =
value in range
}
public class MyRange : KotlinBase {
public init()
}
let myRange = MyRange()
myRange.isRange(value: 2, range: -1...2)
extension MyRange {
func isInRange(value value: Int32,
range range: ClosedRange<Int32>) -> Bool {
return __is(inRangeValue: value,
range: KotlinIntRange(start:
range.lowerBound, endInclusive:
range.lowerBound))
}
}
THE
SOLUTION

Providing only platform specific code
import io.ktor.http.Url
import java.net.URL
fun Url.toURL(): URL =
URL(this.toString())
let url = Service.shared.baseUrl.toURL()
let session = URLSession(configuration: .default)
session.dataTask(with: url) {
...
extension Ktor_httpUrl {
open func toURL() -> URL
}
import io.ktor.http.Url
import platform.Foundation.NSURL
fun Url.toURL(): NSURL =
NSURL(string = toString())
androidMain
iosMain
THE
SOLUTION

Default arguments
fun compareString(a: String, b: String,
ignoreCase: Boolean = false): Boolean {
return a.equals(b, ignoreCase =
ignoreCase)
}
open class func compareString(a:
String, b: String, ignoreCase: Bool) ->
Bool
}
let result1 = ExampleKt.compareString(a: "HELLO", b: "hello", ignoreCase:
true)
// Compile error: Missing argument for parameter 'ignoreCase' in call
let result2 = ExampleKt.compareString(a: "Roma", b: "Londres")
THE
PROBLEM

Exceptions
class Bomb {
fun explode() : Unit =
throw IOException(”Bomb has detonated")
}
public class Bomb : KotlinBase {
public init()
open func explode()
}
func defuseBomb() {
let bomb = Bomb()
do {
bomb.explode()
} catch {
// 'catch' block is unreachable because no errors are thrown in 'do'
block
print("Bomb disarmed!")
}
}
THE
PROBLEM

Function doesn't have or inherit @Throws annotation and thus exception isn't propagated
from Kotlin to Objective-C/Swift as NSError.
It is considered unexpected and unhandled instead. Program will be terminated.
Uncaught Kotlin exception: io.ktor.utils.io.errors.IOException: Bomb has detonated
at
0 shared 0x103e6ec95 kfun:kotlin.Exception#<init>(kotlin.String?;kotlin.Throwable?){} + 133
(/opt/buildAgent/work/f43969c6214a19e7/kotlin/kotlin-native/runtime/src/main/kotlin/kotlin/Exceptions.kt:25:63)
at
1 shared 0x1040c6de7 kfun:io.ktor.utils.io.errors.IOException#<init>(kotlin.String;kotlin.T
hrowable?){} + 119 (/opt/buildAgent/work/8d547b974a7be21f/ktor-io/posix/src/io/ktor/utils/io/errors/IOException.kt:4:58)
at
2 shared 0x1040c6e4d kfun:io.ktor.utils.io.errors.IOException#<init>(kotlin.String){} + 93
(/opt/buildAgent/work/8d547b974a7be21f/ktor-io/posix/src/io/ktor/utils/io/errors/IOException.kt:5:50)
at 3 shared 0x103dc6591 kfun:io.aoriani.kmpapp.Bomb#explode(){} + 145
at 4 shared 0x103dc8c54 objc2kotlin_kfun:io.aoriani.kmpapp.Bomb#explode(){} + 148
at 5 iosApp 0x1027a6c59 $s6iosApp10defuseBombyyF + 57
(/Users/aoriani/Development/Multiplatform/KmpApp/iosApp/iosApp/ContentView.swift:33:14)
at 6 iosApp 0x1027a6f5d $s6iosApp11ContentViewVACycfC + 141
(/Users/aoriani/Development/Multiplatform/KmpApp/iosApp/iosApp/ContentView.swift:10:0)
at 7 iosApp 0x1027a67eb $s6iosApp6iOSAppV4bodyQrvgAA11ContentViewVyXEfU_ + 27
(/Users/aoriani/Development/Multiplatform/KmpApp/iosApp/iosApp/iOSApp.swift:7:4)
at 8 SwiftUI 0x105435a01 get_witness_table
7SwiftUI4ViewRzlAA15ModifiedContentVyxAA25ComplicationIdiomModifierVGAaBHPxAaBHD1__AfA0cH0HPyHCHCTm + 60924
at 9 iosApp 0x1027a66cc $s6iosApp6iOSAppV4bodyQrvg + 156 (/
CRASH!

Exceptions
class Bomb {
@Throws(IOException::class)
fun explode() : Unit =
throw IOException("Error")
}
public class Bomb : KotlinBase {
public init()
open func explode() throws
}
func defuseBomb() {
let bomb = Bomb()
do {
try bomb.explode()
} catch let error as NSError {
switch error.kotlinException
{
case let ioe as
Ktor_ioIOException:
print("Bomb defused:
(ioe.message ?? "")")
default:
print("Sorry could not
defuse!")
}
}
}
- (BOOL)explodeAndReturnError:(NSError
* _Nullable * _Nullable)error
__attribute__((swift_name(”explode()"))
);
[OBJ-C]
Konan
Xcode Bridging
THE
SOLUTION

Kotlin Enums vs Swift Enums
enum class Result {
LOADING,
SUCCESS,
FAILURE
}
public class Result : KotlinEnum<Result> {
open class var loading: Result { get }
open class var success: Result { get }
open class var failure: Result { get }
open class func values() ->
KotlinArray<Result>
open class var entries: [Result] { get
}
}
func testEnum() {
let result: Result = .failure
switch result {
case .loading: print("Loading")
case .success: print("Success")
case .failure: print("Failed")
default: print("I will never be called")
}
}
THE
PROBLEM

Kotlin Sealed Class vs Swift Enums
sealed class Result {
data object Loading: Result()
class Success(val value: String): Result()
class Failure(val throwable: Throwable): Result()
}
open class Result : KotlinBase {}
extension Result {
public class Loading : Result {
public convenience init()
open class var shared: Result.Loading {
get }
open func isEqual(_ other: Any?) -> Bool
open func hash() -> UInt
open func description() -> String
}
public class Failure : Result {
public init(throwable: KotlinThrowable)
open var throwable: KotlinThrowable { get
}
}
public class Success : Result {
public init(value: String)
open var value: String { get }
}
}
func testSealedClass() {
let result: Result =
Result.Loading.shared
switch result {
case is Result.Loading:
print("loading")
case let success as Result.Success:
print("Success: (success.value)")
case let failure as Result.Failure:
print("Failure:
(failure.throwable.message ?? "")")
default: print("I will never be called")
}
}
THE
SOLUTION

Kotlin Sealed Class vs Swift Enums
func testWrappedSealedClass() {
if let result =
SwiftResult(Result.Loading.shared) {
switch result {
case .loading:
print("Loading")
case let .success(value):
print("Success: (value)")
case let .failure(throwable):
print("Failure:
(throwable.message ?? "")")
}
}
}
enum SwiftResult {
case loading
case success(value: String)
case failure(throwable: KotlinThrowable)
}
extension SwiftResult {
init?(_ result: Result) {
switch result {
case is Result.Loading:
self = .loading
case let success as Result.Success:
self = .success(value:
success.value)
case let failure as Result.Failure:
self = .failure(throwable:
failure.throwable)
default: return nil
}
}
}
Libraries that can create the wrapper
for you:
• MOKO KSwift
• Touchlab SKIE
THE
SOLUTION

Generics
@property (nonatomic,strong) NSArray *arrayOfUrls;
@property (nonatomic,strong) NSDictionary *myDictNameToAge;
@property (nonatomic,strong) NSSet *setOfStrings;
open var arrayOfUrls: [Any]
open var myDictNameToAge: [AnyHashable : Any]
open var setOfStrings: Set<AnyHashable>
@property (nonatomic,strong) NSArray<NSURL *> *arrayOfUrls;
@property (nonatomic,strong) NSDictionary<NSString *, NSNumber *> *myDictNameToAge;
@property (nonatomic,strong) NSSet<NSString *> *setOfStrings;
open var arrayOfUrls: [URL]
open var myDictNameToAge: [String : NSNumber]
open var setOfStrings: Set<String>
[OBJ-C]
[OBJ-C]
Before Lightweight Generics
After Lightweight Generics

Generics : Protocols
interface Cat<T> {
val data: T
}
public protocol Cat {
var data: Any? { get }
}

Generics : Classes
class Dog<T>(val data: T)
// Bounded Type Parameter
class Bird<T: Number>(val data: T)
public class Dog<T> : KotlinBase
where T : AnyObject {
public init(data: T?)
open var data: T? { get }
}
//Bounded Typed Parameter
public class Bird<T> : KotlinBase
where T : AnyObject {
public init(data: T)
open var data: T { get }
}

Generics : Methods & Functions
interface MyInterface
class MethodTest {
fun <T: Number> eat(value: T): Int =
value.toInt()
fun <T: MyInterface> drink(value: T) = a
}
public protocol MyInterface {}
public class MethodTest : KotlinBase {
public init()
open func eat(value: Any) -> Int32
open func drink(value: MyInterface) ->
MyInterface
}

Coroutines
class SuspendFunctions {
suspend fun generateInteger(): Int {
delay(1_000)
return 1
}
}
public class SuspendFunctions :
KotlinBase {
public init()
open func
generateInteger(completionHandler:
@escaping (KotlinInt?, Error?) -> Void)
// After Swift 5.5 we also have
open func generateInteger() async
throws -> KotlinInt
}
func usingTask() {
let task = Task { @MainActor in
do {
let object = SuspendFunctions()
let result = try await
object.generateInteger()
print("Result: (result)")
} catch {
print("We had an error:
(error)")
}
}
// This doesn't do what do you think it
would do
task.cancel()
}
THE
PROBLEM

Flows
class Flows {
fun generateIntegers(): Flow<Int> {
return flowOf(1,2,3)
}
}
public class Flows : KotlinBase {
public init()
open func genenerateIntegers() ->
Kotlinx_coroutines_coreFlow
}
THE
PROBLEM

KMP-NativeCouroutines
class CoroutinesAndFlows {
@NativeCoroutineScope
internal val scope = MainScope()
@NativeCoroutines
suspend fun randomInt(): Int {
delay(1_000);
return (1..10).random()
}
@NativeCoroutines
fun flowOfInt(value: Int): Flow<Int> {
return flowOf(value)
}
}
THE
SOLUTION

KMP-NativeCouroutines: Coroutine
import KMPNativeCoroutinesAsync
func usingNativeSuspend() {
let task = Task {
let object = CoroutinesAndFlows()
do {
let v = try await asyncFunction(for: object.randomInt())
} catch {
print("We met an error: (error)")
}
}
//Cancels Swift task and Kotlin Job
task.cancel()
}
THE
SOLUTION

KMP-NativeCouroutines: Flow
import Combine
import KMPNativeCoroutinesCombine
func usingNativeFlow() {
let object = CoroutinesAndFlows()
let publisher = createPublisher(for: object.flowOfInt(value: 3))
let cancellable = publisher.sink { completion in
switch completion {
case .finished:
print("Flow has finished")
case let .failure(err):
print("Flow terminated with error (err)")
}
} receiveValue: { value in
print("Received (value)")
}
// Cancels Kotlin Flow and Combine Publisher
cancellable.cancel()
}
THE
SOLUTION

And if you found everything too complicated…
Just use Compose Multiplatform!😁

Acknowledgement
• Thomas Hultgren
• Daniel Youn
• Paolo Rotolo
• John O’Reilly
• Pamela Hill
• Better Programming
• Towards Devs
• ProAndroidDev
• Android Weekly
• Kotlin Weekly

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