Kotlin/Native as an Apple Framework

Kotlin/Native provides bi-directional interoperability with Objective-C/Swift. Objective-C frameworks and libraries can be used in Kotlin code. Kotlin modules can be used in Swift/Objective-C code too. Besides that, Kotlin/Native has C Interop. There is also the Kotlin/Native as a Dynamic Library tutorial for more information.

In this tutorial, we will look at how to use Kotlin/Native code from Objective-C and Swift applications on macOS and iOS. We will build a framework from Kotlin code.

In this tutorial we'll:

• create a Kotlin Library and compile it to a framework
• examine the generated Objective-C and Swift API code
• use the framework from Objective-C and Swift
• Configure Xcode to use the framework for macOS and iOS

Creating a Kotlin Library

Kotlin/Native compiler can produce a framework for macOS and iOS out of the Kotlin code. The created framework contains all declarations and binaries needed to use it with Objective-C and Swift. The best way to understand the techniques is to try it for ourselves. Let's create a tiny Kotlin library first and use it from an Objective-C program.

We create the hello.kt file with the library contents:

package example

object Object {
  val field = "A"
}

interface Interface {
  fun iMember() {}
}

class Clazz : Interface {
  fun member(p: Int): ULong? = 42UL
}

fun forIntegers(b: Byte, s: UShort, i: Int, l: ULong?) { }
fun forFloats(f: Float, d: Double?) { }

fun strings(str: String?) : String {
  return "That is '$str' from C"
}

fun acceptFun(f: (String) -> String?) = f("Kotlin/Native rocks!")
fun supplyFun() : (String) -> String? = { "$it is cool!" }

While it is possible to use the command line, either directly or by combining it with a script file (i.e., sh or bat file), we should notice, that it does not scale well for big projects that have hundreds of files and libraries. It is then better to use the Kotlin/Native compiler with a build system, as it helps to download and cache the Kotlin/Native compiler binaries and libraries with transitive dependencies and run the compiler and tests. Kotlin/Native can use the Gradle build system through the kotlin-multiplatform plugin.

We covered the basics of setting up an IDE compatible project with Gradle in the A Basic Kotlin/Native Application tutorial. Please check it out if you are looking for detailed first steps and instructions on how to start a new Kotlin/Native project and open it in IntelliJ IDEA. In this tutorial, we'll look at the advanced C interop related usages of Kotlin/Native and multiplatform builds with Gradle.

First, let's create a project folder. All the paths in this tutorial will be relative to this folder. Sometimes the missing directories will have to be created before any new files can be added.

We'll use the following build.gradle build.gradle.kts Gradle build file with the following contents:

plugins {
    id 'org.jetbrains.kotlin.multiplatform' version '1.3.21'
}

repositories {
    mavenCentral()
}

kotlin {
  macosX64("native") {
    binaries {
      framework {
        baseName = "Demo"
      }
    }
  }
}

wrapper {
  gradleVersion = "5.3.1"
  distributionType = "ALL"
}

plugins {
    kotlin("multiplatform") version "1.3.21"
}

repositories {
    mavenCentral()
}

kotlin {
  macosX64("native") {
    binaries {
      framework {
        baseName = "Demo"
      }
    }
  }
}

tasks.withType<Wrapper> {
  gradleVersion = "5.3.1"
  distributionType = Wrapper.DistributionType.ALL
}

The prepared project sources can be directly downloaded from GitHub. GitHub.

Let's move the sources file into the src/nativeMain/kotlin folder under the project. That is the default path, where sources are located, when the kotlin-multiplatform plugin is used. We use the following block to instruct configure the project to generate a dynamic or shared library for us:

binaries {
  framework {
    baseName = "Demo"
  }  
}

Along with macOS X64, Kotlin/Native supports iOS arm32, arm64 and X64 targets. We may replace the macosX64 with respective functions as shown in the table:

Let's run the linkNative Gradle task to build the library in the IDE or by calling the following console command:

./gradlew linkNative

./gradlew linkNative

gradlew.bat linkNative

Depending on the variant, the build generates the framework into the build/bin/native/debugFramework and build/bin/native/releaseFramework folders. Let's see what is inside

Generated Framework Headers

Each of the created frameworks contains the header file in <Framework>/Headers/Demo.h. The headers do not depend on the target platform (at least with Kotlin/Native v.0.9.2). It contains the definitions for our Kotlin code and a few Kotlin-wide declarations.

Note, the way Kotlin/Native exports symbols is subject to change without notice.

Kotlin/Native Runtime Declarations

Let's take a look at Kotlin runtime declarations first:

NS_ASSUME_NONNULL_BEGIN

@interface KotlinBase : NSObject
- (instancetype)init __attribute__((unavailable));
+ (instancetype)new __attribute__((unavailable));
+ (void)initialize __attribute__((objc_requires_super));
@end;

@interface KotlinBase (KotlinBaseCopying) <NSCopying>
@end;

__attribute__((objc_runtime_name("KotlinMutableSet")))
__attribute__((swift_name("KotlinMutableSet")))
@interface DemoMutableSet<ObjectType> : NSMutableSet<ObjectType>
@end;

__attribute__((objc_runtime_name("KotlinMutableDictionary")))
__attribute__((swift_name("KotlinMutableDictionary")))
@interface DemoMutableDictionary<KeyType, ObjectType> : NSMutableDictionary<KeyType, ObjectType>
@end;

@interface NSError (NSErrorKotlinException)
@property (readonly) id _Nullable kotlinException;
@end;

Kotlin classes have a KotlinBase base class in Objective-C, the class extends the NSObject class there. We also have wrappers for collections and exceptions. Most of the collection types are mapped to similar collection types from the other side:

Kotlin Numbers and NSNumber

The next part of the <Framework>/Headers/Demo.h contains number type mappings between Kotlin/Native and NSNumber. We have the base class called DemoNumber in Objective-C and KotlinNumber in Swift. It extends NSNumber. There are also child classes per Kotlin number type:

Every number type has a class method to create a new instance from the related simple type. Also, there is an instance method to extract a simple value back. Schematically, declarations look like that:

__attribute__((objc_runtime_name("Kotlin__TYPE__")))
__attribute__((swift_name("Kotlin__TYPE__")))
@interface Demo__TYPE__ : DemoNumber
- (instancetype)initWith__TYPE__:(__CTYPE__)value;
+ (instancetype)numberWith__TYPE__:(__CTYPE__)value;
@end;

Where __TYPE__ is one of the simple type names and __CTYPE__ is the related Objective-C type, e.g. initWithChar(char).

These types are used to map boxed Kotlin number types into Objective-C and Swift. In Swift, we may simply call the constructor to create an instance, e.g. KotlinLong(value: 42).

Classes and Objects from Kotlin

Let's see how class and object are mapped to Objective-C and Swift. The generated <Framework>/Headers/Demo.h file contains the exact definitions for Class, Interface, and Object:

NS_ASSUME_NONNULL_BEGIN

__attribute__((objc_subclassing_restricted))
__attribute__((swift_name("Object")))
@interface DemoObject : KotlinBase
+ (instancetype)alloc __attribute__((unavailable));
+ (instancetype)allocWithZone:(struct _NSZone *)zone __attribute__((unavailable));
+ (instancetype)object __attribute__((swift_name("init()")));
@property (readonly) NSString *field;
@end;

__attribute__((swift_name("Interface")))
@protocol DemoInterface
@required
- (void)iMember __attribute__((swift_name("iMember()")));
@end;

__attribute__((objc_subclassing_restricted))
__attribute__((swift_name("Clazz")))
@interface DemoClazz : KotlinBase <DemoInterface>
- (instancetype)init __attribute__((swift_name("init()"))) __attribute__((objc_designated_initializer));
+ (instancetype)new __attribute__((availability(swift, unavailable, message="use object initializers instead")));
- (DemoLong * _Nullable)memberP:(int32_t)p __attribute__((swift_name("member(p:)")));
@end;

The code is full of Objective-C attributes, which are intended to help the use of the framework from both Objective-C and Swift languages. DemoClazz, DemoInterface, and DemoObject are created for Clazz, Interface, and Object respectively. The Interface is turned into @protocol, both a class and an object are represented as @interface. The Demo prefix comes from the -output parameter of the kotlinc-native compiler and the framework name. We see here that the nullable return type ULong? is turned into DemoLong* in Objective-C.

Global Declarations from Kotlin

All global functions from Kotlin are turned into DemoLibKt in Objective-C and into LibKt in Swift, where Demo is the framework name and set by the -output parameter of kotlinc-native.

NS_ASSUME_NONNULL_BEGIN

__attribute__((objc_subclassing_restricted))
__attribute__((swift_name("LibKt")))
@interface DemoLibKt : KotlinBase
+ (void)forIntegersB:(int8_t)b s:(int16_t)s i:(int32_t)i l:(DemoLong * _Nullable)l __attribute__((swift_name("forIntegers(b:s:i:l:)")));
+ (void)forFloatsF:(float)f d:(DemoDouble * _Nullable)d __attribute__((swift_name("forFloats(f:d:)")));
+ (NSString *)stringsStr:(NSString * _Nullable)str __attribute__((swift_name("strings(str:)")));
+ (NSString * _Nullable)acceptFunF:(NSString * _Nullable (^)(NSString *))f __attribute__((swift_name("acceptFun(f:)")));
+ (NSString * _Nullable (^)(NSString *))supplyFun __attribute__((swift_name("supplyFun()")));
@end;

We see that Kotlin String and Objective-C NSString* are mapped transparently. Similarly, Unit type from Kotlin is mapped to void. We see primitive types are mapped directly. Non-nullable primitive types are mapped transparently. Nullable primitive types are mapped into Kotlin<TYPE>* types, as shown in the table above. Both higher order functions acceptFunF and supplyFun are included, and accept Objective-C blocks.

More information about all other types mapping details can be found in the Objective-C Interop documentation article

Garbage Collection and Reference Counting

Objective-C and Swift use reference counting. Kotlin/Native has its own garbage collection too. Kotlin/Native garbage collection is integrated with Objective-C/Swift reference counting. We do not need to use anything special to control the lifetime of Kotlin/Native instances from Swift or Objective-C.

Using the Code from Objective-C

Let's call the framework from Objective-C. For that we create the main.m file with the following content:

#import <Foundation/Foundation.h>
#import <Demo/Demo.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        [[DemoObject object] field];
        
        DemoClazz* clazz = [[ DemoClazz alloc] init];
        [clazz memberP:42];
        
        [DemoLibKt forIntegersB:1 s:1 i:3 l:[DemoULong numberWithUnsignedLongLong:4]];
        [DemoLibKt forIntegersB:1 s:1 i:3 l:nil];
        
        [DemoLibKt forFloatsF:2.71 d:[DemoDouble numberWithDouble:2.71]];
        [DemoLibKt forFloatsF:2.71 d:nil];
        
        NSString* ret = [DemoLibKt acceptFunF:^NSString * _Nullable(NSString * it) {
            return [it stringByAppendingString:@" Kotlin is fun"];
        }];
        
        NSLog(@"%@", ret);
        return 0;
    }
}

We call Kotlin classes directly from Objective-C code. A Kotlin object has the class method function object, which allows us to get the only instance of the object and to call Object methods on it. The widespread pattern is used to create an instance of the Clazz class. We call the [[ DemoClazz alloc] init] on Objective-C. We may also use [DemoClazz new] for constructors without parameters. Global declarations from the Kotlin sources are scoped under the DemoLibKt class in Objective-C. All methods are turned into class methods of that class. The strings function is turned into DemoLibKt.stringsStr function in Objective-C, we can pass NSString directly to it. The return is visible as NSString too.

Using the Code from Swift

The framework that we compiled with Kotlin/Native has helper attributes to make it easier to use with Swift. Let's convert the previous Objective-C example into Swift. As a result, we'll have the following code in main.swift:

import Foundation
import Demo

let kotlinObject = Object()
assert(kotlinObject === Object(), "Kotlin object has only one instance")

let field = Object().field

let clazz = Clazz()
clazz.member(p: 42)

LibKt.forIntegers(b: 1, s: 2, i: 3, l: 4)
LibKt.forFloats(f: 2.71, d: nil)

let ret = LibKt.acceptFun { "\($0) Kotlin is fun" }
if (ret != nil) {
  print(ret!)
}

The Kotlin code is turned into very similar looking code in Swift. There are some small differences, though. In Kotlin any object has only one instance. Kotlin object Object now has a constructor in Swift, and we use the Object() syntax to access the only instance of it. The instance is always the same in Swift, so that Object() === Object() is true. Methods and property names are translated as-is. Kotlin String is turned into Swift String too. Swift hides NSNumber* boxing from us too. We pass Swift closure to Kotlin and call a Kotlin lambda function from Swift too.

More documentation on the types mapping can be found in the Objective-C Interop article.

Xcode and Framework Dependencies

We need to configure an Xcode project to use our framework. The configuration depends on the target platform.

Xcode for MacOS Target

First, we need to include the framework in the General section of the target configuration. There is the Linked Frameworks and Libraries section to include our framework. This will make Xcode look at our framework and resolve imports both from Objective-C and Swift.

The second step is to configure the framework search path of the produced binary. It is also known as rpath or run-time search path. The binary uses the path to look for the required frameworks. We do not recommend installing additional frameworks to the OS if it is not needed. We should understand the layout of our future application, for example, we may have the Frameworks folder under the application bundle with all the frameworks we use. The @rpath parameter can be configured in Xcode. We need to open the project configuration and find the Runpath Search Paths section. Here we specify the relative path to the compiled framework.

Xcode for iOS Targets

First, we need to include the compiled framework into the Xcode project. For this we add the framework to the Embedded Binaries block of the General section of the target configuration page.

The second step is to then include the framework path into the Framework Search Paths block of the Build Settings section of the target configuration page. It is possible to use $(PROJECT_DIR) macro to simplify the setup.

The iOS simulator requires a framework compiled for the ios_x64 target, the iOS_sim folder in our case.

The Stack Overflow thread contains few more recommendations. Also, CocoaPods package manager may be helpful to automate the process too.

Next Steps

Kotlin/Native has bidirectional interop with Objective-C and Swift languages. Kotlin objects integrate with Objective-C/Swift reference counting. Unused Kotlin objects are automatically removed. The Objective-C Interop article contains more information on the interop implementation details. Of course, it is possible to import an existing framework and use it from Kotlin. Kotlin/Native comes with a good set of pre-imported system frameworks.

Kotlin/Native supports C interop too. Check out the Kotlin/Native as a Dynamic Library tutorial for that, or have a look at the C Interop documentation article