Interoperability with Swift/Objective-C

Hide Kotlin declarations from Objective-C and Swift

To make your Kotlin code more Objective-C/Swift-friendly, you can hide a Kotlin declaration from Objective-C and Swift with @HiddenFromObjC. The annotation disables a function or property export to Objective-C.

Alternatively, you can mark Kotlin declarations with theinternal modifier to restrict their visibility in the compilation module. Choose @HiddenFromObjC if you only want to hide the Kotlin declaration from Objective-C and Swift, but still keep it visible from other Kotlin modules.

See an example in the Kotlin-Swift interopedia.

Use refining in Swift

@ShouldRefineInSwift helps to replace a Kotlin declaration with a wrapper written in Swift. The annotation marks a function or property as swift_private in the generated Objective-C API. Such declarations get the __ prefix, which makes them invisible from Swift.

You can still use these declarations in your Swift code to create a Swift-friendly API, but they won't be suggested in the Xcode autocomplete.

• For more information on refining Objective-C declarations in Swift, see the official Apple documentation.

• For an example on how to use the @ShouldRefineInSwift annotation, see the Kotlin-Swift interopedia.

Change declaration names

To avoid renaming Kotlin declarations, use the @ObjCName annotation. It instructs the Kotlin compiler to use the custom Objective-C and Swift name for the annotated class, interface, or another Kotlin entity:

See another example in the Kotlin-Swift interopedia.

Provide documentation with KDoc comments

Documentation is essential for understanding any API. Providing documentation for the shared Kotlin API allows you to communicate with its users on matters of usage, dos and don'ts, and so on.

By default, KDocs comments are not translated into corresponding comments when generating an Objective-C header. For example, the following Kotlin code with KDoc:

Will produce an Objective-C declaration without any comments:

To enable export of KDoc comments, add the following compiler option to your build.gradle(.kts):

After that, the Objective-C header will contain a corresponding comment:

You'll be able to see comments on classes and methods in autocompletion, for example, in Xcode. If you go to the definition of functions (in the .h file), you'll see comments on @param, @return, and so on.

Known limitations:

• Dependency documentation is not exported unless it is compiled with -Xexport-kdoc itself. The feature is Experimental, so libraries compiled with this option might be incompatible with other compiler versions.

• KDoc comments are mostly exported as is. Many KDoc features, for example @property, are not supported.

Classes

Initializers

A Swift/Objective-C initializer is imported to Kotlin as constructors or as factory methods named create. The latter happens with initializers declared in the Objective-C category or as a Swift extension, because Kotlin has no concept of extension constructors.

Kotlin constructors are imported as initializers to Swift/Objective-C.

Setters

Writeable Objective-C properties overriding read-only properties of the superclass are represented as setFoo() method for the property foo. The same goes for a protocol's read-only properties that are implemented as mutable.

Top-level functions and properties

Top-level Kotlin functions and properties are accessible as members of special classes. Each Kotlin file is translated into such a class, for example:

You can then call the foo() function from Swift like this:

See a collection of examples on accessing top-level Kotlin declarations in the Kotlin-Swift interopedia:

• Top-level functions

• Top-level read-only properties

• Top-level mutable properties

Method names translation

Generally, Swift argument labels and Objective-C selector pieces are mapped to Kotlin parameter names. These two concepts have different semantics, so sometimes Swift/Objective-C methods can be imported with a clashing Kotlin signature. In this case, the clashing methods can be called from Kotlin using named arguments, for example:

In Kotlin, it's:

Here's how the kotlin.Any functions are mapped to Swift/Objective-C:

See an example with data classes in the Kotlin-Swift interopedia.

You can specify a more idiomatic name in Swift or Objective-C, instead of renaming the Kotlin declaration with the @ObjCName annotation.

Errors and exceptions

All Kotlin exceptions are unchecked, meaning that errors are caught at runtime. However, Swift has only checked errors that are handled at compile time. So, if Swift or Objective-C code calls a Kotlin method that throws an exception, the Kotlin method should be marked with the @Throws annotation, specifying a list of "expected" exception classes.

When compiling to the Objective-C/Swift framework, non-suspend functions that have or inherit the @Throws annotation are represented as NSError*-producing methods in Objective-C and as throws methods in Swift. Representations for suspend functions always haveNSError*/Error parameter in completion handler.

When Kotlin function called from Swift/Objective-C code throws an exception which is an instance of one of the @Throws-specified classes or their subclasses, it is propagated as NSError. Other Kotlin exceptions reaching Swift/Objective-C are considered unhandled and cause program termination.

suspend functions without @Throws propagate only CancellationException (as NSError). Non-suspend functions without @Throws don't propagate Kotlin exceptions at all.

Note that the opposite reversed translation is not implemented yet: Swift/Objective-C error-throwing methods aren't imported to Kotlin as exception-throwing.

See an example in the Kotlin-Swift interopedia.

Enums

Kotlin enums are imported into Objective-C as @interface and into Swift as class. These data structures have properties corresponding to each enum value. Consider this Kotlin code:

You can access the properties of this enum class from Swift as follows:

To use variables of a Kotlin enum in a Swift switch statement, provide a default statement to prevent a compilation error:

See another example in the Kotlin-Swift interopedia.

Suspending functions

Kotlin's suspending functions (suspend) are presented in the generated Objective-C headers as functions with callbacks, or completion handlers in Swift/Objective-C terminology.

Starting from Swift 5.5, Kotlin's suspend functions are also available for calling from Swift as async functions without using the completion handlers. Currently, this functionality is highly experimental and has certain limitations. See this YouTrack issue for details.

• Learn more about the async/await mechanism in the Swift documentation.

• See an example and recommendations on third-party libraries that implement the same functionality in the Kotlin-Swift interopedia.

Extensions and category members

Members of Objective-C categories and Swift extensions are generally imported to Kotlin as extensions. That's why these declarations can't be overridden in Kotlin, and the extension initializers aren't available as Kotlin constructors.

Kotlin extensions to "regular" Kotlin classes are imported to Swift and Objective-C as extensions and category members, respectively. Kotlin extensions to other types are treated as top-level declarations with an additional receiver parameter. These types include:

• Kotlin String type

• Kotlin collection types and subtypes

• Kotlin interface types

• Kotlin primitive types

• Kotlin inline classes

• Kotlin Any type

• Kotlin function types and subtypes

• Objective-C classes and protocols

See a collection of examples in the Kotlin-Swift interopedia.

Kotlin singletons

Kotlin singleton (made with an object declaration, including companion object) is imported to Swift/Objective-C as a class with a single instance.

The instance is available through the shared and companion properties.

For the following Kotlin code:

Access these objects as follows:

See more examples in the Kotlin-Swift interopedia:

• How to access Kotlin objects using shared

• How to access members of Kotlin companion objects from Swift.

NSNumber

Kotlin primitive type boxes are mapped to special Swift/Objective-C classes. For example, the kotlin.Int box is represented as KotlinInt class instance in Swift (or ${prefix}Int instance in Objective-C, where prefix is the framework names prefix). These classes are derived from NSNumber, so the instances are proper NSNumbers supporting all corresponding operations.

NSNumber type is not automatically translated to Kotlin primitive types when used as a Swift/Objective-C parameter type or return value. The reason is that NSNumber type doesn't provide enough information about a wrapped primitive value type, for example, NSNumber is statically not known to be Byte, Boolean, or Double. So Kotlin primitive values should be cast to and from NSNumber manually.

NSMutableString

NSMutableString Objective-C class is not available from Kotlin. All instances of NSMutableString are copied when passed to Kotlin.

Collections

Kotlin collections are converted to Swift/Objective-C collections as described in the table above. Swift/Objective-C collections are mapped to Kotlin in the same way, except for NSMutableSet and NSMutableDictionary.

NSMutableSet isn't converted to a Kotlin MutableSet. To pass an object to Kotlin MutableSet, explicitly create this kind of Kotlin collection. To do this, use, for example, the mutableSetOf() function in Kotlin or the KotlinMutableSet class in Swift and ${prefix}MutableSet in Objective-C (prefix is the framework names prefix). The same is true for MutableMap.

See an example in the Kotlin-Swift interopedia.

Function types

Kotlin function-typed objects (for example, lambdas) are converted to functions in Swift and blocks in Objective-C. See an example of a Kotlin function with a lambda in the Kotlin-Swift interopedia.

However, there is a difference in how types of parameters and return values are mapped when translating a function and a function type. In the latter case, primitive types are mapped to their boxed representation. Kotlin Unit return value is represented as a corresponding Unit singleton in Swift/Objective-C. The value of this singleton can be retrieved the same way as for any other Kotlin object. See singletons in the table above.

Consider the following Kotlin function:

It's represented in Swift as follows:

And you can call it like this:

Generics

Objective-C supports "lightweight generics" defined on classes, with a relatively limited feature set. Swift can import generics defined on classes to help provide additional type information to the compiler.

Generic feature support for Objective-C and Swift differ from Kotlin, so the translation will inevitably lose some information, but the features supported retain meaningful information.

For specific examples on how to use Kotlin generics in Swift, see the Kotlin-Swift interopedia.

Forward declarations

To import forward declarations, use the objcnames.classes and objcnames.protocols packages. For example, to import a objcprotocolName forward declaration declared in an Objective-C library with a library.package, use a special forward declaration package: import objcnames.protocols.objcprotocolName.

Consider two objcinterop libraries: one that uses objcnames.protocols.ForwardDeclaredProtocolProtocol and another with an actual implementation in another package:

To transfer objects between the two libraries, use an explicit as cast in you Kotlin code:

Subclassing Kotlin classes and interfaces from Swift/Objective-C

Kotlin classes and interfaces can be subclassed by Swift/Objective-C classes and protocols.

Subclassing Swift/Objective-C classes and protocols from Kotlin

Swift/Objective-C classes and protocols can be subclassed with a Kotlin final class. Non-final Kotlin classes inheriting Swift/Objective-C types aren't supported yet, so it is not possible to declare a complex class hierarchy inheriting Swift/Objective-C types.

Normal methods can be overridden using the override Kotlin keyword. In this case, the overriding method must have the same parameter names as the overridden one.

Sometimes it is required to override initializers, for example when subclassing UIViewController. Initializers imported as Kotlin constructors can be overridden by Kotlin constructors marked with the @OverrideInit annotation:

The overriding constructor must have the same parameter names and types as the overridden one.

To override different methods with clashing Kotlin signatures, you can add the @ObjCSignatureOverride annotation to the class. The annotation instructs the Kotlin compiler to ignore conflicting overloads, in case several functions with the same argument types, but different argument names, are inherited from the Objective-C class.

By default, the Kotlin/Native compiler doesn't allow calling a non-designated Objective-C initializer as a super() constructor. This behaviour can be inconvenient if the designated initializers aren't marked properly in the Objective-C library. To disable these compiler checks, add the disableDesignatedInitializerChecks = true to the library's .def file.