Kotlin-Swift interopedia

Overview

Kotlin/Native provides bidirectional interoperability with Objective-C. Kotlin is not directly interoperable with Swift but rather indirectly via an Objective-C bridge. The reason for this is twofold:

• It allows all iOS projects (written in Objective-C or Swift) to use shared Kotlin code
• When the decision was made, Swift was still on the road to maturity and mainstream adoption.

The Kotlin/Native compiler generates Objective-C headers, which Swift code can import. This has the following implications for Swift development:

• Some features work exactly as expected
• Some features work with a small workaround
• Some features work better with a community solution
• Some features don’t work optimally right now
• Some features don’t work

Swift export, where the Kotlin API is directly exported as Swift declarations instead of Objective-C headers, is something that the Kotlin team is currently working on, and will resolve some of the difficulties experienced by Swift developers when consuming shared Kotlin code. You can find the table with the latest feature state here.

How to use

Interopedia

This interopedia of the different Kotlin language features is categorized into broad categories, namely:

• Overview
• Functions and properties
• More about functions
• Types
• Classes and interfaces
• Coroutines
• Extensions
• Generics

Each language feature has its own article, consisting of an explanation of the feature, some sample code in Kotlin, how to call this code from Swift (if possible), and additional improvements should the Swift code not be as idiomatic as we’d like.

You could search the interopedia for the particular language feature that you are interested in, or read all the articles in order for a more comprehensive understanding of Kotlin/Swift interoperability.

Kotlin/Swift Interop Playground app

The iOS app is organized into the same broad categories as the interopedia. Clicking on a specific language feature will:

• Display a summary of the interoperability of the feature.
• Run code samples associated with the feature and print the results in the console.

You can edit the code, rerun the app, and see how the output has changed.

Overview

Classes and functions	You can instantiate Kotlin classes and call Kotlin functions from Swift: SimpleClass().simpleFunction().
Top-level functions	You can access a top-level function via the wrapper class: TopLevelFunctionKt.topLevelFunction().
Types	Simple types and custom types can be passed as arguments and returned from function calls.
Collections	Kotlin and Swift have very similar kinds of collections and can be mapped between each other.
Exceptions	If you invoke a Kotlin function that throws an exception and doesn't declare it with `@Throws`, that crashes the app. Declared exceptions are converted to NSError and must be handled.
Public API	Public classes, functions, and properties are visible from Swift. Marking classes, functions, and properties internal will exclude them from the public API of the shared code, and they will not be visible in Swift.
Interop annotation - @ObjCName	Gives better Objective-C/Swift names to Kotlin constructs like classes, functions and so on, without actually renaming the Kotlin constructs. Experimental.
Interop annotations - @HiddenFromObj	Hides a Kotlin declaration from Objective-C/Swift. Experimental.
Interop annotations - @ShouldRefineInSwift	Helps to replace a Kotlin declaration with a wrapper written in Swift. Experimental.
KDoc comments	You can see certain KDoc comments at development time. In Xcode, use Option+Double left click to see the docs. Note that many KDocs features don't work in Xcode, like properties on constructors (@property) aren't visible. In Fleet, use the 'Show Documentation' action.

Functions and properties

Member functions	You can call public member functions from Swift. Internal or private declarations aren't visible.
Constructor	You call constructors to create Kotlin classes from Swift.
Read-only member properties	Member val property is accessible from Swift and is a read-only property in Swift.
Mutable member properties	Member var property is accessible from Swift and is a mutable property in Swift.
Top-level val properties (readonly)	You access a top-level property via the wrapper class: TopLevelPropertyKt.topLevelProperty.
Top-level var properties (mutable)	You access a top-level property via the wrapper class: TopLevelMutablePropertyKt.topLevelProperty.
Functions expecting lambda arguments	You can use a function expecting one or more lambdas as arguments without issues from Swift.
Functions returning function type	You can call a Kotlin function returning a lambda. The result has Swift function type, like () -> String, so you can easily call it.

More about functions

Functions with overloads	There are some peculiarities when using the same parameter names.
Functions with default arguments	You always have to specify all the function arguments. Improved interop available with SKIE.
Constructor with default arguments	You always have to specify all the arguments for a constructor.
Functions expecting lambda with receiver	The extension function turns into a lambda with a parameter.
Functions with receivers	Functions with receivers turn into functions with parameters, which is not as convenient.
Functions with value class parameter	The function appears in the .h file, but the inline class argument is turned into a basic type.
Functions with vararg parameter	varargs are mapped to KotlinArray, not Swift's variardic parameters.
Inline functions	Inline functions are in the .h file, they can be called. However, they are regular functions and not inlined.

Types

Basic types	May require mapping for integer data types and mapping for Char.
Optional basic types	Some basic types require mapping into special optional types.
Collections with custom types	Collections with elements of custom types do not require additional mappings.
Collections with basic types	Collections with elements of basic types (except String) require a wrapper.
Mutable, immutable collections	To adjust mutability, the let and var keywords are used. Additional mappings are required for mutable collections.
Unit and Nothing	The Unit and Nothing types can be used in the same way as in Kotlin: Unit as an object or void, Nothing cannot be created.

Classes and interfaces

Abstract classes	Xcode has no hints to override abstract methods, rather we get a crash when trying to use the method during runtime.
Annotation classes	Annotations are not supported and are not included in the .h file.
Data classes	Some autogenerated functions are converted to Swift: copy to doCopy, equals to isEquals, toString to description. Additional features, like destructuring, are not supported.
Enum classes	No equivalent enum is generated on the Swift side, and default case must be specified in a switch expression. Instead an object with static elements is generated. Improved interop available with SKIE.
Inner classes	Minor differences in creation syntax.
Open classes	Can inherit from open class, use its protected properties, override open, but not override final methods.
Sealed classes	A class with heirs is generated. Passing to a switch statement requires a default case. Improved interop available with SKIE.
Inline classes	This feature is not supported.
Objects	You can access Kotlin object via the shared auxiliary object: MyKotlinObject.shared.myProperty.
Companion objects	You can access members of Kotlin companion objects from Swift explicitly through the `companion` auxiliary object: ClassWithCompanionObject.companion.CONST_VAL_EXAMPLE.
Fun interfaces	You can't create an anonymous class in Swift.
Interfaces	The interface has become @protocol. Xcode turns val property into var when generating the stubs.
Sealed interfaces	Separate protocols are generated that are not related to each other.

Coroutines

Suspend functions	Translated into callback, experimentally - into async / await. Libraries like SKIE and KMP-NativeCoroutines can be used to improve the interop and provide cancellation support.
Flows	Translated into callback, experimentally - into async / await. Generic type arguments are lost. Libraries like SKIE and KMP-NativeCoroutines can be used to improve the interop and provide cancellation support.

Extensions

Extension function over platform class	A wrapper class appears with a function that accepts an object of the desired class.
Extension function over usual class	The function can be used on a class object.
Extension properties over platform class	A wrapper class appears with a function that accepts an object of the desired class.
Extension properties over usual class	The property can be accessed through the class object.
Extension properties for companion object of platform class	There is a property in the .h file, but in Swift it’s impossible to use.
Extension properties for companion object of usual class	The property can be accessed through the companion object.

Generics

Generic classes	Some features are supported.
Generic functions	Automatic type inference and nullability are not supported.
Bounded generics	The generic type restriction is not supported.
Contravariant generics	Requires a type cast.
Covariant generics	Requires a type cast.
Reified functions	The reified function crashes at runtime.
Star projection	Requires a type cast.
Generic interfaces	Generic interfaces are not supported.

Feature Support Refresher

Some features work exactly as expected

• Classes and functions
• Member properties (Readonly or Mutable)
• Higher-order functions (lambda as Parameter or Return)
• Collections with custom types
• Unit and Nothing
• Abstract classes
• Open classes
• Interfaces
• Extension function over usual class
• Extension properties over usual class

Some features work with a small workaround

• Top-level functions
• Top-level properties (Read-only or Mutable)
• Exceptions
• Function overloads
• Functions with default arguments
• Functions expecting lambda with receiver
• Functions with receivers
• Basic types
• Optional basic types
• Collections with basic types
• Mutable / immutable collections
• Enum classes
• Inner classes
• Sealed classes
• Objects
• Companion objects
• Sealed interfaces
• Extension function over platform class
• Extension properties over platform class
• Extension properties for companion object of usual class

Some features work better with a community solution

• Suspend functions
• Flows

Some features don’t work optimally right now (use with care)

• Data classes
• Generic classes
• Generic functions
• Contravariant generics
• Covariant generics
• Star projections

Some features don’t work (don’t use)

• Functions with value class parameter
• Functions with a vararg parameter
• Inline functions
• Annotations
• Inline classes
• Fun interfaces
• Extension properties for companion object of platform class
• Bounded generics
• Reified functions
• Generic interfaces