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.
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.
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 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.
Gives better Objective-C/Swift names to Kotlin constructs like classes, functions and so on, without actually renaming the Kotlin constructs. Experimental.
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.
Some autogenerated functions are converted to Swift: copy to doCopy, equals to isEquals, toString to description. Additional features, like destructuring, are not supported.
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.
You can access members of Kotlin companion objects from Swift explicitly through the `companion` auxiliary object: ClassWithCompanionObject.companion.CONST_VAL_EXAMPLE.
Translated into callback, experimentally - into async / await. Libraries like SKIE and KMP-NativeCoroutines can be used to improve the interop and provide cancellation support.
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.
