Introduction to Kotlin Multiplatform
Support for multiplatform programming is one of Kotlin's key benefits. It reduces time spent writing and maintaining the same code for different platforms while retaining the flexibility and benefits of native programming.
Kotlin Multiplatform allows you to share code across different platforms, whether it's mobile, web, or desktop. The platforms to which the code is compiled are defined by the list of targets.
Each target has a corresponding source set which represents a set of source files with its own dependencies and compiler options. Platform-specific source sets, for example jvmMain for the JVM, can make use of platform-specific libraries and APIs.
To share code among a subset of targets, intermediate source sets are used. For example, the appleMain source set represents the code shared among all Apple platforms. The code shared among all platforms and compiled to all declared targets has its own source set, commonMain. It cannot use platform-specific APIs, but can take advantage of multiplatform libraries.
When compiling for a specific target, Kotlin combines the common source set, the relevant intermediate source sets, and the target-specific source set.
For more details on this topic, see:
It's sometimes more convenient to share code among a subset of similar targets. Kotlin Multiplatform provides a way to simplify their creation with a default hierarchy template. It includes a pre-defined list of intermediate source sets that are created based on the targets you specified in your project.
To access platform-specific APIs from shared code, you can use another Kotlin mechanism, expected and actual declarations. This way, you can declare that you expect a platform-specific API in common code but provide a separate actual implementation for each target platform. You can use this mechanism with different Kotlin concepts, including functions, classes, and interfaces. For example, you can define a function in common code, but provide its implementation using a platform-specific library in a corresponding source set.
For more details on this topic, see:
A Kotlin Multiplatform project can depend on external libraries and other multiplatform projects. For common code, you can add dependencies on multiplatform libraries in the common source set. Kotlin automatically resolves and adds the appropriate platform-specific parts to other source sets. If only a platform-specific API is required, add the dependency to the corresponding source sets.
Adding Android-specific dependencies to a Kotlin Multiplatform project is similar to adding them in a pure Android project. When working with iOS-specific dependencies, you can seamlessly integrate Apple SDK frameworks without extra configuration. For external libraries and frameworks, Kotlin offers interoperability with Objective-C and Swift.
For more details on this topic, see:
If your multiplatform project targets iOS, you can set up integration of the Kotlin Multiplatform shared module with your iOS app.
For that, you generate an iOS framework and then add it as a local or remote dependency to the iOS project:
Local integration: connect your multiplatform and Xcode projects directly with a special script or use the CocoaPods dependency manager for setups involving local Pod dependencies.
Remote integration: set up an SPM dependency using XCFrameworks or distribute shared modules through CocoaPods.
For more details on this topic, see iOS integration methods.
Every target can have multiple compilations for different purposes, typically for production or testing, but you can also define custom compilations.
With Kotlin Multiplatform, you can configure all compilations in the project, set up specific compilations within a target, and even create individual compilations. When configuring compilations, you can modify compiler options, manage dependencies, or configure interoperability with native languages.
For more details on this topic, see Configure compilations.
By default, a target is compiled to a .klib artifact, which can be consumed by Kotlin/Native itself as a dependency but cannot be executed or used as a native library. However, Kotlin Multiplatform provides additional mechanisms for building final native binaries.
You can create executable binaries, shared and static libraries, or Objective-C frameworks, each configurable for different build types. Kotlin also provides a way to build universal (fat) frameworks and XCFrameworks for iOS integration.
For more details on this topic, see Build native binaries.
You can create a multiplatform library with common code and its platform-specific implementations for JVM, web, and native platforms.
Publishing a Kotlin Multiplatform library involves specific configurations in your Gradle build script. You can use a Maven repository and the maven-publish plugin for publication. Once published, a multiplatform library can be used as a dependency in other cross-platform projects.
For more details on this topic, see Publish a multiplatform library.