Kotlin expect and actual Keywords

Tags:

Multiplatform development

The expect and actual keywords are special keywords used in Kotlin Multiplatform projects. They take advantage of Kotlin’s design for use across multiple platforms.

In a Kotlin Multiplatform project, code is generally shared across multiple platforms such as JVM, Android, iOS, and JavaScript. However, some cases require a platform-specific implementation. In such cases, the expect and actual keywords can be used to provide different implementations for each platform.

  • expect
    • The expect keyword is used to declare how a particular interface, class, or function should be implemented on a platform.
    • In other words, it defines the expected implementation. It is generally used in shared code and defines APIs that must be provided by a specific platform.
  • actual
    • The actual keyword maps to the expect keyword and provides the concrete implementation of that interface, class, or function.
    • This is the specific implementation provided per platform. Therefore, platform-specific code can be written for each platform.

The following is a simple example that uses the expect and actual keywords to provide different implementations by platform.

// common module
expect fun platformSpecificFunction(): String

// JVM module
actual fun platformSpecificFunction(): String {
    return "This is the JVM implementation"
}

// iOS module
actual fun platformSpecificFunction(): String {
    return "This is the iOS implementation"
}

// Usage
fun main() {
    println(platformSpecificFunction()) // Output depends on the platform
}

In the example above, platformSpecificFunction is declared with expect in the shared common module. The actual implementation for each platform is then provided with the actual keyword. As a result, the appropriate implementation is used each time the code runs on a platform.