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Kotlin Academy · Lesson

expect/actual Mechanism for Platform APIs

Declare expected declarations in common code and provide actual implementations per platform.

The expect/actual Pattern

expect declares an API contract in commonMain. Each platform provides an actual implementation. The compiler enforces that all targets have an actual.

// commonMain:
expect fun platformName(): String

// androidMain:
actual fun platformName(): String = "Android ${android.os.Build.VERSION.SDK_INT}"

// iosMain:
actual fun platformName(): String = UIDevice.currentDevice.systemName()

expect class

expect class declares a class with its API in commonMain. Each platform provides the full implementation.

// commonMain:
expect class DateTime {
    fun format(pattern: String): String
    val timestamp: Long
}

// androidMain:
actual class DateTime {
    actual val timestamp = System.currentTimeMillis()
    actual fun format(pattern: String): String =
        java.text.SimpleDateFormat(pattern).format(java.util.Date(timestamp))
}

All lessons in this course

  1. KMP Project Structure: commonMain, androidMain, iosMain
  2. expect/actual Mechanism for Platform APIs
  3. Sharing Repository and Use Case Layers
  4. Dependency Injection in KMP with Koin
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