extract DarwinFormatter and add test

build.gradle.kts

@@ -188,6 +188,11 @@ kotlin {
         val darwinMain by creating {
             dependsOn(commonMain)
         }
+        val darwinTest by creating {
+            dependencies {
+                implementation(kotlin("test"))
+            }
+        }
         linuxTargets.forEach {
             getByName("${it.targetName}Main") {
                 dependsOn(linuxMain)

src/darwinMain/kotlin/io/github/oshai/kotlinlogging/DarwinKLogger.kt

@@ -2,6 +2,7 @@
 
 package io.github.oshai.kotlinlogging
 
+import io.github.oshai.kotlinlogging.internal.DarwinFormatter
 import kotlinx.cinterop.ExperimentalForeignApi
 import kotlinx.cinterop.ptr
 import platform.darwin.OS_LOG_TYPE_DEBUG
@@ -20,17 +21,7 @@ public class DarwinKLogger(override val name: String, override val underlyingLog
   override fun at(level: Level, marker: Marker?, block: KLoggingEventBuilder.() -> Unit) {
     if (isLoggingEnabledFor(level, marker)) {
       KLoggingEventBuilder().apply(block).run {
-        val formattedMessage = buildString {
-          marker?.getName()?.let {
-            append(it)
-            append(" ")
-          }
-          append(message)
-          cause?.stackTraceToString()?.let {
-            append('\n')
-            append(it)
-          }
-        }
+        val formattedMessage: String = DarwinFormatter.getFormattedMessage(this, marker)
         _os_log_internal(
           __dso_handle.ptr,
           underlyingLogger,

src/darwinMain/kotlin/io/github/oshai/kotlinlogging/internal/DarwinFormatter.kt

@@ -0,0 +1,21 @@
+package io.github.oshai.kotlinlogging.internal
+
+import io.github.oshai.kotlinlogging.KLoggingEventBuilder
+import io.github.oshai.kotlinlogging.Marker
+
+internal object DarwinFormatter {
+
+  internal fun getFormattedMessage(builder: KLoggingEventBuilder, marker: Marker?): String {
+    return buildString {
+      marker?.getName()?.let {
+        append(it)
+        append(" ")
+      }
+      append(builder.message)
+      builder.cause?.stackTraceToString()?.let {
+        append('\n')
+        append(it)
+      }
+    }
+  }
+}

src/darwinTest/kotlin/io/github/oshai/kotlinlogging/internal/DarwinFormatterTest.kt

@@ -0,0 +1,13 @@
+package io.github.oshai.kotlinlogging.internal
+
+import kotlin.test.*
+
+class DarwinFormatterTest {
+
+  @Test
+  fun simpleTest() {
+    KLoggingEventBuilder().apply { message = "msg" }.run {
+      assertEquals("msg1", DarwinFormatter.getFormattedMessage(this, null))
+    }
+  }
+}