kmp-logging

KMP Logging (Kermit)

Overview

Kermit (Touchlab) is the standard KMP logging library — log from commonMain and it writes to Logcat on Android and OSLog on iOS automatically (console on JS). Current version 2.0.7. Logs flow through composable LogWriters, so you can also send them to Crashlytics/Bugsnag.

When to use

• Logging from shared code (commonMain)
• Tagged loggers per class/feature
• Sending warnings/errors to crash reporting
• Controlling verbosity per build (minSeverity)

Dependency

commonMain.dependencies {
    implementation("co.touchlab:kermit:2.0.7")
    // implementation("co.touchlab:kermit-crashlytics:2.0.7") // optional
}

Usage

// Global logger — lambda message is only built if it will actually be logged
Logger.i { "App started" }
Logger.e(throwable) { "Failed to load user $id" }

// Tagged logger per class
class UserRepository {
    private val log = Logger.withTag("UserRepository")
    fun load() { log.d { "loading…" } }
}

Severities, low → high: v d i w e a.

Configure writers + severity (once, at startup)

Logger.setMinSeverity(if (isDebug) Severity.Verbose else Severity.Warn)
Logger.setLogWriters(platformLogWriter(), CrashlyticsLogWriter())

// or a fully custom instance:
val log = Logger(
    config = StaticConfig(
        minSeverity = Severity.Info,
        logWriterList = listOf(platformLogWriter()),
    ),
    tag = "MyApp",
)

Common mistakes

Mistake	Fix
String concatenation in hot log paths	Use the lambda form { "x=$x" } — skipped below minSeverity
Same verbosity in release	Raise minSeverity for release builds
Expecting iOS logs in Logcat	iOS writes to OSLog (Console.app / Xcode), not Logcat
Crash logs not appearing	Add CrashlyticsLogWriter to the writer list

See also

• Android Logcat / ADB / ANR debugging: android-skills:android-debugging
• Analytics events (distinct from logs): analytics-patterns