Message Formatting

To make message formatting more uniform and flexible, many of the LogWriter instances can take a MessageStringFormatter parameter. This allows you to control how log messages are presented and apply common formatting.

TL;DR

If you don't want tags to be printed in your iOS or JS log statements, give your LogWriter instances NoTagFormatter. This will suppress printing tag in log message strings. Tag will still be sent to Android log messages.

For the global Logger and the platform LogWriter, config would look like the following:

Logger.setLogWriters(platformLogWriter(NoTagFormatter))

For most use cases, this is all you need to know about formatters. The rest of this doc explains them in more detail.

Log Message Components

There are 3 parts to a message: Severity, Tag, and the Message itself. All loggers accept a log message string, but support for severity and tag vary between systems. Here is a breakdown of support for a few commonly used logging systems.

System	Severity	Tag
Android	✅	✅
OSLog (iOS)	✅	❌
JS-Console	✅	❌
SystemWriter (jvm)	❌	❌
Common (println)	❌	❌

The main purpose of MessageStringFormatter is to manage situations where the logging system does not support either Severity or Tag. The LogWriter instance will pass null's to the MessageStringFormatter in cases where the LogWriter natively supports Severity or Tag.

For example, the DefaultLogFormatter will result in the following message strings depending on platform:

// Log call
Logger.w(tag = "ATag") { "A Log Message" }

// Android
// 'A Log Message'

// OSLog
// '(ATag) A Log Message'

// Common
// 'Warn: (ATag) A Log Message'

info

You can use MessageStringFormatter to implement custom message strings. Maybe, say, add a special date format to each line. However, Kermit uses them to manage Severity and Tag for systems that don't natively support those constructs. Note: the Android LogcatWriter does not currently use a MessageStringFormatter. You would need to create a custom implementation to support that.

Implementations

Kermit provides a few implementations out of the box.

DefaultLogFormatter

This is the standard format that all compatible LogWriter instances get by default. Messages are formatted as in the examples above.

NoTagFormatter

Tags are really an Android convention. Other platforms can feel cluttered with them. You can simply ignore them with NoTagFormatter. Logging will function the same for Android, and other platforms will ignore the Tag.

SimpleLogFormatter

This formatter skips tags and severity. It just prints the message.

Configuration

To simplify setting platform LogWriter instances, you can pass a MessageStringFormatter to platformLogWriter.

platformLogWriter(NoTagLogFormatter)