Add support for os_log on Apple platforms

[conradev]

Apple platforms (macOS, iOS, watchOS, tvOS) have a unified logging system called os_log. I implemented support for os_log and made it the default on Apple platforms. This is similar to how slf4j-android implements slf4j-api to write to the Android system log.

I kept the console output appender still available on Apple platforms because it might still be desirable to use it in, for example, a command line tool.

os_log has two key concepts – a subsystem and a category. A subsystem is intended to be constant for an entire project (i.e. the reverse DNS notation for the project) and the category is supposed to change based on the component. Neither are usually part of the log message string itself, but rather used for filtering the log messages. If you have a subsystem and a category, you can create a log object with os_log_create(2), but if you don't have either you can just use OS_LOG_DEFAULT

My implementation makes OS_LOG_DEFAULT the default for Apple platforms, but you can specify a subsystem with:

KotlinLoggingConfiguration.appender = OSLogSubsystemAppender("com.example.app")

If you are using the default log, the loggerName is logged as a part of the message, but if you are using a specific subsystem, the loggerName will instead be used as the category. Similar to the subsystem and the category os_log also makes the level a parameter of the message itself (and something you can filter on), instead of as a part of the message text.

I hide the log level and loggerName if you are using a OSLogSubsystemAppender, but this part of the diff is a bit rough, and I'd love your thoughts on which direction it should go. For the time being, it works for my needs :)

os_log also has the ability to disable log levels for performance, and my implementation supports that as well.

Thank you for writing this library, it is working great for me! Feel free to suggest changes, or to just make them yourself – the branch is yours 😊

One small note is that Kotlin multiplatform handles the artifact multiplexing automatically – I am able to just specify the single io.github.microutils:kotlin-logging dependency in my commonMain and It Just Works™

[oshai]

Thanks for the PR! I will look into it as soon as possible.

[oshai]

LGTM!
Do you want to also create a documentation for that (in the wiki?) once merged?

[conradev]

Yes, I can do that! I will also make sure that the README is updated in the commit before merge

[oshai]

Thanks for the PR!