Companion object with extension function in kotlin?

Question

I would like to have extension function and use logger from kotlin-logging and have constants inside companion object.

My function:

fun String.toFoo(): Foo {

    logger.debug { "Mapping [$this] to Foo" }

    if(MY_CONST.equals(this) { 
        ...
}

Question is where I should put val logger = KotlinLogging.logger {} and MY_CONST since I cannot use companion object with an extension function?

cmaynard · Accepted Answer · 2019-03-08T13:32:06.840

1

If you just want you logger to be a singleton you can make an object that contains and instance of the logger and reach it from there.

Object LoggerSingleton( val logger = KotlinLogging.logger{})

Then in your extension function

fun String.toFoo(): Foo {

LoggerSingleton.logger.debug { "Mapping [$this] to Foo" }

if(MY_CONST.equals(this) { 
 }

Since an Object in Kotlin is guaranteed to have only one instance you won't have a different logger for each use of toFoo.

EDIT To keep the desired class name Use this signature Like so:

Object StringLoggerSingleton( val logger = KotlinLogging.logger("String"))

edited Mar 08 '19 at 13:32

answered Jan 29 '19 at 14:29

cmaynard

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and where to store constants that would be used in extension function? – pixel Jan 29 '19 at 14:54
If they're really constants then store them inside the function, if they come from a configuration just load them from the config file into a singleton and reference that singleton using the same pattern as the logger. – cmaynard Jan 29 '19 at 15:14
Argh, I've just found out that 'kotlin-logging' takes name of class from the context where it's initialized. So making a singleton would hide real class name :( – pixel Jan 30 '19 at 06:45
1

Just give it the class name instead of using the lambda signature https://github.com/MicroUtils/kotlin-logging/blob/master/kotlin-logging-jvm/src/main/kotlin/mu/KLogging.kt#L46 – cmaynard Jan 30 '19 at 12:04

Roland · Answer 2 · 2019-01-30T10:34:57.843

I do not know what you want to accomplish with your logger, but I show you what I did already ;-)

Usually I put extension functions in its own file named similar to what the function is actually extending (e.g. either StringExtensionFunction or if is more related to its purpose and maybe only available if certain dependencies are available, I also did something like, e.g. JsoupExtensionFunctions (where there was a String.toJsoupHtml(), File.toJsoupXml(), etc.)).

If I then need constants I just place them within that file, e.g. by just writing something like:

private const val MY_CONST = "my_const_value"

No surrounding class, no surrounding object.

Regarding the logger... as loggers are usually tied to a certain name/class, I usually put a logger inside every (important) class or associate some logger to specific names... So I am not completely sure what your intent is here... If it's ok for you that the logger is returning the container of your extension function (maybe StringExtensionFunction.kt), then you can also put a logger-val inside that file similar to what I showed with MY_CONST.

If your intention was rather to reuse the callers logger, that might not work so easily... (the easiest would then probably be to pass it to the function, but usually you do not want that)... and other mechanisms may not really be worth it ;-)

Companion object with extension function in kotlin?

2 Answers2