Hello, How can we create a Singleton in Kotlin which takes some parameters? The reason I am asking t...

# multiplatform

[Amritansh]

Hello, How can we create a Singleton in Kotlin which takes some parameters? The reason I am asking this in this channel is because in kotlin native we can not have var in an object class unless you annotate it with ThreadLocal. I am not sure how to save value in a member variables of a singleton object class.

[Tijl]

AtomicReference and annotate with @SharedImmutable

[Amritansh]

I can not access AtomicReference in CommonMain

[Amritansh]

I am working on a Kotlin Multiplatform Mobile Project

[Tijl]

you can use expect/actual to access it quite easily

[Tijl]

you can also use the stately library

[Tijl]

one way to do it without a library is to expect a var, and on Native implement get and set and redirect it to an AtomicReference

[Amritansh]

ah, interesting. Let me try that

[Benjamin Charais]

Another approach:

class MyVarSingleton private constructor(
    val title: String,
    val description: String
) {

    @Suppress("VARIABLE_IN_SINGLETON_WITHOUT_THREAD_LOCAL")
    companion object {
        private var instance: MyVarSingleton? = null
        private val mutex = Mutex(false)

        suspend fun getInstance(): MyVarSingleton {
            mutex.withLock {
                return instance ?: throw AssertionError("Must call Init before getting instance")
            }
        }

        suspend fun setInstance(title: String, description: String) {
            if (instance != null) throw AssertionError("Can not re-initialize singleton instance")
            mutex.withLock {
                instance = MyVarSingleton(title, description)
            }
        }

    }
}

[Benjamin Charais]

Thread safe singleton with a mutex and can be initialized with arguments

[Benjamin Charais]

I would not suggest this as a good or best practice to be fair. I would instead suggest using environment variables, const definitions, or to have concurrent patterns such as an actor to produce and expose these values.

[Amritansh]

thanks for replying. I went down the route of using expect/ actual and storing the instance in AtomicReference and it worked.