SingletonHolder.kt

open class SingletonHolder<out T, in A>(private val constructor: (A) -> T) {

    @Volatile
    private var instance: T? = null

    fun getInstance(arg: A): T {
        return when {
            instance != null -> instance!!
            else -> synchronized(this) {
                if (instance == null) instance = constructor(arg)
                instance!!
            }
        }
    }
}

/*usage :
* class MyManager private constructor(context: Context) {

    fun doSomething() {
        ...
    }

    companion object : SingletonHolder<MyManager, Context>(::MyManager)
}
*
*
* Finally:
*
* MyManager.getInstance(context).doSomething()
*/