Singleton.kt

open class Singleton<in T, out R>(private val creator: (T) -> R) {
    @Volatile private var instance: R? = null

    fun of(context: T): R {
        return instance ?: synchronized(this) {
            val i = instance
            if (i != null) {
                i
            } else {
                val i2 = creator(context)
                instance = i2
                i2
            }
        }
    }
}