SingletonHolder Kotlin Andorid

SingletonHolder.kt

package com.kotlin.utils.singleton

/**
 * What if you need pass some arguments for initialization jus like parameterized constructor?
 * Since we can't use constructor with the object keyword. So, we need to find some other way of
 * doing the same.
 *
 * We can archive this by using a SingletonHolder class.
 * Also, to make it thread-safe, we need to have a way of synchronized a double check locking principle.
 */
open class SingletonHolder<out T : Any, in A>(creator: (A) -> T) {
    private var creator: ((A) -> T)? = creator

    @Volatile
    private var instance: T? = null

    protected fun getInstanceInternal(arg: A): T {
        val checkInstance = instance
        if (checkInstance != null) return checkInstance
        return synchronized(this) {
            val checkInstanceAgain = instance
            if (checkInstanceAgain != null) checkInstanceAgain
            else {
                val created = creator!!(arg)
                instance = created
                creator = null
                created
            }
        }
    }
}

/**
 * If you need to pass only ONE argument to the constructor of the singleton class.
 * Make companion object extended from [SingleArgSingletonHolder] for best match.
 * Ex:
    class AppRepository private constructor(private val db: Database) {
        companion object : SingleArgSingletonHolder<AppRepository, Database>(::AppRepository)
    }
 * Uses:
    val appRepository =  AppRepository.getInstance(db)
 */
open class SingleArgSingletonHolder<out T : Any, in A>(creator: (A) -> T) :
    SingletonHolder<T, A>(creator) {
    fun getInstance(arg: A): T = getInstanceInternal(arg)
}

/**
 * If you need to pass TWO arguments to the constructor of the singleton class.
 * Extended from [PairArgsSingletonHolder] for best match.
 * Ex:
    class AppRepository private constructor(private val db: Database, private val apiService: ApiService) {
        companion object : PairArgsSingletonHolder<AppRepository, Database, ApiService>(::AppRepository)
    }
 *
 * Uses:
    val appRepository =  AppRepository.getInstance(db, apiService)
 */
open class PairArgsSingletonHolder<out T : Any, in A, in B>(creator: (A, B) -> T) :
    SingletonHolder<T, Pair<A, B>>(creator = { (a, b) -> creator(a, b) }) {

    fun getInstance(arg1: A, arg2: B) = getInstanceInternal(Pair(arg1, arg2))
}


/**
 * If you need to pass THREE arguments to the constructor of the singleton class.
 * Extended from [TripleArgsSingletonHolder] for the best match.
 *
 * Ex:
    class AppRepository private constructor(
       private val db: Database,
       private val apiService: ApiService,
       private val storage : Storage
    ) {
       companion object : TripleArgsSingletonHolder<AppRepository, Database, ApiService, Storage>(::AppRepository)
    }
 *
 * Uses:
    val appRepository =  AppRepository.getInstance(db, apiService, storage)
 */
open class TripleArgsSingletonHolder<out T : Any, in A, in B, in C>(creator: (A, B, C) -> T) :
    SingletonHolder<T, Triple<A, B, C>>(creator = { (a, b, c) -> creator(a, b, c) }) {

    fun getInstance(arg1: A, arg2: B, arg3: C) = getInstanceInternal(Triple(arg1, arg2, arg3))
}