Using room as singleton in kotlin

Question

I\'m trying to use Room as singleton so I didn\'t have to invoke Room.databaseBuilder() -which is expensive- more than once.

@Database(entities = ar         


        

Answer 1

I found a solution so here is the answer for future me and any one who might have the same problem.

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After some research, I found that I have two options.

1. use Double-checked locking
2. use Initialization-on-demand holder idiom

so I considered implementing one of them, but this doesn't feel right in kotlin too much boilerplate code :p

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so after more research, I stumbled upon this great article which provides an excellent solution which uses Double-checked locking but in an eligant way.

my code becomes like this:

companion object : SingletonHolder({
       Room.databaseBuilder(it.applicationContext, AppDatabase::class.java, "train.db").build()
})

from the article:

A reusable Kotlin implementation:

We can encapsulate the logic to lazily create and initialize a singleton with argument inside a SingletonHolder class. In order to make that logic thread-safe, we need to implement a synchronized algorithm and the most efficient one — which is also the hardest to get right — is the double-checked locking algorithm.

open class SingletonHolder(creator: (A) -> T) {
    private var creator: ((A) -> T)? = creator
    @Volatile private var instance: T? = null

    fun getInstance(arg: A): T {
        val i = instance
        if (i != null) {
            return i
        }

        return synchronized(this) {
            val i2 = instance
            if (i2 != null) {
                i2
            } else {
                val created = creator!!(arg)
                instance = created
                creator = null
                created
            }
        }
    }
}

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Extra: if you want Singleton with two arguments

open class SingletonHolder2(creator: (A, B) -> T) {
    private var creator: ((A, B) -> T)? = creator
    @Volatile private var instance: T? = null

    fun getInstance(arg0: A, arg1: B): T {
        val i = instance
        if (i != null) return i

        return synchronized(this) {
            val i2 = instance
            if (i2 != null) {
                i2
            } else {
                val created = creator!!(arg0, arg1)
                instance = created
                creator = null
                created
            }
        }
    }
}