SingletonHolder.kt

// Those are two tasks. 

// TASK 1 
open class SingletonHolder<out T, in A>(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
            }
        }
    }

}


// Used Like This 

@Database(entities = [User::class, Product::class], version = 1, exportSchema = false)
abstract class ECommerceDatabase : RoomDatabase()
{
    companion object : SingletonHolder<ECommerceDatabase, Context>({
        Room.databaseBuilder(
            it.applicationContext,
            ECommerceDatabase::class.java,
            "ecommerce"
        ).build()
    })
}


// ----------------- 

// second task 
// Using higher order function find the solution : 
// Describe what this function is doing 

fun <T> Activity.startActivityFinishThis(target: Class<T>) {
    val intent = Intent(this, target)
    intent.flags = Intent.FLAG_ACTIVITY_NEW_TASK or Intent.FLAG_ACTIVITY_CLEAR_TASK
    // I want to add some custom behaviour on the intent in each activity I use this in.
    startActivity(intent)
}