How to setup and fetched Strings Array in CoreData? - ios

Question

I have a method which need an Array of Strings for further calculations. For now I set up this array through UserDefaults, because it should store small (3-characters string) amount of data:
func setRow(for currency: Currency, in currencies: [Currency]) {
// initialise array from UserDefaults
var currencyRowsArray = UserDefaults.standard.stringArray(forKey: "currencyRowsArray") ?? [String]()
// calculations with the array (add or remove string object to\from it
if currency.isForConverter {
currencyRowsArray.append(currency.shortName!)
} else {
guard let row = currencyRowsArray.firstIndex(of: currency.shortName!) else { return }
currencyRowsArray.remove(at: row)
currency.rowForConverter = 0
}
for (row, object) in currencyRowsArray.enumerated() {
for currency in currencies {
if object == currency.shortName {
currency.rowForConverter = Int32(row)
}
}
}
// save array back to UserDefaults after calculations
UserDefaults.standard.set(currencyRowsArray, forKey: "currencyRowsArray")
}
But since I have a CoreData implemented also I decided to store it in CoreData rather than UserDefaults. Here is how I tried to implement that:
1.I have entity Currency, so I created an attribute currencyRowsArray: Type - Transformable, Custom Class - [String], Transformer - NSSecureUnarchiveFromData, Optional - YES (because I don't need it to initialize every time a created a Currency object with other attributes)
2.In the setRow method:
let request: NSFetchRequest<Currency> = Currency.fetchRequest()
request.predicate = NSPredicate(format: "currencyRowsArray IN %#", currencyRowsArray)
3.Initialize the empty array to fill it later with fetched Data:
currencyRowsArray = [String]()
4.Fetch the array:
do {
let fetchResult = try context.fetch(request)
currencyRowsArray = fetchedResult as! [String]
} catch {
print(error)
}
5.After calculations save the changes:
do {
try context.save()
} catch {
print(error)
}
Full changed code of the setRow method:
func setRow(for currency: Currency, in currencies: [Currency]) {
var currencyRowsArray = [String]()
let request: NSFetchRequest<Currency> = Currency.fetchRequest()
request.predicate = NSPredicate(format: "currencyRowsArray IN %#", currencyRowsArray)
do {
let fetchResult = try context.fetch(request)
currencyRowsArray = fetchResult as! [String]
} catch {
print(error)
}
if currency.isForConverter {
currencyRowsArray.append(currency.shortName!)
} else {
guard let row = currencyRowsArray.firstIndex(of: currency.shortName!) else { return }
currencyRowsArray.remove(at: row)
currency.rowForConverter = 0
}
for (row, object) in currencyRowsArray.enumerated() {
for currency in currencies {
if object == currency.shortName {
currency.rowForConverter = Int32(row)
}
}
}
do {
try context.save()
} catch {
print(error)
}
}
But every time the array loads as empty and when I make change, array doesn't add or remove items in it. What I did wrong?