Making NSDecimalNumber Codable

Question

Is it possible to extend NSDecimalNumber to conform Encodable & Decodable protocols?

What have you tried so far? What are your requirements? What are you trying to encode / decode? — Lorenzo B, Sep 19 '17 at 07:45
@LorenzoB I checked Apple's documentation https://developer.apple.com/documentation/foundation/archives_and_serialization/encoding_and_decoding_custom_types for encoding and decoding custom types. I'm trying to parse responses from server which I'd loose precision if I used Double. — Arda Keskiner, Sep 19 '17 at 07:54

score 13 · Accepted Answer · answered Sep 20 '17 at 07:59

It is not possible to extend NSDecimalNumber to conform to Encodable & Decodable protocols. Jordan Rose explains it in the following swift evolution email thread.

If you need NSDecimalValue type in your API you can build computed property around Decimal.

struct YourType: Codable {
    var decimalNumber: NSDecimalNumber {
        get { return NSDecimalNumber(decimal: decimalValue) }
        set { decimalValue = newValue.decimalValue }
    }
    private var decimalValue: Decimal
}

Btw. If you are using NSNumberFormatter for parsing, beware of a known bug that causes precision loss in some cases.

let f = NumberFormatter()
f.generatesDecimalNumbers = true
f.locale = Locale(identifier: "en_US_POSIX")
let z = f.number(from: "8.3")!
// z.decimalValue._exponent is not -1
// z.decimalValue._mantissa is not (83, 0, 0, 0, 0, 0, 0, 0)

Parse strings this way instead:

NSDecimalNumber(string: "8.3", locale: Locale(identifier: "en_US_POSIX"))

score 3 · Answer 2 · answered Sep 19 '17 at 08:07

In swift you should use Decimal type. This type confirms to protocols Encodable & Decodable from the box.

If you have NSDecimalNumber type in your code it's easy to cast it to Decimal

let objcDecimal = NSDecimalNumber(decimal: 10)
let swiftDecimal = (objcDecimal as Decimal)

score 0 · Answer 3 · answered Sep 02 '21 at 12:38

With Swift 5.1 you can use property wrappers to avoid the boilerplate of writing a custom init(from decoder: Decoder) / encode(to encoder: Encoder).

@propertyWrapper
struct NumberString {
    private let value: String
    var wrappedValue: NSDecimalNumber

    init(wrappedValue: NSDecimalNumber) {
        self.wrappedValue = wrappedValue
        value = wrappedValue.stringValue
    }
}

extension NumberString: Decodable {
    init(from decoder: Decoder) throws {
        value = try String(from: decoder)
        wrappedValue = NSDecimalNumber(string: value)
    }
}

extension NumberString: Encodable {
    func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(wrappedValue.stringValue)
    }
}

extension NumberString: Equatable {}

Usage:

struct Foo: Codable {
    @NumberString var value: NSDecimalNumber
}

Making NSDecimalNumber Codable

3 Answers3

Linked