Swift: Decode imprecise decimal correctly

问题

I need to decode (Decodable protocol) an imprecise decimal value correctly, from this question I understand how to properly handle the Decimal instantiation, but how can I do this when decoding?

If trying to init any number as a String

if let value = try! container.decode(String.self, forKey: .d) {
    self.taxAmount = Decimal(string: value)
}

I get Fatal Error: "Expected to decode String but found a number instead."

And if try to init 130.43 as a Decimal

if let value = try! container.decode(Decimal.self, forKey: .d) {
    //value.description is 130.43000000000002048
    self.d = Decimal(string: value.description)
    //making subtotal to be also 130.43000000000002048 and not 130.43
}

Is there any way to use either of this constructors when decoding?

NSDecimalNumber(string: "1.66")
NSDecimalNumber(value: 166).dividing(by: 100)
Decimal(166)/Decimal(100)
Decimal(sign: .plus, exponent: -2, significand: 166)

Here is a simplified version of the JSON I receive from the external service:

{
   "priceAfterTax": 150.00,
   "priceBeforeTax": 130.43,
   "tax": 15.00,
   "taxAmount": 19.57
}

Note: I can't change what is being received to be decoded, I'm stuck working with decimal numbers.

回答1:

You can implement your own decoding method and format your double with maximum two fraction digits and use the string to initialize your decimal properties:

struct Root: Codable {
    let priceAfterTax, priceBeforeTax, tax, taxAmount: Decimal
}

extension Root {
    public init(from decoder: Decoder) throws {
        var container = try decoder.container(keyedBy: CodingKeys.self)

        let priceAfterTaxDouble = try container.decode(Double.self, forKey: .priceAfterTax)
        priceAfterTax = Decimal(string: Formatter.decimal.string(for: priceAfterTaxDouble)!) ?? 0

        let priceBeforeTaxDouble = try container.decode(Double.self, forKey: .priceBeforeTax)
        priceBeforeTax = Decimal(string: Formatter.decimal.string(for: priceBeforeTaxDouble)!) ?? 0

        let taxDouble = try container.decode(Double.self, forKey: .tax)
        tax = Decimal(string: Formatter.decimal.string(for: taxDouble)!) ?? 0

        let taxAmountDouble = try container.decode(Double.self, forKey: .taxAmount)
        taxAmount = Decimal(string: Formatter.decimal.string(for: taxAmountDouble)!) ?? 0
    }
}

extension Formatter {
    static let decimal: NumberFormatter = {
        let formatter = NumberFormatter()
        formatter.numberStyle = .decimal
        formatter.decimalSeparator = "."
        formatter.maximumFractionDigits = 2
        return formatter
    }()
}

let data = Data("""
{
"priceAfterTax": 150.00,
"priceBeforeTax": 130.43,
"tax": 15.00,
"taxAmount": 19.57
}
""".utf8)

let decodedObj = try! JSONDecoder().decode(Root.self, from: data)
decodedObj.priceAfterTax   // 150.00
decodedObj.priceBeforeTax  // 130.43
decodedObj.tax             // 15.00
decodedObj.taxAmount       // 19.57

来源：https://stackoverflow.com/questions/55131400/swift-decode-imprecise-decimal-correctly

标签

swift

decodable

nsdecimalnumber