Cached NumberFormatter

FormattedAsCurrency.swift

/// NumberFormatter is quite expensive to run each time, 
/// so lets make a cache to hold it
///
struct FormatterCache {
    static let currency: NumberFormatter = {
        let formatter = NumberFormatter()
        formatter.locale = Locale(identifier: "en_US")
        formatter.numberStyle = .currency
        formatter.minimumFractionDigits = 0
        formatter.maximumFractionDigits = 0
        formatter.allowsFloats = false
        formatter.roundingMode = .ceiling
        formatter.alwaysShowsDecimalSeparator = false
        return formatter
    }()
}

// Add extension on double 
extension Double {
    var formattedAsCurrency: String {
        return FormatterCache.currency.string(from: NSNumber(value: self)) ?? "$???"
    }
}

// Usage
let baseSalary = 95_000
print (baseSalary.formattedAsCurrency)

// -- possible alternative to handle different currencies --

struct AltFormatterCache {
    private static var formatters: [String: NumberFormatter] = [:]

    static func currency(for localeID: String) -> NumberFormatter {
        if let cached = formatters[localeID] { return cached }

        let formatter = NumberFormatter()
        formatter.locale = Locale(identifier: localeID)
        formatter.numberStyle = .currency
        formatter.minimumFractionDigits = 0
        formatter.maximumFractionDigits = 0
        formatters[localeID] = formatter
        return formatter
    }
}

let locale = "en_GB"  // this could be from user profile, region settings, etc.
let salary = 95000.0 
let formatted = AltFormatterCache.currency(for: locale).string(from: NSNumber(value: salary)) ?? "?"
print (formatted)