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Swift value types for working with money & currency
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README.md

Build status Coverage Status CocoaPods Compatible Carthage compatible Platform

Money

Money is a Swift framework for iOS, watchOS, tvOS and OS X. It provides types and functionality to represent, calculate and convert money in the 298 ISO currencies.


Usage

The Money framework defines the type Money, which represents money in the device’s current locale. The following code:

import Money

let money: Money = 100
print("I'll give \(money) to charity.”)

will print out

I'll give $100.00 to charity

when the region is set to United States

I'll give £100.00 to charity

when the region is set to United Kingdom

I'll give CN¥100.00 to charity

when the region is set to China

You get the idea. See Localized Formatting for more info.

Money is IntegerLiteralConvertible and FloatLiteralConvertible. Which means values can be initialized using literal Ints and Doubles as shown in these code snippets.

Specific Currency

Under the hood, Money is a typealias for _Money<Currency.Local> where Currency.Local is a specific CurrencyType which represents the currency for the current locale. This means that it is strongly typed to the local currency.

In a similar way, there are 298 foreign currency types supported.

let pounds: GBP = 99.99
let euros: EUR = 149.50

print(“You have \(pounds / 2) and \(euros + 30)”)

You have £ 50.00 and € 179.50

Because the currencies are typed, it means that they cannot be combined together.

let money = pounds + euros

Binary operator '+' cannot be applied to operands of type 'GBP' (aka '_Money<Currency.GBP>') and 'EUR' (aka '_Money<Currency.EUR>')

Of course, Money supports the usual suspects of decimal arithmetic operators, so you can add, subtract, multiply, divide values of the same type, and values with Int and Double with the expected limitations.

Convenience initializers

Money (and its friends) can be initialized with Ints (and friends) andDoubles.

let anIntegerFromSomewhereElse: Int = getAnInteger()
let money = Money(anIntegerFromSomewhereElse)

let aDoubleFromSomewhere: Double = getAnotherDouble()
let pounds = GBP(aDoubleFromSomewhere)

Minor Units

Money can be initialized using the smallest units of currency:

let dollars = USD(minorUnits: 3250)
let yuen = JPY(minorUnits: 3000)

print(“You have \(dollars) and \(yuen)”)

You have $32.50 and ¥3,000

Localized Formatting

When displaying money values, it is important that they be correctly localized for the user. In general, it’s best to use the Money type to always work in currency of the user’s current locale.

When printing a MoneyType value, the .description uses the current locale with .CurrencyStyle number style, in conjunction with NSNumberFormatter. The code snippets throughout this README uses .description whenever the value of money is printed.

However, to specify a different style for the number formatter, use the formattedWithStyle method, like this:

let money: Money = 99.99
print("She has \(money.formattedWithStyle(.CurrencyPluralStyle))")

For an American in Russia, this would print out:

She has 99,99 Russian roubles

Working with Locales

A locale is the codification of associated regional and linguistic attributes. A locale varies by language and region. Each locale has an identifier, which is the concatenation of language, country and modifier codes.

The language code is two or three lowercase letters. English is en, French is fr. There is a long list. Some languages are spoken in more than one country, in which case a country code (two uppercase letters) is appended (with an underscore). For example, English in the United States is en_US, which is the default locale in the iOS Simulator. English in the United Kingdom is en_GB.

Lastly, a locale identifier can be modified, say for example to set the currency code to “USD”, for Portuguese speaking user in Brazil, the locale identifier would be pt_BR@currency=USD.

In total, NSLocale has support for ~ 730 distinct locales. Typically when creating a specific NSLocale it is done with the locale identifier. The NSLocale is like a dictionary with an objectForKey method which returns AnyObject!.

Formatting for specific Locale

I think NSLocale is an amazing class, but it’s very easy to make mistakes, and not that easy to construct. Therefore, to support arbitrary locales, but remove the need for framework consumers to construct locale identifiers, a new Locale type is provided. This is an enum which means that it is type safe, and indexable for code completion in Xcode. Its cases are all the languages which NSLocale supports. For those languages which are spoken in more than one country, there is an associated value of country names of only those counties.

To format money for a specific locale we can use the Locale enum. The following code uses Locale.Chinese(.China) to represent the "zh_CN" locale.

let money: Money = 99.99
print("She has \(money.formattedWithStyle(.CurrencyPluralStyle, forLocale: .Chinese(.China)))")

Now, for our American in Russia, (or any user with a region set to Russia) we get:

She has 99.99俄罗斯卢布

In this case, because our type is Money, and the user’s region is set to Russia, we’re working with RUB currency. But equally, if we need money in a specific currency, we can. Here’s Australian dollars, for a SwissGerman speaking user, in France.

let dollars: AUD = 39.99
print("You’ll need \((dollars / 2.5).formattedWithStyle(.CurrencyPluralStyle, forLocale: .SwissGerman(.France)))")

Regardless of the user’s current locale, this will print out:

You’ll need 16.00 Auschtralischi Dollar

 Pay

On iOS (not watchOS, tvOS or OS X), there is support in Money for using Money with  Pay.

Create a PaymentSummaryItem in lieu of PKPaymentSummaryItem with a suitable MoneyType:

import PassKit

typealias DollarItem = PaymentSummaryItem<USD>

let items = [
    DollarItem(label: "Something fancy.", cost: 9.99),
    DollarItem(label: "Something less fancy.", cost: 5.99)
]

let request = PKPaymentRequest(items: items, sellerName: "Acme, Inc.")

The convenience initializer receives an array of PaymentSummaryItem values and a seller name. It sets the currency code and payment summary items. Following the  Pay guidelines, will append a total summary item using the provided seller name.

PaymentSummaryItem conforms to Hashable and ValueCoding.

Bitcoin

Money has support for Bitcoin types, the popular BTC and the unofficial ISO 4217 currency code XBT.

In November 2015, the Unicode consortium accepted U+20BF as the Bitcoin symbol. However, right now that means it is not available in Foundation. Therefore, currently the Bitcoin currency type(s) use Ƀ, which is also a popular symbol and available already within Unicode.

To work with Bitcoin, use the following:

let bitcoin: BTC = 0.1234_5678
print(“You have \(bitcoin)”)

You have Ƀ0.12345678

Foreign Exchange (FX)

The FX support which was previously part of this framework has been moved into its own, called FX.

Creating custom currencies

If your app has its own currency e.g. ⭐️s or 💎s or even 🐝s, you might want to consider making a type for it.

Lets imagine we’re making Hive.app - where you compete with your friends to see who can get the biggest hive (measured in number of 🐝s).

To create a custom currency, just conform to CurrencyType.

protocol HiveCurrencyType: CustomCurrencyType { }

extension Currency {
    final class Bee: HiveCurrencyType {

        static let code: String = "BEES"
        static let symbol: String = "🐝"
        static let scale: Int  = 0
    }
}

typealias Bees = _Money<Currency.Bee>

Just make sure that your currency code doesn’t clash with a real one - make it more than three characters to be sure.

Now it’s possible to work with your own app’s currency as a proper money type.

let bees: Bees = 10_000
print(“I have \(bees)”)

I have 🐝10,000

And of course if you have an IAP for purchasing in-app currency, then I’m sure a custom FX provider would be handy.

Take a look at the example project, Custom Money, for an example of a custom local FX provider to exchange your 🐝s.

Installation

Money builds as a cross platform (iOS, OS X, watchOS) extension compatible framework. It is compatible with Carthage. It is also available via CocoaPods.

pod ‘Money

At of writing there are some issues with the CocoaDocs generator for pure Swift 2 projects. This means that the project doesn’t have a page/docs in CocoaPods sites, however they are available through Xcode.


Architectural style

Swift is designed to have a focus on safety, enabled primarily through strong typing. This framework fully embraces this ethos and uses generics heavily to achieve this goal.

At the highest level currency is modeled as a protocol, CurrencyType. The protocol defines a few static properties like its symbol, and currency code. Therefore money is represented as a decimal number with a generic currency. Additionally, we make CurrencyType refine the protocol which defines how the decimal number behaves.

Finally, we auto-generate the code which defines all the currencies and money typealiases.

Implementation Details

Cocoa has two type which can perform decimal arithmetic, these are NSDecimalNumber and NSDecimal. NSDecimal is faster, but is trickier to work with, and doesn’t have support for limiting the scale of the numbers (which is pretty important when working with currencies).

DecimalNumberType is a protocol which refines SignedNumberType and defines its own functions, add, subtract etc to support the arithmetic. It is generic over two types, the underlying storage, and the behaviors.

DecimalNumberType.DecimalStorageType exists so that conforming types can utilize either NSDecimalNumber or NSDecimal as their underling storage type.

DecimalNumberBehavior is a protocol which exposes a NSDecimalNumberBehaviors which should be used in calculations. This includes rounding style, scale, and when to throw exceptions.

_Decimal

Which leads us to _Decimal<Behavior: DecimalNumberBehavior> which is a value type implementing DecimalNumberType with an NSDecimalNumber storage type.

There are two public typealiases for convenience.

/// `Decimal` with plain decimal number behavior
public typealias Decimal = _Decimal<DecimalNumberBehavior.Plain>

/// `BankersDecimal` with banking decimal number behavior
public typealias BankersDecimal = _Decimal<DecimalNumberBehavior.Bankers>

This means, that Decimal is more than likely the type to use for most things.

_Money

The _Money<C: CurrencyType> type composes a _Decimal<C>. Its behavior is provided via its generic CurrencyType which refines DecimalNumberBehavior. _Money also conforms to DecimalNumberType which means that it can also be used with the operators.

Why not use NSDecimal?

NSDecimal would be a better storage type for _Decimal, however it doesn’t have the full NSDecimalNumberBehaviors support that NSDecimalNumber enjoys. In particular, specifying the scale is problematic. If anyone has any smart ideas, please get in touch. I’ve added an equivalent extension on NSDecimal as for NSDecimalNumber.

ValueCoding

Both _Decimal, _Money and FXTransaction all conform to ValueCoding which means they can be encoded and stored inside archives.

Author

Daniel Thorpe @danthorpe.

Feel free to get in contact if you have questions, queries, suggestions, or need help. Especially get in contact via an Issue here or on Twitter if you want to add support for another FX service provider.

I wrote an introductory blog post about money here.

License

Money is available under the MIT license. See the LICENSE file for more info.

Disclaimer

Usage of this framework prevents the author, Daniel Thorpe, from being held liable for any losses incurred by the user through their use of the framework.