How to add custom init for String extension?

Question

How can I add custom init method for String extension?

extension String {
    init(_ amount: Double, decimalPlaces: UInt) {
        self.init()
        let decimalFormat = "%0.\(String(decimalPlaces))f"
        let currencyAmount = String(format: decimalFormat, amount)
        let currencySign = NSLocalizedString("Defaults.CurrencySign", comment: "currency sign")
        let formattedString = "\(currencySign)\(currencyAmount)"
        // How to set self to `formattedString` ?
    }
}

As result I want to see something like this:

let price = Double(155.15)
let formattedPrice = String(price, decimalPlaces: 2) // formattedPrice = "$155.15"

UPDATED: Final solution

extension String {
    init?(currencyAmount: Double) {
        let formatter = NumberFormatter()
        formatter.numberStyle = .currency
        formatter.locale = Locale(identifier: NSLocalizedString("Defaults.LocaleCurrencyFormat", comment: "currency sign")) // Defaults.LocaleCurrencyFormat equal "es_US" for US
        let formattedAmount = formatter.string(from: NSNumber(value: currencyAmount)) ?? ""
        self.init(formattedAmount)
    }
}

"*How to set self to `formattedString`*?" How do you normally perform an assignment? (*hint hint*) — Hamish, Feb 28 '17 at 21:34
@Hamish Funny enough, this didn't at all occur to me the first time I needed it. From my Java background, I was forced into a very particular, misleading mindset. — Alexander, Feb 28 '17 at 21:36
@VasiliiMuravev Not all locale print the currency sign before the currency magnitude. — Alexander, Feb 28 '17 at 21:44
@Alexander, yes, but currently app used only English, so it's ok. In future it'll be easy to fix. — Vasilii Muravev, Feb 28 '17 at 21:48
@VasiliiMuravev I figured you were aiming for correct internationalization, given that you used `NSLocalizedString` to get the correct currency sign. — Alexander, Feb 28 '17 at 21:53
@VasiliiMuravev you don't need `let _ = currencySignLast,` in your last conditional. `if currencySignLast == true {` — Leo Dabus, Feb 28 '17 at 22:04
Also take a look a this post: http://stackoverflow.com/questions/24960621/struggling-with-nsnumberformatter-in-swift-for-currency — BallpointBen, Feb 28 '17 at 22:05
oneliner `self = currencySignLast == true ? "\(currencyAmount)\(currencySign)" : "\(currencySign)\(currencyAmount)"` — Leo Dabus, Feb 28 '17 at 22:07
@VasiliiMuravev Is there any particular reason for currencySignLast being optional? Why don't you just assign a default value to it , `currencySignLast: Bool = false)` ? I think would be much better to use NumberFormatter so it would be naturally localized — Leo Dabus, Feb 28 '17 at 22:13
@LeoDabus Better: `self = currencySignLast ? currencyAmount+currencySign : currencySign+currencyAmount` — Alexander, Feb 28 '17 at 22:19
@VasiliiMuravev Not all locales use `.` for the decimal separator. — Alexander, Feb 28 '17 at 22:20
@VasiliiMuravev you should use `NSNumberFormatter `. See http://stackoverflow.com/a/5039433/3141234 — Alexander, Feb 28 '17 at 22:21
@LeoDabus yea, actually I already rewrite function with @BallpointBen link http://stackoverflow.com/questions/24960621/struggling-with-nsnumberformatter-in-swift-for-currency Most important that now I know how to rewrite `init` methods. — Vasilii Muravev, Feb 28 '17 at 22:21
@Alexander currencySignLast being optional (as it was before edit) `== true` is required — Leo Dabus, Feb 28 '17 at 22:30
BallpointBen, LeoDabus, Alexander, thanks, fixed with formatter :) — Vasilii Muravev, Feb 28 '17 at 22:40

Alexander · Accepted Answer · 2019-08-08T14:27:00.940

Edit: Don't format currencies yourself.

However you might think currencies are formatted, you're almost certainly wrong. Just compare:

US/Canada: $3,490,000.89
French Canadian: 3 490 000,89 $
France: 3 490 000,89 €
Germany: 3.490.000,89 €

Instead, use NumberFormatter with numberStyle set to .currency, with a specified locale.

let currencyFormatter = NumberFormatter()
currencyFormatter.usesGroupingSeparator = true
currencyFormatter.numberStyle = .currency
currencyFormatter.locale = Locale.current
let priceString = currencyFormatter.string(from: 9999.99)!
print(priceString) // Displays $9,999.99 in the US locale

Original answer:

The initializers (and mutating methods) of value types can simply assign directly to self:

import Foundation

extension String {
    init(_ amount: Double, decimalPlaces: UInt) {
        let currencyAmount = String(format: "%\(decimalPlaces).f", amount)
        let currencySign = NSLocalizedString("Defaults.CurrencySign", comment: "currency sign")
        self = "\(currencySign)\(currencyAmount)"
    }
}

let price = Double(155.15)
let formattedPrice = String(price, decimalPlaces: 2) // formattedPrice = "$155.15"

Using a String initializer inside a String interpolation is redundant — Leo Dabus, Feb 28 '17 at 21:40
@LeoDabus I just copied the original code, didn't notice. Fixed. — Alexander, Feb 28 '17 at 21:43

How to add custom init for String extension?

1 Answers1

Edit: Don't format currencies yourself.

Original answer:

Linked