UITextField’s numerical pad: dot instead of comma for float values

Swift 4

extension String {
    static let numberFormatter = NumberFormatter()
    var doubleValue: Double {
        String.numberFormatter.decimalSeparator = "."
        if let result =  String.numberFormatter.number(from: self) {
            return result.doubleValue
        } else {
            String.numberFormatter.decimalSeparator = ","
            if let result = String.numberFormatter.number(from: self) {
                return result.doubleValue
            }
        }
        return 0
    }
}

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"2.25".doubleValue // 2.25
"2,25".doubleValue // 2.25

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Localized approach using NumberFormatter:

extension NumberFormatter {
    static let shared = NumberFormatter()
}
extension StringProtocol {
    var doubleValue: Double? {
        return NumberFormatter.shared.number(from: String(self))?.doubleValue
    }
}

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Playground testing

// User device's default settings for current locale (en_US)
NumberFormatter.shared.locale            // en_US (current)
NumberFormatter.shared.numberStyle       // none
NumberFormatter.shared.decimalSeparator  // "."
"2.7".doubleValue  // 2.7
"2,7".doubleValue  // nil
"$2.70".doubleValue  // nil

NumberFormatter.shared.numberStyle  = .currency
"2.7".doubleValue  // nil
"2,7".doubleValue  // nil
"$2.70".doubleValue  // 2.7


NumberFormatter.shared.locale  = Locale(identifier: "pt_BR") // pt_BR (fixed)
"2.7".doubleValue     // nil
"2,7".doubleValue     // nil
"R$2,70".doubleValue  // 2.7

NumberFormatter.shared.numberStyle = .none
"2.7".doubleValue      // nil
"2,7".doubleValue      // 2.7
"R$2,70".doubleValue   // nil