Force NSTextField to Only Accept Decimal (#.#) Numbers and Periods - swift3

Question

I'm trying to make it so an NSTextField will only accept numbers and periods like 12.4 and 3.6 in a Mac app.
I feel like I'm getting pretty close after reviewing other SO questions, but I can't quite get it. The below code works except that it won't allow . characters. It returns true and doesn't beep at me when I type a . but it won't let the character appear in the field.
class decimalFormatter: NumberFormatter {
override func isPartialStringValid(_ partialString: String, newEditingString newString: AutoreleasingUnsafeMutablePointer<NSString?>?, errorDescription error: AutoreleasingUnsafeMutablePointer<NSString?>?) -> Bool {
//Allows the text to be deleted
if partialString.isEmpty {
return true
}
//Check for #.# numbers
let charSet = NSCharacterSet(charactersIn: "1234567890.").inverted
if partialString.rangeOfCharacter(from: charSet) != nil{
NSBeep()
return false
}else{
return true
}
}
}
Any idea what I'm doing wrong?

Answer 1

I found a simpler way to do it. Inside controlTextDidChange I just did this:
let charSet = NSCharacterSet(charactersIn: "1234567890.").inverted
let chars = fieldDuration.stringValue.components(separatedBy: charSet)
fieldDuration.stringValue = chars.joined()
It works great!

Answer 2

#Clifton Labrum solution is really great but it doesn't reduce the field to Decimal (#.#), you can stil put some inputs as 1.2.4 which would lead to an error when trying tu cast it to Float.
Here is a draft of an extension that worked fine for me ( In Swift 4 )
public override func controlTextDidChange(_ obj: Notification) {
if let textfield = obj.object as? NSTextField,
textfield == self.quantityTextField {
var stringValue = textfield.stringValue
// First step : Only '1234567890.' - #Clifton Labrum solution
let charSet = NSCharacterSet(charactersIn: "1234567890.").inverted
let chars = stringValue.components(separatedBy: charSet)
stringValue = chars.joined()
// Second step : only one '.'
let comma = NSCharacterSet(charactersIn: ".")
let chuncks = stringValue.components(separatedBy: comma as CharacterSet)
switch chuncks.count {
case 0:
stringValue = ""
case 1:
stringValue = "\(chuncks[0])"
default:
stringValue = "\(chuncks[0]).\(chuncks[1])"
}
// replace string
textfield.stringValue = stringValue
}
}
This prevent multiple occurences of . , even if I know that's not the best algorithmic way to do this. For instance 1.2.4 becomes 1.2 when pasted, and by keyboard you can't add another .