Simple? Bronze Solution

iOS Programming: The Big Nerd Ranch Guide (6th Edi 4. Text Input and Delegation
1

I have tried to make it as simple as I possibly could and have treated them as separate events.

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    let allowedCharacterSet = CharacterSet(charactersIn: "0123456789.")
        
    let existingTextHasDecimalSeparator = textField.text?.range(of: ".")
    let replacementTextHasDecimalSeparator = string.range(of: ".")
        
    for i in string.unicodeScalars {
        if allowedCharacterSet.contains(i) == false {
            return false
        }
    }
        
    if existingTextHasDecimalSeparator != nil, replacementTextHasDecimalSeparator != nil {
        return false
    } else {
        return true
    }
}

Hope this helps you guys!

2

Work properly, almost I obtain the same answer than you. Great job!!

3

Nice solution,

suggestion, change this:
allowedCharacterSet.contains(i) == false
by
!allowedCharacterSet.contains(i).