Yet Another Bronze Solution

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

This solution prevents non-decimal character input while allowing the user to delete previously entered characters:

let validCharacters = CharacterSet(charactersIn: ".0123456789")

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    let replacementTextIsBackspace = (string.characters.count == 0 ? true : false)
    
    if replacementTextIsBackspace {
        return true
    } else if string.rangeOfCharacter(from: validCharacters) != nil {
        let existingTextHasDecimalSeparator = textField.text?.range(of: ".")
        let replacementTextHasDecimalSeparator = string.range(of: ".")
        
        if existingTextHasDecimalSeparator != nil,
            replacementTextHasDecimalSeparator != nil {
            return false
        } else {
            return true
        }
    } else {
        return false
    }
}