Force patterns in textfields - swift

Question

(Swift)
So the problem is:
My app (which is a kind of calculator) is crashing when the user puts in the textfield things that can't be calculated.
For example, if he types " -4-.", the app won't be able to do the math.
So, a pattern must be followed.
The following characters are allowed: 1234567890.-
The minus sign can only be typed when it is the first character in the textfield and cannot be typed again.
The point can only be typed after a number, and cannot be typed again.

Answer 1

Well you would have to determine:
When the user clicks on a number/digit/character, you would have to do a:
//Goes at top of one of your classes
var decimalCount:Int = 0
//At location of tap for character
if(decimalCount < 1) {
textField.text += "."
decimalCount += 1
}
This ideology could be applied to "-" as well.

Answer 2

Some how i have understood your question. According to my assumption our task is to validate the input for proper math function.Ok here we go.
First of all declare a bool variable at top of your class
var isNonNumericCharactersAllowes = true
At first we need to make our textfield to respond according to user input.So add delegate to text field and add the following delegate method.
func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {
//1. To make sure that this is applicable to only particular textfield add tag.
if textField.tag == 1 {
let char = string.cStringUsingEncoding(NSUTF8StringEncoding)!
let isBackSpace = strcmp(char, "\\b")
//Helps to react only while typing and not while clearing text
if (isBackSpace != -92) {
let numbersOnly = NSCharacterSet(charactersInString: "1234567890")
let characterSetFromTextField = NSCharacterSet(charactersInString: string)
let Validate:Bool = numbersOnly .isSupersetOfSet(characterSetFromTextField)
if !Validate {
if isNonNumericCharactersAllowes {
isNonNumericCharactersAllowes = false
return true
}
return false
}
isNonNumericCharactersAllowes = true
}
}
return true
}
The above method stops unusual text entry's such as 0..012,--4,4++ etc..
Now while hitting calculate button we need to some validation.Add the following code in IBAction.
#IBAction func calculate(sender: AnyObject) {
let textContent:String!
textContent = textFieldTwo.text
var characterContainer = textContent.characters.map { String($0) }
let numbersOnly = NSCharacterSet(charactersInString: "1234567890")
let lastObjectOfString = NSCharacterSet(charactersInString: characterContainer.last!)
let Validate:Bool = numbersOnly .isSupersetOfSet(lastObjectOfString)
if !Validate {
characterContainer .removeLast()
textFieldTwo.text = characterContainer .joinWithSeparator("")
}
}
This above validation helps in removing things like 30+20+,4+4+, etc.. i.e removes unused operators at the end.