Textfield Validation using CharacterSet

I have written below code to validate text input in textfield.

else if (textField == txtField_Password)
    {
        let charSet = CharacterSet(charactersIn: "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789@$&*!")
        let charLength = (txtField_Password.text!.count) + (string.count) - range.length

        for i in 0..<string.count
        {
            let c = (string as NSString).character(at: i)
            if (!((charSet as NSCharacterSet).characterIsMember(c)))
            {
                return false
            }
        }
        return (charLength > 20) ? false : true
    }

Can anyone help me to convert character(at:) and characterIsMember() part to its swift equivalent in the above code.

You can simplify the logic just by checking the range of the inverted character set. If the string contains only allowed characters the function returns nil.

else if textField == txtField_Password {
    let charLength = txtField_Password.text!.utf8.count + string.utf8.count - range.length
    let charSet = CharacterSet(charactersIn: "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789@$&*!")
    return string.rangeOfCharacter(from: charSet.inverted) == nil && charLength < 21
}