func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {
        
        let futureString: NSMutableString = NSMutableString(string: textField.text!)
        futureString.insertString(string, atIndex: range.location)

        var flag = 0;
        
        let limited = 2;//小數(shù)點后需要限制的個數(shù)
        
        if !futureString.isEqualToString("") {
            for i in (futureString.length - 1).stride(through: 0, by: -1) {
                let char = Character(UnicodeScalar(futureString.characterAtIndex(i)))
                if char == "." {
                    if flag > limited {
                        return false
                    }
                    break
                }
                flag += 1
            }
        }

        
        return true
    }