How to Test Character Input to UITextField as the User Enters Characters and Prevent Invalid Characters

Question

First, I setup up the keyboard for the UITextField to use the number with decimal style. So the user can only enter numbers and a single decimal.

What I want to do i

Answer 1

After much experimentation and looking at other's solutions (which seem to be overly complex and fiddly) I came up with the following, which prevents the user from entering anything but a valid currency amount. Basically, let an instance of NSNumberFormatter (created elsewhere in the viewController) do the hard work:

• 1) convert the text to a number (if nil, then there are invalid characters, so return NO)
• 2) then convert the number to currency
• 3) then convert the currency string back to number and, if different to the original (means too many decimals entered), return NO
• 4) then make an NSDecimalNumber, which is what I wanted. May be different for you, of course. Works for me so far - hope that this helps someone.

Firstly, I set the keyboard to UIKeyboardTypeDecimalPad. Then I used the following code

  -(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string

NSString *textString = [textField.text stringByReplacingCharactersInRange:range withString:string];

[numberFormatter setNumberStyle:NSNumberFormatterDecimalStyle];
NSNumber *amountNumber = [numberFormatter numberFromString:textString];

if ([textString length] > 0) {
    if (!amountNumber) {
        return NO;
    }
} else {
    amountNumber = @0;
}


[numberFormatter setNumberStyle:NSNumberFormatterCurrencyStyle];
NSString *amountStringer = [numberFormatter stringFromNumber:amountNumber];
NSNumber *amountAgain = [numberFormatter numberFromString:amountStringer];

//
//make sure that the number obtained again is the same....prevents too many decimals....
if (![amountNumber isEqualToNumber:amountAgain]) {
    return NO;
}

[numberFormatter setNumberStyle:NSNumberFormatterDecimalStyle];
amountShow.text = amountStringer;

self.amount = [NSDecimalNumber decimalNumberWithDecimal:[amountAgain decimalValue]];
NSLog(@"decimal Amount is %@", self.amount);

return YES;

}