gistfile1.m

- (void)viewDidLoad
{
    self.formatter = [NSNumberFormatter new];
    self.formatter.numberStyle = NSNumberFormatterCurrencyStyle;
    self.formatter.partialStringValidationEnabled = YES;
    
    self.textField.delegate = self;
    
    self.textField.text = [self.formatter editingStringForObjectValue:self.number];
}

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    NSString *oldString = textField.text;
    NSString *newString = [oldString stringByReplacingCharactersInRange:range withString:string];
    
    NSRange newRange = NSMakeRange(range.location + string.length, 0);
    
    NSString *error;
    
    BOOL isValid = [self.formatter isPartialStringValid:&newString
                                  proposedSelectedRange:&newRange
                                         originalString:oldString
                                  originalSelectedRange:range
                                       errorDescription:&error];
    NSLog(@"valid: %@, error: %@", isValid?@"YES":@"NO", error);
    
    self.textField.text = newString;
    
    UITextPosition *startPos = [textField positionFromPosition:textField.beginningOfDocument offset:newRange.location];
    UITextPosition *endPos = [textField positionFromPosition:startPos offset:newRange.length];
    
    [textField setSelectedTextRange:[textField textRangeFromPosition:startPos toPosition:endPos]];
    
    return NO;
}