Objective-C：仅数字文本字段？

-(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRangE)range replacementString:(NSString *)String 
{
    /*  limit to only numeric characters  */
    NSCharacterSet *myCharSet = [NSCharacterSet characterSetWithCharactersinstring:@"0123456789"];
    for (int i = 0; i < [String length]; i++) {
        unichar c = [String characterATindex:i];
        if ([myCharSet characterIsMember:c]) {
            return YES;
        }
    }

    /*  limit the users input to only 9 characters  */
    NSUInteger newLength = [customTextField.text length] + [String length] - range.length;
    return (newLength > 9) ? NO : YES;
}