UITextField should allow only numbers.

TextFieldAllowNumberOnly.m

#define ACCEPTABLE_CHARACTERS @"0123456789"
- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
    
    if (!string.length)
        return YES;
    
    if (textField == self.quantityTextField) {
        NSCharacterSet *numbersOnly = [[NSCharacterSet characterSetWithCharactersInString:ACCEPTABLE_CHARACTERS] invertedSet];
        NSString *filtered = [[string componentsSeparatedByCharactersInSet:numbersOnly] componentsJoinedByString:@""];
        BOOL stringIsValid = [string isEqualToString:filtered];
        
        return stringIsValid;
    }
    
    return YES;
}