How do I format UITextField text in real time, while it's being edited?

Add a "textFieldDidChange" notification method to the text field control.

[textField addTarget:self
              action:@selector(textFieldDidChange:)
    forControlEvents:UIControlEventEditingChanged];


-(void)textFieldDidChange:(UITextField *)theTextField
{
    NSLog(@"text changed: %@", theTextField.text);

    NSString *textFieldText = [theTextField.text stringByReplacingOccurrencesOfString:@"," withString:@""];

    NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init];
    [formatter setNumberStyle:NSNumberFormatterDecimalStyle];
    NSString *formattedOutput = [formatter stringFromNumber:[NSNumber numberWithInt:[textFieldText integerValue]]];
    textField.text=formattedOutput;            
}

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init] ;
    if([string length]==0)
    {
        [formatter setGroupingSeparator:@","];
        [formatter setGroupingSize:4];
        [formatter setUsesGroupingSeparator:YES];
        [formatter setSecondaryGroupingSize:3];
        NSString *num = textField.text ;
        num = [num stringByReplacingOccurrencesOfString:@"," withString:@""];
        NSString *str = [formatter stringFromNumber:[NSNumber numberWithDouble:[num doubleValue]]];
        textField.text = str;
        return YES;
    }
    else {
        [formatter setGroupingSeparator:@","];
        [formatter setGroupingSize:2];
        [formatter setUsesGroupingSeparator:YES];
        [formatter setSecondaryGroupingSize:3];
        NSString *num = textField.text ;
        if(![num isEqualToString:@""])
        {
            num = [num stringByReplacingOccurrencesOfString:@"," withString:@""];
            NSString *str = [formatter stringFromNumber:[NSNumber numberWithDouble:[num doubleValue]]];
            textField.text = str;
            NSLog(@"add:%@",str);
        }
        return YES;
    }
}

P.S. This code is in ARC, take care of memory management in non arc environment.