I am using the QLPreviewController in my project, but cannot use pinch gestures to zoom in. What do I need to set up first? This is how I create the view controller:

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
    QLPreviewController *previewer = [[QLPreviewController alloc] init];
    [previewer setDataSource:self];
    [previewer setCurrentPreviewItemIndex:indexPath.row];
    [[self navigationController] pushViewController:previewer animated:YES];
}