[Mac / iOS] 按下Home key時可以close某Class

class MainViewController 是一個 FormSheet Modal

呼叫方式：

UINavigationController *modalDialog = [[UINavigationController alloc]initWithRootViewController:MainViewController];
    MainViewController.modalPresentationStyle = UIModalPresentationFormSheet;

    modalDialog.modalPresentationStyle = UIModalPresentationFormSheet;

    [self presentModalViewController:modalDialog animated:YES];

期望按下Home key時可以close MainViewController：
- (void)viewDidLoad
{

    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(dismissWithoutAnim) name:UIApplicationDidEnterBackgroundNotification object:nil];

}

- (void)dismissWithoutAnim {

    [self.navigationController dismissModalViewControllerAnimated:NO];

    MainViewController *mainViewController = [[MainViewController alloc] initWithNibName:@"MainViewController" bundle:nil];

    [self.mainViewController dismissModalViewControllerAnimated:NO];

}