iOS AirPlay：我的应用程序仅在镜像开启时通知外部显示器？

这是我的问题：如果我从AirPlay按钮选择“AppleTV”,我的应用程序不会收到通知.我的应用程序获得通知的唯一一次是当我离开我的应用程序时,转到操作系统级别的AirPlay按钮,在那里选择“AppleTV”并打开镜像.如果我关闭镜像,我的应用程序会被告知外部显示器已经消失.

所以：

>当我从中选择外部显示器时,为什么我的应用程序不会收到通知
在我的应用程序？
>为什么我的应用程序会收到通知
我开镜像时的外部显示……而不是之前？我显然误解了一些东西,但是看起来像镜像应该通知我的应用程序外部显示器已经消失(而不是现在可用,因为操作系统现在应该使用外部显示器进行镜像.)

代码示例如下.在此先感谢您的帮助！

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];

     // Override point for customization after application launch.

     // Is there already an external screen?
     if (UIScreen.screens.count > 1)]
     {
          [self prepareExternalScreen:UIScreen.screens.lastObject];
     }

     // Tell us when an external screen is added or removed.
     [[NSNotificationCenter defaultCenter] addobserver:self selector:@selector(externalScreenDidConnect:) name:UIScreenDidConnectNotification object:nil];
     [[NSNotificationCenter defaultCenter] addobserver:self selector:@selector(externalScreenDiddisconnect:) name:UIScreenDiddisconnectNotification object:nil];

     self.viewController = [[ViewController alloc] initWithNibName:@"ViewController" bundle:nil];
     self.window.rootViewController = self.viewController;

     // Add AirPlay control to view controller.
     MPVolumeView* airplayButtonView = [[MPVolumeView alloc] init];
     airplayButtonView.frame = CGRectMake(300,300,50,50);
     airplayButtonView.backgroundColor = [UIColor blackColor];
     airplayButtonView.showsVolumeSlider = NO;
     airplayButtonView.showsRouteButton = YES;

     [self.viewController.view addSubview:airplayButtonView];


    [self.window makeKeyAndVisible];

    return YES;
}

#pragma mark - External screen handling

- (void)externalScreenDidConnect:(NSNotification*)notification
{
     [self prepareExternalScreen:notification.object];
}

- (void)externalScreenDiddisconnect:(NSNotification*)notification
{
     // Don't need these anymore.
     self.externalWindow = nil;
}

- (void)prepareExternalScreen:(UIScreen*)externalScreen
{
     NSLog(@"PREPPING EXTERNAL SCREEN.");
     self.viewController.view.backgroundColor = [UIColor blueColor];
     CGRect frame = externalScreen.bounds;
     self.externalWindow = [[UIWindow alloc] initWithFrame:frame];
     self.externalWindow.screen = externalScreen;
     self.externalWindow.hidden = NO;
     self.externalWindow.backgroundColor = [UIColor redColor];
}