NSNotification not working from another class - ios

Question

Part of an app I'm working on involves putting a blank screen over the current content if the user becomes inactive. As such after x seconds a blank page view controller is opened:
(From within ViewController.m and triggered by detecting an NSNotification from ScreenBlank.m)
UIPageViewController *blankPage = [self.storyboard instantiateViewControllerWithIdentifier:#"BlankPageViewController"];
[self presentViewController:blankPage animated:YES completion:nil];
This blank screen is then removed when a user touches the screen (And thus confirms their activity), like so:
(From inside the touchesBegan callback of ViewController.m)
[self dismissViewControllerAnimated:YES completion:nil];
The problem I'm having, is that I now want to trigger removing this screen blanking elsewhere, such as when the user logs out. This is done by removing a card from a plugged in card-reader, and means that it is called from a separate class (And one that is instantiated from within ActionMgr.m).
The class in question (CardWatcher) is created like so:
CardWatcher *newInstance = [[CardWatcher alloc] init];
[newInstance StartCardChecker];
And that instance of CardWatcher issues a notification when the card is removed, like so:
[[NSNotificationCenter defaultCenter] postNotificationName:#"logout" object:nil];
This notification is then listened for in ViewController, and on being triggered, executes EXACTLY the same code as is used to blank the screenearlier:
[self dismissViewControllerAnimated:YES completion:nil];
By using logging, I have determined that the notification is being received fine, and is executing the dismissViewControllerAnimated code, but for some reason, said code is not actually dismissing the view controller.
The only reason I can think of, is that it is (In the long run) called form within an instance of a class, but even then it's being passed via an NSNotification, and so in my eyes its source should be irrelevant?
Any help would be much appreciated!

Answer 1

It sounds like the notification is not generated on the main thread and you are hence trying to dismiss the page from a secondary thread. Make sure that you are posting the notification on the main thread and not from secondary threads. When a notification triggers UI events, such as this one, the code that manipulates the UI is required to be executed on the main thread.
Delivering Notifications To Particular Threads
Regular notification centers deliver notifications on the thread in
which the notification was posted. Distributed notification centers
deliver notifications on the main thread. At times, you may require
notifications to be delivered on a particular thread that is
determined by you instead of the notification center. For example, if
an object running in a background thread is listening for
notifications from the user interface, such as a window closing, you
would like to receive the notifications in the background thread
instead of the main thread. In these cases, you must capture the
notifications as they are delivered on the default thread and redirect
them to the appropriate thread.
Just try the following code and let me know if it worked for you.
dispatch_async(dispatch_get_main_queue(),^{
[[NSNotificationCenter defaultCenter] postNotificationName:#"logout" object:nil];
});