NSNotification not being sent when postNotificationName: called

Question

I'm trying to get one instance of using NSNotificationCenter with addObserver and postNotificationName but I can't work out why it won't work.

I have 2 lines to code to add the observer and send the message in 2 different classes

[[NSNotificationCenter defaultCenter]addObserver:self selector:@selector(newEventLoaded:) name:@"Event" object:nil];

and

[[NSNotificationCenter defaultCenter]postNotificationName:@"Event" object:self];

If I set the name to nil it works fine becuase it's just a broadcast, when i try and define a notification name the messages never get through.

Answer 1

All my code makes use of NSNotifications like so:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(updateView) name:@"ScanCompleted" object:nil];

[[NSNotificationCenter defaultCenter] postNotificationName:@"ScanCompleted" object:nil];

The first one is registering the notification and the second posting of the notification.

Answer 2

Basically it's all to do with the order of execution. If you've executed postNotificationName before addObserver, then this is an easy problem to have. Use breakpoints and step through the code :)

Your first breakpoint should stop here:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(updateView:) name:@"ScanCompleted" object:nil];

Then here:

[[NSNotificationCenter defaultCenter]postNotificationName:@"ScanCompleted" object:self];

Also, make sure the selector has a colon on. Because it's method signature will be:

- (void)updateView:(NSNotification *)notification;

Answer 3

I had the same problem. The reason is that I called removeObserver method at

- (void)viewDidDisappear:(BOOL)animated{

    NSNotificationCenter *notificationCenter = [NSNotificationCenter defaultCenter];

   [notificationCenter removeObserver:self];

}

So check whether if you had called removeObserver before postNotification.

Tips: You can search the keyword "removeObserver" to find if you had called this function.

Answer 4

Change this:

[[NSNotificationCenter defaultCenter]postNotificationName:@"Event" object:self];

to this:

[[NSNotificationCenter defaultCenter]postNotificationName:@"Event" object:nil];

If your first notification is registered properly, newEventLoaded should be called.

Answer 5

I had a similar issue and my problem was due to the notification being called on another thread. This solved my problem.

dispatch_async(dispatch_get_main_queue(),^{
    [[NSNotificationCenter defaultCenter]postNotificationName:@"Event" object:self];
});

Answer 6

Have you tried any other names but @"Event" and nil? Just to be sure, you could define your event names in one file and include that into both notification registration and sending. For example:

Header file:

extern NSString * const NOTE_myEventName;

Source file:

NSString * const NOTE_myEventName = @"MyEventName";

Registration:

[[NSNotificationCenter defaultCenter]
 addObserver:self
    selector:@selector(handleMyEvent:)
        name:NOTE_myEventName
      object:nil];

Notification sending:

[[NSNotificationCenter defaultCenter]
    postNotificationName:NOTE_myEventName object:nil];

Answer 7

I successfully fixed my "NSNotification not being sent when postNotificationName: called" crash.

I found the real bug is in notification message handler.

The postNotificationName and addObserver are all right as the first post of this thread.