Problems with NSNotificationCenter and UIPickerView - ios

Question

I hope I have better luck with someone helping me on this one:
I have a UIPickerView where a user makes a selection and then presses a button. I can gladly obtain the users choice, as shown in my NSLog, and when this is done, I want to send a notification to another view controller that will show a label with the option selected. Well, although it seems everything is done right, somehow it does not work and the label stays intact. Here is the code:
Broadcaster:
if ([song isEqualToString:#"Something"] && [style isEqualToString:#"Other thing"])
{
NSLog (#"%#, %#", one, two);
[[NSNotificationCenter defaultCenter] postNotificationName:#"Test1" object:nil];
ReceiverViewController *receiver = [self.storyboard instantiateViewControllerWithIdentifier:#"Receiver"];
[self presentModalViewController:receiver animated:YES];
}
Observer:
- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
if (self)
{
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(receiveNotification) name:#"Test1" object:nil];
}
return self;
}
-(void)receiveNotification:(NSNotification*)notification
{
if ([[notification name] isEqualToString:#"Test1"])
{
[label setText:#"Success!"];
NSLog (#"Successfully received the test notification!");
}
else
{
label.text = #"Whatever...";
}
}

Answer 1

I think you have a syntax error in your selector: #selector(receiveNotification). It should probably be #selector(receiveNotification:) with the colon since your method accepts the NSNotification *notification message. Without it, it's a different signature.

Answer 2

The issue is likely that the notification is sent (and therefore received) on a different thread than the main thread. Only on the main thread will you be able to update UI elements (like a label).
See my answer to this question for some insight into threads and NSNotifications.
Use something like:
NSLog(#"Code executing in Thread %#",[NSThread currentThread] );
to compare your main thread versus where your recieveNotifcation: method is being executed.
If it is the case that you are sending the notification out on a thread that is not the main thread, a solution may be to broadcast your nsnotifications out on the main thread like so:
//Call this to post a notification and are on a background thread
- (void) postmyNotification{
[self performSelectorOnMainThread:#selector(helperMethod:) withObject:Nil waitUntilDone:NO];
}
//Do not call this directly if you are running on a background thread.
- (void) helperMethod{
[[NSNotificationCenter defaultCenter] postNotificationName:#"SOMENAME" object:self];
}
If you only care about the label being updated on the main thread, you can perform that operation on the main thread using something similar to:
dispatch_sync(dispatch_get_main_queue(), ^(void){
[label setText:#"Success!"];
});
Hope that was helpful!