Problems with NSNotificationCenter and UIPickerView

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...";
}


}

score 1 · Answer 1 · edited Jan 29 '14 at 04:52

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.

edited Jan 29 '14 at 04:52

Bob Gilmore

12,608
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answered Jan 29 '14 at 04:30

Robert

11
2

score 0 · Accepted Answer · edited May 23 '17 at 12:11

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!

Problems with NSNotificationCenter and UIPickerView

2 Answers2