setText in UIlabel after notification call has no effect - ios

Question

i want update the text of my label every time i receive notification from nsmanageObjContext.
this is my code for add the observer:
- (IBAction)requestFotPhoto {
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(updateLabel) name:NSManagedObjectContextDidSaveNotification
object:self.facebook.managedObjectContext];
and this is the method for update the label:
-(void)updateLabel
{
NSString *text = [NSString stringWithFormat:#"Downalad %i pictures",[Photo NumeberOfAllPhotosFromContext:self.facebook.managedObjectContext]];
dispatch_async(dispatch_get_main_queue(), ^{
//UIKIT method
NSLog(#"text %#",text);
[self.downlaodLabel setText:text];
});
}
i assume that updateLabel is execute in a another thread, so i execute the instructions for update the label on the main thread, but this code has no effect. where is the problem?
obviously the NSlog print the right message!
thanks!

Answer 1

In your situation you don't need to use dispatch_async, because notification handlers are run in the main thread. They are executed in a main loop on idle moments — sorry if I'm wrong with techincal words, english is not native for me.
And one more thing: you should't reference self from blocks, because self points to your block, and block points to self — they're not going to be released. If you really want to do it, you can read this question.

Answer 2

seems like:
your should move your NSNotificationCenter addObserver code, from your (IBAction)requestFotPhoto (seems is some button click event handler, which only run after user tapped) to viewDidLoad
shold like this:
- (void)viewDidLoad {
[super viewDidLoad];
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(updateLabel) name:NSManagedObjectContextDidSaveNotification object:self.facebook.managedObjectContext];
}
and for noficacation handler, not use dispatch_async
should like this:
- (void)updateLabel:(NSNotification *) notification {
NSLog (#"updateLabel: notification=%#", notification);
if ([[notification name] isEqualToString: NSManagedObjectContextDidSaveNotification]) {
NSDictionary *passedInUserInfo = notification.userInfo;
NSString *yourText = [passedInUserInfo objectForKey:#"dataKey"];
//UIKIT method
NSLog(#"yourText=%#",yourText);
[self.downlaodLabel setText:yourText];
}
}
and somewhere else should send the text:
NSString *newText = #"someNewText";
NSDictionary *passedInfo = #{#"dataKey": newText};
[[NSNotificationCenter defaultCenter] postNotificationName:NSManagedObjectContextDidSaveNotification object: self userInfo:passedInfo];
for more detail pls refer another post answer