Removing a NSNotificationCenter observer in iOS 5 ARC
Solution 1
It's pretty clear your dealloc
method isn't being called (nor is the removeObserver
call).
Why not remove your UIViewController's observer in the viewDidUnload:
or viewWillDisappear:
methods?
Solution 2
If your dealloc isn't being called, it's likely because someone is still holding a reference to the view controller. Perhaps you need to mark something as __weak
? You can use the allocations instrument to help track down what's holding on to your view controller.
Solution 3
"I also need the notification callbacks to still be fired if the view is off-screen" -> you may need to register UIApplicationWillEnterForegroundNotification. If so, let try this:
- (void)viewWillAppear:(BOOL)animated {
NSLog(@"viewWillAppear");
[super viewWillAppear:animated];
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(applicationDidEnterBackground:)
name:UIApplicationDidEnterBackgroundNotification
object:nil];
}
- (void)viewWillDisappear:(BOOL)animated {
NSLog(@"viewWillDisappear");
[super viewWillDisappear:animated];
[[NSNotificationCenter defaultCenter] removeObserver:self name:UIApplicationDidEnterBackgroundNotification object:nil];
}
- (void)applicationWillEnterForeground:(UIApplication *)application {
NSLog(@"applicationWillEnterForeground");
[[NSNotificationCenter defaultCenter] removeObserver:self];
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(applicationDidEnterBackground:)
name:UIApplicationDidEnterBackgroundNotification
object:nil];
// do your stuff here
}
- (void)applicationDidEnterBackground:(UIApplication *)application {
NSLog(@"applicationDidEnterBackground");
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(applicationWillEnterForeground:)
name:UIApplicationWillEnterForegroundNotification
object:nil];
}
The idea is adding or removing UIApplicationDidEnterBackgroundNotification whenever coming in and out of your screen. We just register UIApplicationWillEnterForegroundNotification when the app enter background and remove once it's back. Be noticed that we just remove UIApplicationDidEnterBackgroundNotification when viewWillDisappear.
My dealloc() is not called by somehow, so I found this way, hope it useful for you too.
Enjoy :)
Skoota
Updated on June 10, 2022Comments
-
Skoota about 2 years
I have an iOS 5 ARC-based project, and am having difficulty about where I should be removing the observer for the
NSNotificationCenter
observations which I have registered within aUIViewController
. Similar posts on SO have said this should be done in the-dealloc
method. Even though this method is not required in ARC projects I have added it with the following code:- (void)dealloc { [[NSNotificationCenter defaultCenter] removeObserver:self]; }
As a test, I open the
UIViewController
(within aUINavigationController
), do some things which trigger the notifications, and then pop it off the stack by tapping the Back button. I then reopen theUIViewController
, and do some more things to trigger the notifications, but notice that each callback is being called twice - an indication that the previous notifications have not been deregistered. Repeating this procedure just causes each callback to be called more than more times, so they appear to never be deregistering.Any help would be appreciated!
-
Skoota over 12 years
viewDidUnload:
is only called in low memory conditions, as far as I know. I also need the notification callbacks to still be fired if the view is off-screen, so that means I can't useviewWillAppear:
andviewWillDisappear:
-
Michael Dautermann over 12 yearsb.t.w., if you're popping your view controller off the stack, that should be deallocing it. If it isn't, is something else retaining your view controller? Check instruments to be sure. Oh, and here is a related question that might be helpful for you
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GregJaskiewicz over 11 yearsyou forgot to call [super ...] in those. Per documentation.
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thanhbinh84 over 11 yearsAre they required? I don't see any problem without them since I am using this code in my app.
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GregJaskiewicz over 11 yearsYou'll see a problem when the super object will perform its own actions. Get in the habit of doing it, because the doc says so. Otherwise one day you might get into weird issues.
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thanhbinh84 over 11 yearsThanks Greg, you are right. They mention in the api doc developer.apple.com/library/ios/#documentation/uikit/reference/…
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Johan Karlsson over 9 years@doraemon Yes, you will sooner or later run into problems if you do not call super in these methods. They are in fact doing some "magic stuff". I have seen several problems that have been solved by adding calls to super.
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Chris Nolet over 9 yearsInstead of adding and removing these notifications, why not just add them in
init
orviewDidLoad
and remove all observers indealloc
? Or if you really must have them removed when the view disappears, add them both inviewWillAppear
and remove them inviewDidDisappear
. No need to juggle :)