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POWarning for iOS/iPhone users about duplicate NSNotification observations
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copied!<p>This isn't a question so much as a warning to others to save them some time.</p> <p>NSNotificationCenter on iOS 3/iPhone OS 3 (I'm assuming also Mac OS X and iOS 4) has the following behavior:</p> <p>If you register yourself multiple times for the exact specific notification, NSNotificationCenter will <strong>NOT</strong> recognize the redundancy and instead will fire off as many notifications to you as you've registered an observation for.</p> <p>This is almost never the behavior you want to see and is almost always accidental.</p> <p>Example:</p> <p>I want my view controller to receive notifications from a singleton network object when new data comes in:</p> <pre><code>- (void) viewDidLoad { [super viewDidLoad]; [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(newDataArrived:) name:NewDataArrivedNotification object:[NetworkListener sharedNetworkListener]]; } </code></pre> <p>but earlier I'd already put the same thing in <code>viewWillAppear</code>:</p> <pre><code>- (void) viewWillAppear { [super viewWillAppear]; [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(newDataArrived:) name:NewDataArrivedNotification object:[NetworkListener sharedNetworkListener]]; } </code></pre> <p>Note that it's <strong>exactly</strong> the same notification, resolving to the same observer, sender and notification name.</p> <p>In this case, if I don't remove one of those addObserver calls, I'll get duplicate notifications to my view controller.</p> <p>In a multi-threaded environment, this is a world of hurt. Trust me.</p> <p>Just putting this out there in case there are others who run into something like this.</p>

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