StackOverflow2013

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copied!<p>The <code>NSNotificationCenter</code> and <code>gcd</code> & <code>dispatch_get_main_queue()</code> serve very different purposes. I don't thing "vs" is truly applicable. </p> <p><code>NSNotificationCenter</code> provides a way of de-coupling disparate parts of your application. For example the <code>kReachabilityChangedNotification</code> in Apple's Reachability sample code is posted to the notification center when the system network status changes. And in turn you can ask the notification center to call your selector/invocation so you can respond to such an event.(Think Air Raid Siren)</p> <p><code>gcd</code> on the other hand provides a quick way of assigning work to be done on a queue specified by you. It lets you tell the system the points at which your code can be dissected and processed separately to take advantage of multiple-threads and of multi-core CPUs.</p> <p>Generally (almost always) the notifications are observed on the thread on which they are posted. With the notable exception of one piece of API...</p> <p>The one piece of API where these to concepts intersect is <code>NSNotificationCenter</code>'s:</p> <pre><code>addObserverForName:object:queue:usingBlock: </code></pre> <p>This is essentially a convenient method for ensuring that a given notification is observed on a given thread. Although the "<code>usingBlock</code>" parameter gives away that behind the scenes it's using <code>gcd</code>.</p> <p>Here is an example of its usage. Suppose somewhere in my code there is an <code>NSTimer</code> calling this method every second:</p> <pre><code>-(void)timerTimedOut:(NSTimer *)timer{ dispatch_async(dispatch_get_global_queue(0, 0), ^{ // Ha! Gotcha this is on a background thread. [[NSNotificationCenter defaultCenter] postNotificationName:backgroundColorIsGettingBoringNotification object:nil]; }); } </code></pre> <p>I want to use the <code>backgroundColorIsGettingBoringNotification</code> as a signal to me to change my view controller's view's background color. But it's posted on a background thread. Well I can use the afore mentioned API to observe that only on the main thread. Note <code>viewDidLoad</code> in the following code:</p> <pre><code>@implementation NoWayWillMyBackgroundBeBoringViewController { id _observer; } -(void)observeHeyNotification:(NSNotification *)note{ static NSArray *rainbow = nil; static dispatch_once_t onceToken; dispatch_once(&onceToken, ^{ rainbow = @[[UIColor redColor], [UIColor orangeColor], [UIColor yellowColor], [UIColor greenColor], [UIColor blueColor], [UIColor purpleColor]]; }); NSInteger colorIndex = [rainbow indexOfObject:self.view.backgroundColor]; colorIndex++; if (colorIndex == rainbow.count) colorIndex = 0; self.view.backgroundColor = [rainbow objectAtIndex:colorIndex]; } - (void)viewDidLoad{ [super viewDidLoad]; self.view.backgroundColor = [UIColor redColor]; __weak PNE_ViewController *weakSelf = self; _observer = [[NSNotificationCenter defaultCenter] addObserverForName:backgroundColorIsGettingBoringNotification object:nil queue:[NSOperationQueue mainQueue] usingBlock:^(NSNotification *note){ [weakSelf observeHeyNotification:note]; }]; } -(void)viewDidUnload{ [super viewDidUnload]; [[NSNotificationCenter defaultCenter] removeObserver:_observer]; } -(void)dealloc{ [[NSNotificationCenter defaultCenter] removeObserver:_observer]; } - (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation{ return (interfaceOrientation == UIInterfaceOrientationPortrait); } @end </code></pre> <p>The primary advantage of this API seems to be that your observing block will be called during the <code>postNotification...</code> call. If you used the standard API and implemented <code>observeHeyNotification:</code> like the following there would be no guarantee how long it would be before your dispatch block was executed:</p> <pre><code>-(void)observeHeyNotification:(NSNotification *)note{ dispatch_async(dispatch_get_main_queue(), ^{ // Same stuff here... }); } </code></pre> <p>Of course in this example you could simply not post the notification on a background thread, but this might come in handy if you are using a framework which makes no guarantees about on which thread it will post notifications.</p>

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