Lifetime of the NSNotification object

Question

Let's say I have this posting from one class that fires rapidly:

[[NSNotificationCenter defaultCenter] postNotificationName:kGotData object:nil userInfo:someDictionaryObject];

And an observer from another class:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(gotData:) name:kGotData object:nil];

-(void)gotData:(NSNotification *)notification
{
    NSDictionary *myUserInfo = notification.userInfo;

    // more code to process userInfo, etc.
}

What if gotData take longer time to process myUserInfo than it did receive calls from the notification center?

score 4 · Answer 1 · answered Oct 07 '13 at 23:25

4

All posted notifications are sent to observers and handled on the same thread and immediately. So, if gotData: takes time and doesn't push it's processing onto a different thread it will simply be blocking whatever is going to post the next notification. So the next notification will be posted later in time (than you might expect).

answered Oct 07 '13 at 23:25

Wain

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Yours and Tommy answers both have cleared up my misconception of NSNotification. Thank you. – user523234 Oct 08 '13 at 21:31

score 3 · Accepted Answer · answered Oct 07 '13 at 23:25

3

It can't take longer — calls are entirely synchronous. The notification centre acts as a one-to-many messaging clearing house that decouples actors that might want to listen from actors that might want to announce an event.

Supposing you had only exactly the one observer, then your code is equivalent to just calling gotData: directly. So no further notification can occur until the previous has finished being processed.

answered Oct 07 '13 at 23:25

Tommy

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what if I have two observers, one can process faster than the other? – user523234 Oct 07 '13 at 23:28
@user523234 then it's exactly the same thing as if the object posting the notification called each in turn. So it'll be the total time of both bits of processing before the `postNotificationName:` call returns. – Tommy Oct 07 '13 at 23:30

Lifetime of the NSNotification object

2 Answers2