Calling a method either the second time a condition occurs OR after a timeout, whichever comes first - objective-c

Question

After plenty of coding in the past days, I'm finally (um) pretty much stuck.
My application listens for an external NSNotification sent by iTunes. The notification is sent out whenever the current playing status changes, in this case most interestingly when the current stream title changes. When you connect to a new radio station, two notifications are usually sent - one as soon as iTunes connects, with the station name as the title, and one soon thereafter (a second or so) with the actual artist and title of the current song. I'm only interested in the artist/title combo, or the second notification. Or, if there's still only one notification sent after a two second pause, use the first one. (Since there's no way of knowing whether there will be one or two beforehand, the timeout is the only way I can think of.)
In fewer words, I want to call a method only the second time a condition occurs, OR after a two-second timeout if only one notification is sent. It should reset back to do the whole deal again after the two seconds have passed.
Any ideas?

Answer 1

This isn't too complicated. You just need to hang on to the first notification until either the timer fires or the second notification comes in. The comments I put in the code should explain the procedure.
- (void)awakeFromNib {
// Register for the notification you're interested in
[[NSDistributedNotificationCenter defaultCenter] addObserver:self
selector:#selector(iTunesNoteCallback:)
name:NSTheiTunesNotificationImInterestedIn
object:nil]; // #"iTunes"?
}
- (void)iTunesNoteCallback:(NSNotification *)note {
// Check whether there's been a notification already
if( !gotFirstNote ){
// If so, hang on to it,
gotFirstNote = YES;
self.currNote = note; // With currNote declared as a retained property
// and start a timer.
noteTimer = [NSTimer scheduledTimerWithTimeInterval:2.0
target:self
selector:#selector(actOniTunesInfo:)
userInfo:nil repeats:NO];
}
else {
// However, if we got a notification already,
// hang on to the new one,
self.currNote = note;
// stop the timer,
[noteTimer invalidate];
// and call the same method the timer would have
[self actOniTunesInfo:nil];
}
}
- (void)actOniTunesInfo:(NSTimer *)timer {
// Reset the flag
gotFirstNote = NO;
// Use currNote; doesn't matter which one it is,
// it's the best info we've gotten
}
If there are two types of notifications, it's even simpler. You no longer need the flag, you just register two callbacks, and as soon as the second one is called, you can invalidate the timer, release the first notification, and use the info you've just gotten.

Answer 2

You need to create a data model that can store and model the notfications as well as their order and timing.
First you would need a data object to store each notification and data about the notification.
- the notification object
- the time stamp when the notification arrived
- type of notification
Then you would need a container-object that could hold the data-objects in an array as well as start and catch timers. So, when a notification arrives, it is classified, stored in a data object which is then pushed onto the array. If it is the first type of notification the container object starts a two second timer. If another notification of the second type arrives, the container-object kills the timer but if the timer fires, then it returns the data-object of the last first type notification that is more than two seconds old.
Once you've triggered an action, empty the container-object and start over.