接上一段回复内容：

dispatch_sync(dispatch_get_global_queue(0, 0), ^{

[[NSOperationQueue mainQueue] addOperationWithBlock:^{
NSLog(@"First");
}];

[[NSOperationQueue currentQueue] addOperationWithBlock:^{
NSLog(@"Three");
}];

__block id observer = [[NSNotificationCenter defaultCenter] addObserverForName:@"MyNotif" object:nil queue:[NSOperationQueue mainQueue] usingBlock:^(NSNotification *note) {
NSLog(@"Receive Notif");
[[NSNotificationCenter defaultCenter] removeObserver:observer];
}];

[[NSOperationQueue currentQueue] addOperationWithBlock:^{
NSLog(@"Forth");
}];

[[NSNotificationCenter defaultCenter] postNotificationName:@"MyNotif" object:self];

[[NSOperationQueue currentQueue] addOperationWithBlock:^{
NSLog(@"Five");
}];
});