Are NSNotificationCenter events received synchrono

问题:

If a class registers for NSNotificationCenter events of a certain type and another class posts an event of that type, will the code in the receiver execute before (synchronously) or after (asynchronously) the posting class continues?

- (void)poster {
    [[NSNotificationCenter defaultCenter]
        postNotificationWithName:@"myevent"
        object:nil];
    NSLog(@"Hello from poster");
}

- (void)receiver {
    [[NSNotificationCenter defaultCenter]
        addObserver:self
        selector:@selector:(mySelector)
        name:@"myevent"
        object:nil];
}

- (void) mySelector:(NSNotification *) notification {
    NSLog(@"Hello from receiver");
}

In the code example above, will "Hello from receiver" be printed before or after "Hello from caller"?

回答1:

As stated in the documentation for NSNotificationCenter NSNotificationCenter Class Reference notifications are posted synchronously.

A notification center delivers notifications to observers synchronously. In other words, the postNotification: methods do not return until all observers have received and processed the notification. To send notifications asynchronously use NSNotificationQueue.

In a multithreaded application, notifications are always delivered in the thread in which the notification was posted, which may not be the same thread in which an observer registered itself.

Hope it helps you.