Are NSNotificationCenter events received synchronously or asynchronously?
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.
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