The documentation (https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/Notifications/Articles/NotificationQueues.html ) says:
Does this mean that each observer's handler must run to completion or are they dispatched asynchronously?Using the NSNotificationCenter’s postNotification: method and its variants, you can post a notification to a notification center. However, the invocation of the method is synchronous: before the posting object can resume its thread of execution, it must wait until the notification center dispatches the notification to all observers and returns.