ios編程：NSNotificationCenter多線程的情況

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.

這句話是說，notification都是在post時的線程中被傳遞和接收，即使add observer時是 在另外一個線程中。

例如: A, B兩個線程都addObserver。如果A中post，則無論此時B線程是否退出，回調都 在A中觸發，且觸發2次。

來源：http://blog.csdn.net/lwl_ls/archive/2011/04/20/6336083.aspx