iOS-watchdog(看門狗)機制

為了防止一個應用佔用過多的系統資源，開發iOS的蘋果工程師門設計了一個“看門狗”的機制。在不同的情境下，“看門狗”會監測應用的效能。如果超出了該情境所規定的已耗用時間，“看門狗”就會強制終結這個應用的進程。開發人員們在crashlog裡面，會看到諸如0x8badf00d這樣的錯誤碼。

在ARC及手機型號比較低的情況下，這種現象尤為突出。

The exception code 0x8badf00d indicates that an application has been terminated by iOS because a watchdog timeout occurred. The application took too long to launch, terminate, or respond to system events. One common cause of this is doing synchronous networking on the main thread. Whatever operation is on Thread 0: needs to be moved to a background thread, or processed differently, so that it does not block the main thread.

Apple doc地址；  https://developer.apple.com/library/ios/technotes/tn2151/_index.html