關於註冊了通知／監聽 必須登出，註冊通知監聽登出

關於註冊了通知／監聽 必須登出，註冊通知監聽登出
An instance 0x15618e00 of class UITextView was deallocated while key value observers were still registered with it. Observation info was leaked, and may even become mistakenly attached to some other object. Set a breakpoint on NSKVODeallocateBreak to stop here in the debugger. Here's the current observation info:<NSKeyValueObservationInfo 0x159b3b70> (<NSKeyValueObservance 0x159b3b30: Observer: 0x14c68c00, Key path: contentSize, Options: <New: YES, Old: NO, Prior: NO> Context: 0x0, Property: 0x159cab50>)
http://www.tuicool.com/articles/yEVzE3E

我對UITextView進行了監聽，卻沒有登出
註冊了，一定要登出。還要判斷是否存在，存在才登出，不存在不能對其進行沒有的操作（有的本身沒有判斷是否存在的，可用一個變數去做判斷，eg：bool，int）