Registration notification listening must be canceled when a notification/listener is registered

Registration notification listening must be canceled when a notification/listener is registered
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: <symbol 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

I listened to UITextView but did not log out.
Registered, must be canceled. You also need to determine whether the operation exists before canceling the operation. If the operation does not exist, you cannot perform the operation. (Some do not determine whether the operation exists. You can use a variable to determine whether the operation exists. For example: bool, int)