An instance 0x172b8600 of class UITableView was deallocated while key value

从带有tableview并且使用Mj下拉刷新的视图调用popViewControllerAnimated的时候，控制台打印这个:
An instance 0x15d7aa00 of class UITableView 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 0x16835900> (
<NSKeyValueObservance 0x16835a40: Observer: 0x16834ed0, Key path: contentOffset, Options: <New: YES, Old: NO, Prior: NO> Context: 0x0, Property: 0x15748160>
)

解决办法：

- ( void )dealloc{

　　[ _header free ];

　　[ _footer free ];

}