導致VC不能釋放的幾個原因

調用了performSelector，退出時沒有cancelPerformSelectorsWithTarget

This method sets up a timer to perform the aSelector message on the current thread’s run loop. The timer is configured to run in the default mode (NSDefaultRunLoopMode).

performSelector其實就是定時器，會強引用object。如果delay很長的話，就會被導致object很長時間不會被釋放。