导致VC不能释放的几个原因

1. delegate的属性不是weak
2. NSTimer没有invalidate
3. block中的强引用

4. 调用了performSelector，退出时没有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其实就是定时器，会强引用object。如果delay很长的话，就会被导致object很长时间不会被释放。