在dealloc中使用weakSelf会崩溃,apple给出的解释:
Cannot form weak reference to instance (0x137e06750) of class NewViewController. It is possible that this object was over-released, or is in the process of deallocation.
翻译:在对象正在释放的过程中,或者对象已经释放后,是不允许使用weak来引用实例变量的。这可能是出于防止野指针的出现。