有个疑问，就是在使用gcd的时候，为了怕循环引用，用 ____weak来修饰。
可是比如要是该任务花费的时间很长，万一____weak修饰的变量被释放掉了，那回调的
block体里要如何做呢？
__weak的作用不是这个吧
A function with this attribute has its name emitted as a weak
symbol instead of a global name. This is primarily for the naming
of library routines that can be overridden by user code.
weak symbol
Having two or more global symbols of the same name will not cause a conflict as long as all but one of them are declared as being weak symbols. The linker ignores the definitions of the weak symbols and uses the normal global symbol definition to resolve all references, but the weak symbols will be used if the normal global symbol is not available. A weak symbol can be used to name functions and data that can be overridden by user code. A weak symbol is also referred to as a weak alias, or simply weak.