__weak在mrc下可用

@HppZ 请注意：__weak是iOS4.2的之后ARC产物。你可以在MRC是使用__weak测试下。会爆警告：'__weak' only applies to Objective-C object or block pointer types; type here is 'int'。而block块内操作的变量会爆红：Variable is not assignable (missing __block type specifier)。

@maquannene __weak修饰符的语义：指定一个引用，该引用不会使引用的对象保持活着状态。 当没有强引用时，弱引用设置为零，指针置空nil。官方文档：https://developer.apple.com/library/content/releasenotes/ObjectiveC/RN-TransitioningToARC/Introduction/Introduction.html#//apple_ref/doc/uid/TP40011226-CH1-SW4

对于楼主的issue第一句话“ARC 下 __weak 可避免循环引用，__block 不行”认同。
第二句话“__block 仅在 MRC 下可解决循环引用的问题，但是会有野指针的问题。”不认同。
__block在MRC下会有野指针？是吗？请好好看看我发的文档链接。也可自行测试。

It waste my 5 minutes to prove my hesitation.

In order to prevent I get this rude question to wast my time again, i paste official docs which @HaiTeng-Wang provided:

In manual reference counting mode, __block id x; has the effect of not retaining x. In ARC mode, __block id x; defaults to retaining x (just like all other values). To get the manual reference counting mode behavior under ARC, you could use __unsafe_unretained __block id x;. As the name __unsafe_unretained implies, however, having a non-retained variable is dangerous (because it can dangle) and is therefore discouraged. Two better options are to either use __weak (if you don’t need to support iOS 4 or OS X v10.6), or set the __block value to nil to break the retain cycle.

@ChenYilong 既然ARC下__block无法避免循环引用，那为什么不修正答案呢？补充一点，OC高级编程一书中有说__block可以避免循环引用，但前提是要及时的将__block变量置为nii.

已更正，并在文中附带了该 issue 讨论链接