这会导致保留周期

Question

xCode 5.1, iOS 7.1, ARC Would this cause a retain cycle on topics? I don't think so, as setting event=nil will deallocate the whole tree, so topics will also get deallocated, but I am not sure.

 event -> subEvent -> Topics
  |                     ^
  |_____________________|

Answer 1

Referring to your graphic:

Both 'arrows' go from event to topics. It would be a cycle if the lines/arrows would describe a cycle. (If topics would retain the event.)

The graphic doesn't show a cycle thus there cannot be a retain cycle in there - so no.