[RFC] ASResponderChainEnumerator threading requirement

[nguyenhuy]

As reported in #341, ASResponderChainEnumerator can sometimes be initiated off the main thread, which seems to be expected base on the comment here. However, a couple of main thread assertions were added recently here.

I think ideally we should remove the assertions if we know -[UIResponder nextResponder] is thread-safe, and the new Main Thread Checker might give us the answer. Otherwise, we sadly have to avoid printing the view controller if it's off main.

[Adlai-Holler]

Unfortunately -[UIView nextResponder] is implemented as [self superview] which is main-thread only, so responder chain traversing is main-thread-only. I think #348 should make things tolerable.

[Adlai-Holler]

@nguyenhuy Feel absolutely free to reopen this but I believe #348 gives us as much flexibility as safely possible.

[nguyenhuy]

@Adlai-Holler So I'm hitting this complaint from the Main Thread checker while running our test suite :(