Posts: enable polling with leading fetch

[rralian]

I tried this in #2965 but I had to revert before it hit production. It worked fine on mounting, for pagination, and for polling queries, but I had removed the mechanism by which the component triggered a new query with different props. Whoops.

So now I'm using a leading polling query, but I've added some protection against double-fetching. The check in fetchNextPage isn't strictly necessary for my purposes, but seems like a good protection to have in general.

/cc @gwwar

[retrofox]

LGTM.

Just I wonder if here we have a dangerous condition when we're setting the flag used to return the value of postListStore.isFetchingNextPage() method.

Its value by default is false. When we start to fetch data this flag is set to true in the FETCH_NEXT_POSTS_PAGE action.

When the response is here the flag is set to false but it also depends of a if condition above (L135).

TBH I think you've already aware of this situation. It's just my two cents.

[rralian]

Thanks for the review @retrofox.

I think the code you point out is correct. The isFetchingNextPage() method only checks whether we are fetchingNextPage of the _activeList. That check makes sure that a fetch response for a prior list does not clear out the fetchingNextPage value of the current _activeList. E.g., let's say we fire off a fetch for one list, then immediately change to a new _activeList and fire off a request for that second list. When the first response comes back, we don't want to set _activeList.fetchingNextPage to false, because that request for the second list (which is now _activeList) is still in-flight. And therefore we would not want to repeat it.