This refactoring basically splits off two functions from update_unread_counts(),
which then becomes a simple three-liner.
The function get_unread_counts() is extracted, and it's purely functional
computation. It paves the way for a more pull-based approach to getting "unread"
counts, where other parts of the program can just call it to get values as
needed without worrying about side effects. It is staying in zephyr.js for
now.
The other function is stream_list.update_dom_with_unread_counts(), which
has a new home in stream_list.js. It handles all the DOM manipulation
aspect of unread counts in the left pane, mostly by delegating to smaller
functions within stream_list. Some of those smaller functions can now
be turned into private methods FWIW, but I'm not sure it's worth the
trouble.
(imported from commit 799f9ebbaed8d530829a4741ef14be04bd8abf5a)
The optimizations are:
* Sort over the list of subscriptions instead of the DOM li elements.
This requires storing the li elements for each sub on the sub object.
* Do a bulk insert of the li elements instead of doing them one by one.
(imported from commit 1a987799930fc677e25f0bc2dcf66f83a4ac3163)
