Commits on Feb 5, 2022

1. Switch back to Subscription in useSelector to fix unsubscribe perf …

   The Subscription impl originally used an array of listeners just
   like the actual Redux store, but in #1523 we changed it to be a
   linked list to fix perf issues. The use of `indexOf+splice` caused
   a quadratic cost to unmounting many components.
   
   In v8, I originally dropped use of `Subscription` to potentially
   save on bundle size. However, `Provider` still uses `Subscription`,
   so that will always be in the bundle.
   
   For this issue, a user pointed out that directly subscribing to the
   store brought back the original quadratic unsubscription behavior,
   which I've confirmed locally.
   
   Swapping `store.subscribe` for `subscription.addNestedSub` fixes
   that issue.
   
   I've added a unit test that mounts and unmounts a massive tree and
   measures the elapsed time. There's a distinct difference between
   the "correct" and quadratic behavior. Weirdly, there's also a big
   diff in "correct" time between React 18, and 17 + the "compat" entry
   point, and I have no idea why.  It's not as bad as the quadratic
   time, but it's still pretty expensive.
   
   I've written the test to set an acceptable max unmount time based on
   which React version we're running against, with some buffer added.

   

   markerikson committed Feb 5, 2022
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