Commits on Nov 17, 2022

1. Force unwind work loop during selective hydration …

   When an update flows into a dehydrated boundary, React cannot apply the
   update until the boundary has finished hydrating. The way this currently
   works is by scheduling a slightly higher priority task on the boundary,
   using a special lane that's reserved only for this purpose. Because
   the task is slightly higher priority, on the next turn of the work loop,
   the Scheduler will force the work loop to yield (i.e. shouldYield starts
   returning `true` because there's a higher priority task).
   
   The downside of this approach is that it only works when time slicing
   is enabled. It doesn't work for synchronous updates, because the
   synchronous work loop does not consult the Scheduler on each iteration.
   
   We plan to add support for selective hydration during synchronous
   updates, too, so we need to model this some other way.
   
   I've added a special internal exception that can be thrown to force the
   work loop to interrupt the work-in-progress tree. Because it's thrown
   from a React-only execution stack, throwing isn't strictly necessary —
   we could instead modify some internal work loop state. But using an
   exception means we don't need to check for this case on every iteration
   of the work loop. So doing it this way moves the check out of the
   fast path.
   
   The ideal implementation wouldn't need to unwind the stack at all — we
   should be able to hydrate the subtree and then apply the update all 
   within a single render phase. This is how we intend to implement it in
   the future, but this requires a refactor to how we handle "stack"
   variables, which are currently pushed to a per-render array. We need to
   make this stack resumable, like how context works in Flight and Fizz.

   

   acdlite committed Nov 17, 2022
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