Commits on Aug 4, 2022

1. Add API to force Scheduler to yield for macrotask …

   We need a way to yield control of the main thread and schedule a
   continuation in a separate macrotask. (This is related to some Suspense
   optimizations we have planned.)
   
   Our solution needs account for how Scheduler is implemented. Scheduler
   tasks are not 1:1 with real browser macrotasks — many Scheduler "tasks"
   can be executed within a single browser task. If a Scheduler task yields
   control and posts a continuation, but there's still time left in the
   frame, Scheduler will execute the continuation immediately
   (synchronously) without yielding control back to the main thread. That's
   not what we want — we want to schedule a new macrotask regardless of
   where we are in the browser's render cycle.
   
   There are several ways we could approach this. What I ended up doing was
   adding a new Scheduler method `unstable_requestYield`. (It's similar to
   the existing `unstable_requestPaint` that we use to yield at the end of
   the frame.)
   
   It works by setting the internal start time of the current work loop to
   a large negative number, so that when the `shouldYield` call computes
   how much time has elapsed, it's guaranteed to exceed the deadline. The
   advantage of doing it this way is that there are no additional checks in
   the normal hot path of the work loop.
   
   The existing layering between Scheduler and React DOM is not ideal. None
   of the APIs are public, so despite the fact that Scheduler is a separate
   package, I consider that a private implementation detail, and think of
   them as part of the same unit.
   
   So for now, though, I think it makes sense to implement this macrotask
   logic directly inside of Scheduler instead of layering it on top.
   
   The rough eventual plan for Scheduler is turn it into a `postTask`
   prollyfill. Because `postTask` does not yet have an equivalent for
   `shouldYield`, we would split that out into its own layer, perhaps
   directly inside the reconciler. In that world, the macrotask logic I've
   added in this commit would likely live in that same layer. When the
   native `postTask` is available, we may not even need any additional
   logic because it uses actual browser tasks.

   

   acdlite committed Aug 4, 2022
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