[React 19] useTransition()'s pending state does not go back to false (revision 94eed63c49-20240425)

[alexeyraspopov]

Summary

I am excited to start using React v19 as it has so many features and QoL improvements I've been waiting for!

There is a bug (new bug comparing to v18.2.0) that I found while reproducing https://github.com/facebook/react/issues/26814. When using useTransition() with use(), pending flag of transition correctly becomes true in the beginning, but doesn't go back to false after transition is complete, which means any pending state artifacts in the UI remain visible.

Repository for reproducing: https://github.com/alexeyraspopov/react-beta-bugs.

Basic code (since the repo contains more than just this bug):

function SimpleAsyncFlow() {
  // this state holds instance of a promise that resolves after a second
  // the same `delayed()` function is used to trigger state update inside SimpleControlledDisplay
  let [value, setValue] = useState(() => delayed(Math.random()));
  return (
    <Suspense fallback={<p>Loading</p>}>
      <SimpleControlledDisplay promise={value} onChange={setValue} />
    </Suspense>
  );
}

function SimpleControlledDisplay({
  promise,
  onChange,
}: {
  promise: Promise<number>;
  onChange: (value: Promise<number>) => void;
}) {
  let value = use(promise);
  let [pending, startTransition] = useTransition();
  let click = () => {
    // this will trigger state update in the parent with the new 1 second promise 
    // that suppose to suspend this component, so transition should prevent it
    startTransition(() => {
      onChange(delayed(Math.random()));
    });
  };
  return (
    <div>
      <button onClick={click}>Refresh</button>
      {/* as UI "pending" state I update text style to become half transparent */}
      <p style={{ opacity: pending ? 0.5 : 1 }}>{value}</p>
    </div>
  );
}

function delayed<T>(value: T) {
  return new Promise<T>((resolve) => setTimeout(resolve, 1000, value));
}

Additionally here's the video reproduction, using the code from the repo I mentioned:

https://github.com/facebook/react/assets/858295/47fc5678-6c99-48f7-a55d-8b48b9bcee35

[eps1lon]

Does this reproduce in dev and prod?

[alexeyraspopov]

@eps1lon I made production build via npx vite build and the behavior remains the same.

[edunomatseye]

Maybe not a good idea to pass an async value directly or either through a function into state as this behaviour is synchronous. this could result to state inconsistency and also concurrency issues. looking at the delayed function passed into state.

[nakjun12]

Resolving the useTransition Hook Issue: Following Official Documentation

In my opinion, the issue mentioned above is not an actual issue according to the official React documentation. I resolved the problem using the following method:

Solution:

I moved the useTransition hook to the top level of the component, which resolved the error. (https://react.dev/reference/react/useTransition)

Reason:

The official documentation states: "Call useTransition at the top level of your component to mark some state updates as Transitions."

Conclusion:

When using the useTransition hook, it is essential to declare it at the top level of the component.

[eps1lon]

I moved the useTransition hook to the top level of the component, which resolved the error.

@nakjun12 Can you provide a repro or diff illustrating what you did? From what I can tell, the original repro is valid.

[alexeyraspopov]

@eps1lon I did try doing so in my repro code. In SimpleControlledDisplay I swapped two lines of code moving transition hook to the beginning.

function SimpleControlledDisplay({
  promise,
  onChange,
}: {
  promise: Promise<number>;
  onChange: (value: Promise<number>) => void;
}) {
+  let [pending, startTransition] = useTransition();
  let value = use(promise);
-  let [pending, startTransition] = useTransition();

And the transition worked as expected, resetting pending flag after transition is complete. Not sure if this is expected behavior, certainly very confusing one. I will attempt to test this later with RC.

[nakjun12]

@eps1lon Please refer to the comment above.