Atom effect onSet() causing infinite loop

In the atom effects documentation - it is noted that the onSet() listener will not be called due to changes triggered from setSelf()

// The callback is not called due to changes from this effect’s own setSelf().

I seem to be getting an infinite loop when using setSelf() - but maybe I’m missing something important.

I have created a contrived example on codesandbox which adds random numbers and illustrates the behaviour.

Cheers

B

[drarmstr]

Atom effects aren’t intended to act as middleware. They allow you to observe sets, but not intercept and modify a value before it is being set. So, if you observe a set an initiate a new set it can cause a loop. Atom sets are queued, so you can’t guarantee if the effect observation handlers are called before or after other components see the new value, though we reserve the ability to make guarantees and maybe allow this in the future.

If you want a temporary value while something is pending you could use useRecoilCallback():

const startAThink = useRecoilCallback(({set}) => async () => {
  set(myAtom, pendingValue);
  const newValue = await fetchLongRunningThing();
  set(myAtom, newValue);
});

or maybe wrap with a selector

const wrappedState = selector({
  key: 'MyWrappedState',
  get: ({get}) => {
    const longRunningLoadable = get(noWait(longRunningSelector));
    return longRunningLoadable.state === 'hasValue'
      ? longRunningLoadable.contents
      : pendingValue;
  },
});

[justintoman]

That is what will happen if you kick off the long running async process from inside the onSet handler. The value will be eagerly set, and then some async thing will happen.

Although, I’m not sure how error handling is supposed to work. If I update my atom with an invalid state and my server responds with a 400 (or the service is down or messed up and I get a 500), how am I supposed to revert the change back to the previous valid value?