MutationObserver is reading old React State - reactjs

Question

I'm attempting to use a MutationObserver with the Zoom Web SDK to watch for changes in who the active speaker is. I declare a state variable using useState called participants which is meant to hold the information about each participant in the Zoom call.
My MutationObserver only seems to be reading the initial value of participants, leading me to believe the variable is bound/evaluated rather than read dynamically. Is there a way to use MutationObserver with React useState such that the MutationCallback reads state that is dynamically updating?
const [participants, setParticipants] = useState({});
...
const onSpeechMutation = (mutations) => {
mutations.forEach((mutation) => {
// identify name of speaker
if(name in participants) {
// do something
} else {
setParticipants({
...participants,
[name] : initializeParticipant(name)
})
}
})
}
...
useEffect(() => {
if(!speechObserverOn) {
setSpeechObserverOn(true);
const speechObserver = new MutationObserver(onSpeechMutation);
const speechConfig = {
attributes: true,
attributeOldValue: true,
attributeFilter: ['class'],
subtree: true,
}
const participantsList = document.querySelector('.participants-selector');
if(participantsList) {
speechObserver.observe(participantsList, speechConfig);
}
}
}, [speechObserverOn])

Answer 1

If you are dealing with stale state enclosures in callbacks then generally the solution is to use functional state updates so you are updating from the previous state and not what is closed over in any callback scope.
const onSpeechMutation = (mutations) => {
mutations.forEach((mutation) => {
// identify name of speaker
if (name in participants) {
// do something
} else {
setParticipants(participants => ({
...participants, // <-- copy previous state
[name]: initializeParticipant(name)
}));
}
})
};
Also, ensure to include a dependency array for the useEffect hook unless you really want the effect to trigger upon each and every render cycle. I am guessing you don't want more than one MutationObserver at-a-time.
useEffect(() => {
if(!speechObserverOn) {
setSpeechObserverOn(true);
const speechObserver = new MutationObserver(onSpeechMutation);
const speechConfig = {
attributes: true,
attributeOldValue: true,
attributeFilter: ['class'],
subtree: true,
}
const participantsList = document.querySelector('.participants-selector');
if(participantsList) {
speechObserver.observe(participantsList, speechConfig);
}
}
}, []); // <-- empty dependency array to run once on component mount
Update
The issue is that if (name in participants) always returns false
because participants is stale
For this a good trick is to use a React ref to cache a copy of the current state value so any callbacks can access the state value via the ref.
Example:
const [participants, setParticipants] = useState([.....]);
const participantsRef = useRef(participants);
useEffect(() => {
participantsRef.current = participants;
}, [participants]);
...
const onSpeechMutation = (mutations) => {
mutations.forEach((mutation) => {
// identify name of speaker
if (name in participantsRef.current) {
// do something
} else {
setParticipants(participants => ({
...participants,
[name]: initializeParticipant(name)
}));
}
})
};