Question: Why does navigating appear to change the readyState of the previous EventSources?
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Explanation: I'm working on a frontend (React) in which the user can enter a sequence of search queries (i.e. strings) and for each search query, my backend (Flask) will return a sequence of URLs. For each search query, I've decided to receive the server's response via an EventSource. Specifically, I first create a React state array of backendEventSources:
const [backendEventSources, setBackendEventSources] = useState([]);
Then I update the backendEventSources when a new prompt comes in:
useEffect(() => {
console.log('Inside useEffect')
// Take 0 for the newest prompt.
const newBackendEventSource = new EventSource(
`https://localhost:8080/generate?counter=${promptsResultsArray[0].counter}&prompt=${promptsResultsArray[0].prompt}`,
{withCredentials: false})
newBackendEventSource.addEventListener('open', () => {
console.log('SSE opened!');
});
newBackendEventSource.addEventListener('error', (e) => {
console.log('SSE error!');
console.error('Error: ', e);
});
newBackendEventSource.addEventListener('close', (e) => {
console.log('SSE closed!');
const data = JSON.parse(e.data);
console.log("close data: ", data);
newBackendEventSource.close();
});
newBackendEventSource.addEventListener('message', (e) => {
const data = JSON.parse(e.data);
console.log("message data: ", data);
// https://stackoverflow.com/a/47580775/4570472
const newPromptsResultsArray = [...promptsResultsArray];
// Since we preprend new results, we need to compute the right index from
// the counter with the equation: length - counter - 1.
// e.g., For counter 2 of a length 3 array, we want index 0.
// e.g., For counter 2 of a length 4 array, we want index 1.
// Recall, the counter uses 0-based indexing.
const index = newPromptsResultsArray.length - data.counter - 1
newPromptsResultsArray[index].URIs = [data.uri];
newPromptsResultsArray[index].isLoading = false;
setPromptsResultsArray(newPromptsResultsArray);
// Instantiating the element and setting the src property starts preloading the image.
// for (const newImgURI of newImgURIs) {
// const imageElement = new Image();
// imageElement.src = newImgURI;
// }
// setTimeout(() => {setImgURIs(newImgURIs)}, 8000);
});
// Add new backend event source to state for persistence.
setBackendEventSources(backendEventSources => [
newBackendEventSource,
...backendEventSources])
return () => {
newBackendEventSource.close();
};
}, [prompt]);
I use URL params for React navigation:
const navigateToGenerateResults = (promptString) => {
console.log('Adding new prompt results to promptsResultsArray');
// State doesn't update immediately (or even synchronously). To ensure we can immediately
// access the new values, we create a newPromptsResults.
// https://stackoverflow.com/a/62900445/4570472
const newPromptsResults = {
isLoading: true,
prompt: promptString,
counter: promptsResultsArray.length,
URIs: ["https://simtooreal-public.s3.amazonaws.com/white_background.png"]
}
// Prepend the new prompt to our promptsResultsArray
// https://stackoverflow.com/a/60792858/4570472
setPromptsResultsArray(promptsResultsArray => [
newPromptsResults, ...promptsResultsArray])
console.log('Setting prompt to: ' + newPromptsResults.prompt)
setPrompt(newPromptsResults.prompt)
console.log('Navigating from /generate to /generate with prompt: ' + newPromptsResults.prompt)
navigate(`/generate?counter=${newPromptsResults.counter}&prompt=${newPromptsResults.prompt}`)
}
However, I've discovered that as soon as I navigate from one URL to another, the previous EventSource's ready state switches from 0/1 to 2. Additionally, my newBackendEventSource.addEventListener('close' function is never triggered.
Why does navigating appear to change the readyState of the previous EventSources?
