stacker.news/components/use-live-comments.js

import preserveScroll from './preserve-scroll'
import { GET_NEW_COMMENTS } from '../fragments/comments'
import { useEffect, useState } from 'react'
import { SSR, COMMENT_DEPTH_LIMIT } from '../lib/constants'
import { useQuery, useApolloClient } from '@apollo/client'
import { commentsViewedAfterComment } from '../lib/new-comments'
import {
  updateItemQuery,
  updateCommentFragment,
  getLatestCommentCreatedAt,
  updateAncestorsCommentCount,
  calculateDepth
} from '../lib/comments'

const POLL_INTERVAL = 1000 * 5 // 5 seconds

// prepares and creates a fragment for injection into the cache
// also handles side effects like updating comment counts and viewedAt timestamps
function prepareComments (item, cache, newComment) {
  const existingComments = item.comments?.comments || []

  // is the incoming new comment already in item's existing comments?
  // if so, we don't need to update the cache
  if (existingComments.some(comment => comment.id === newComment.id)) return item

  // count the new comment (+1) and its children (+ncomments)
  const totalNComments = newComment.ncomments + 1

  const itemHierarchy = item.path.split('.')
  // update all ancestors comment count, but not the item itself
  const ancestors = itemHierarchy.slice(0, -1)
  updateAncestorsCommentCount(cache, ancestors, totalNComments)
  // update commentsViewedAt to now, and add the number of new comments
  const rootId = itemHierarchy[0]
  commentsViewedAfterComment(rootId, Date.now(), totalNComments)

  // add a flag to the new comment to indicate it was injected
  const injectedComment = { ...newComment, injected: true }

  // an item can either have a comments.comments field, or not
  const payload = item.comments
    ? {
        ...item,
        ncomments: item.ncomments + totalNComments,
        comments: {
          ...item.comments,
          comments: [injectedComment, ...item.comments.comments]
        }
      }
    // when the fragment doesn't have a comments field, we just update stats fields
    : {
        ...item,
        ncomments: item.ncomments + totalNComments
      }

  return payload
}

function cacheNewComments (cache, rootId, newComments, sort) {
  for (const newComment of newComments) {
    const { parentId } = newComment
    const topLevel = Number(parentId) === Number(rootId)

    // if the comment is a top level comment, update the item, else update the parent comment
    if (topLevel) {
      updateItemQuery(cache, rootId, sort, (item) => prepareComments(item, cache, newComment))
    } else {
      // if the comment is too deep, we can skip it
      const depth = calculateDepth(newComment.path, rootId, parentId)
      if (depth > COMMENT_DEPTH_LIMIT) continue
      // inject the new comment into the parent comment's comments field
      updateCommentFragment(cache, parentId, (parent) => prepareComments(parent, cache, newComment))
    }
  }
}

// useLiveComments fetches new comments under an item (rootId),
// that are newer than the latest comment createdAt (after), and injects them into the cache.
export default function useLiveComments (rootId, after, sort) {
  const latestKey = `liveCommentsLatest:${rootId}`
  const { cache } = useApolloClient()
  const [latest, setLatest] = useState(after)
  const [initialized, setInitialized] = useState(false)

  useEffect(() => {
    if (typeof window !== 'undefined') {
      const storedLatest = window.sessionStorage.getItem(latestKey)
      if (storedLatest && storedLatest > after) {
        setLatest(storedLatest)
      } else {
        setLatest(after)
      }
    }

    // Apollo might update the cache before the page has fully rendered, causing reads of stale cached data
    // this prevents GET_NEW_COMMENTS from producing results before the page has fully rendered
    setInitialized(true)
  }, [after])

  const { data } = useQuery(GET_NEW_COMMENTS, SSR || !initialized
    ? {}
    : {
        pollInterval: POLL_INTERVAL,
        // only get comments newer than the passed latest timestamp
        variables: { rootId, after: latest },
        nextFetchPolicy: 'cache-and-network'
      })

  useEffect(() => {
    if (!data?.newComments?.comments?.length) return

    // directly inject new comments into the cache, preserving scroll position
    // quirk: scroll is preserved even if we are not injecting new comments due to dedupe
    preserveScroll(() => cacheNewComments(cache, rootId, data.newComments.comments, sort))

    // update latest timestamp to the latest comment created at
    // save it to session storage, to persist between client-side navigations
    const newLatest = getLatestCommentCreatedAt(data.newComments.comments, latest)
    setLatest(newLatest)
    if (typeof window !== 'undefined') {
      window.sessionStorage.setItem(latestKey, newLatest)
    }
  }, [data, cache, rootId, sort, latest])
}