HEY's Stimulus Pagination Controller

pagination_controller.js

/*
  ERB template chunk from The Feed's display of emails:

  <section class="postings postings--feed-style" id="postings"
      data-controller="pagination" data-pagination-root-margin-value="40px">
    <%= render partial: "postings/snippet", collection: @page.records, as: :posting, cached: true %>

    <%= link_to(spinner_tag, url_for(page: @page.next_param),
          class: "pagination-link", data: { pagination_target: "nextPageLink", preload: @page.first? }) unless @page.last? %>
  </section>
*/

import { delay, nextFrame, request } from "helpers"

export default class extends ApplicationController {
  static targets = [ "nextPageLink" ]
  static values = { manualLoad: Boolean, rootMargin: String }

  initialize() {
    this.observeNextPageLink()
  }

  async loadNextPage(event) {
    event?.preventDefault()

    this.element.setAttribute("aria-busy", "true")

    const html = await this.loadNextPageHTML()
    await nextFrame()
    this.element.setAttribute("aria-busy", "false")
    this.nextPageLink.outerHTML = html

    await delay(500)
    this.observeNextPageLink()
  }

  // Private

  async observeNextPageLink() {
    if (this.manualLoadValue) return

    await nextFrame()
    const { nextPageLink, intersectionOptions } = this
    if (!nextPageLink) return

    if (nextPageLink.dataset.preload == "true") {
      this.loadNextPage()
    } else {
      await nextIntersection(nextPageLink, intersectionOptions)
      this.loadNextPage()
    }
  }

  async loadNextPageHTML() {
    const html = await request.get(this.nextPageLink.href)
    const doc = new DOMParser().parseFromString(html, "text/html")
    const element = doc.querySelector(`[data-controller~="${this.identifier}"]`)
    return element ? element.innerHTML.trim() : ""
  }

  get nextPageLink() {
    const links = this.nextPageLinkTargets
    if (links.length > 1) console.warn("Multiple next page links", links)
    return links[links.length - 1]
  }

  get intersectionOptions() {
    const options = {
      root: this.scrollableOffsetParent,
      rootMargin: this.rootMarginValue
    }
    for (const [ key, value ] of Object.entries(options)) {
      if (value) continue

      delete options[key]
    }

    return options
  }

  get scrollableOffsetParent() {
    const root = this.element.offsetParent
    return root && root.scrollHeight > root.clientHeight ? root : null
  }
}

function nextIntersection(element, options = {}) {
  return new Promise(resolve => {
    new IntersectionObserver(([ entry ], observer) => {
      if (!entry.isIntersecting) return
      observer.disconnect()
      resolve()
    }, options).observe(element)
  })
}