class PreloadObserver {
  started = false
  attributeName = "data-turbo-preload"

  constructor(delegate, element) {
    this.delegate = delegate
    this.element = element
    this.mutationObserver = new MutationObserver(this.#notifyDelegateOfMutations)
  }

  start() {
    if (!this.started) {
      this.mutationObserver.observe(this.element, { attributeFilter: [this.attributeName], subtree: true })

      for (const element of this.element.querySelectorAll(`[${this.attributeName}]`)) {
        this.#notifyDelegate(element)
      }
  
      this.started = true
    }
  }

  stop() {
    if (this.started) {
      this.mutationObserver.disconnect()
      this.started = false
    }
  }

  #notifyDelegateOfMutations = (mutationList) => {
    for (const { target } of mutationList) {
       this.#notifyDelegate(target)
    }
  }

  #notifyDelegate = (element) => {
    if (element instanceof HTMLAnchorElement) {
      // this is a bad method name, but communicates the point
      this.delegate.canPreloadAnchor(element)
    }
  }
}

export class Session {
  // ...
  preloadObserver = new PreloadObserver(this, document.body)

  start() {
    // ...
    this.preloadObserver.start()
    // ...
  }

  stop() {
    // ...
    this.preloadObserver.stop()
    // ...
  }

  // Preload observer delegate

  canPreloadAnchor(anchor) {
    this.preloader.preloadURL(anchor)
  }
}