Function observe

src/observe.ts:118–171

(
  element: Element,
  callback: ObserverInstanceCallback,
  options: IntersectionObserverInit = {},
  fallbackInView = unsupportedValue,
)

 * @return Function - Cleanup function that should be triggered to unregister the observer
 */
export function observe(
  element: Element,
  callback: ObserverInstanceCallback,
  options: IntersectionObserverInit = {},
  fallbackInView = unsupportedValue,
) {
  if (
    typeof window.IntersectionObserver === "undefined" &&
    fallbackInView !== undefined
  ) {
    const bounds = element.getBoundingClientRect();
    callback(fallbackInView, {
      isIntersecting: fallbackInView,
      target: element,
      intersectionRatio:
        typeof options.threshold === "number" ? options.threshold : 0,
      time: 0,
      boundingClientRect: bounds,
      intersectionRect: bounds,
      rootBounds: bounds,
    });
    return () => {
      // Nothing to cleanup
    };
  }
  // An observer with the same options can be reused, so lets use this fact
  const { id, observer, elements } = createObserver(options);

  // Register the callback listener for this element
  const callbacks = elements.get(element) || [];
  if (!elements.has(element)) {
    elements.set(element, callbacks);
  }

  callbacks.push(callback);
  observer.observe(element);

  return function unobserve() {
    // Remove the callback from the callback list
    callbacks.splice(callbacks.indexOf(callback), 1);

    if (callbacks.length === 0) {
      // No more callback exists for element, so destroy it
      elements.delete(element);
      observer.unobserve(element);
    }

    if (elements.size === 0) {
      // No more elements are being observer by this instance, so destroy it
      observer.disconnect();
      observerMap.delete(id);
    }
  };
}