Function onViewport

packages/core/primitives/defer/src/triggers.ts:150–211

(
  trigger: Element,
  callback: VoidFunction,
  observerFactoryFn: (options?: IntersectionObserverInit) => IntersectionObserver,
  options?: IntersectionObserverInit,
)

 * intersectionObserver to null if there are no more Elements to observe
 */
export function onViewport(
  trigger: Element,
  callback: VoidFunction,
  observerFactoryFn: (options?: IntersectionObserverInit) => IntersectionObserver,
  options?: IntersectionObserverInit,
): VoidFunction {
  const key = getIntersectionObserverKey(options);
  let entry = viewportTriggers.get(trigger)?.get(key);

  if (!intersectionObservers.has(key)) {
    intersectionObservers.set(key, {observer: observerFactoryFn(options), count: 0});
  }

  const config = intersectionObservers.get(key)!;

  if (!entry) {
    entry = new DeferEventEntry();
    config.observer.observe(trigger);

    let triggerConfig = viewportTriggers.get(trigger);

    if (triggerConfig) {
      triggerConfig.set(key, entry);
    } else {
      triggerConfig = new Map();
      viewportTriggers.set(trigger, triggerConfig);
    }

    triggerConfig.set(key, entry);
    config.count++;
  }

  entry.callbacks.add(callback);

  return () => {
    if (!viewportTriggers.get(trigger)?.has(key)) {
      return;
    }

    entry!.callbacks.delete(callback);

    if (entry!.callbacks.size === 0) {
      config.observer.unobserve(trigger);
      config.count--;

      const triggerConfig = viewportTriggers.get(trigger);

      if (triggerConfig) {
        triggerConfig.delete(key);

        if (triggerConfig.size === 0) {
          viewportTriggers.delete(trigger);
        }
      }
    }

    if (config.count === 0) {
      config.observer.disconnect();