Function ɵɵdeferPrefetchOnViewport

packages/core/src/defer/instructions.ts:809–849

(
  triggerIndex: number,
  walkUpTimes?: number | null,
  options?: IntersectionObserverInit,
)

 * @codeGenApi
 */
export function ɵɵdeferPrefetchOnViewport(
  triggerIndex: number,
  walkUpTimes?: number | null,
  options?: IntersectionObserverInit,
) {
  const lView = getLView();
  const tNode = getCurrentTNode()!;

  if (ngDevMode) {
    const args: string[] = [];
    if (walkUpTimes !== undefined && walkUpTimes !== -1) {
      args.push('<target>');
    }
    if (options) {
      args.push(JSON.stringify(options));
    }
    trackTriggerForDebugging(
      lView[TVIEW],
      tNode,
      `prefetch on viewport${args.length === 0 ? '' : `(${args.join(', ')})`}`,
    );
  }

  if (!shouldAttachTrigger(TriggerType.Prefetch, lView, tNode)) return;

  const tView = lView[TVIEW];
  const tDetails = getTDeferBlockDetails(tView, tNode);

  if (tDetails.loadingState === DeferDependenciesLoadingState.NOT_STARTED) {
    registerDomTrigger(
      lView,
      tNode,
      triggerIndex,
      walkUpTimes,
      onViewportWrapper,
      () => triggerPrefetching(tDetails, lView, tNode),
      TriggerType.Prefetch,
      options,
    );
  }
}

/**
 * Creates runtime data structures for the `on viewport` hydrate trigger.