Function ɵɵdeferOnViewport

packages/core/src/defer/instructions.ts:761–801

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

 * @codeGenApi
 */
export function ɵɵdeferOnViewport(
  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,
      `on viewport${args.length === 0 ? '' : `(${args.join(', ')})`}`,
    );
  }

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

  renderPlaceholder(lView, tNode);

  // Avoid adding event listeners when this instruction is invoked on the server.
  if (!(typeof ngServerMode !== 'undefined' && ngServerMode)) {
    registerDomTrigger(
      lView,
      tNode,
      triggerIndex,
      walkUpTimes,
      onViewportWrapper,
      () => triggerDeferBlock(TriggerType.Regular, lView, tNode),
      TriggerType.Regular,
      options,
    );
  }
}

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