Nexus.Reader/src/NexusReader.UI.Shared/wwwroot/js/readerObserver.js

export function initObserver(dotNetHelper, containerSelector, itemSelector) {
    const options = {
        root: null,
        rootMargin: '0px',
        threshold: 0.6 // 60% of the block must be visible
    };

    const observer = new IntersectionObserver((entries) => {
        entries.forEach(entry => {
            if (entry.isIntersecting) {
                const id = entry.target.id;
                const content = entry.target.innerText;
                dotNetHelper.invokeMethodAsync('HandleBlockReached', id, content);
            }
        });
    }, options);

    const items = document.querySelectorAll(itemSelector);
    items.forEach(item => observer.observe(item));

    return observer;
}

export function initScrollListener(dotNetHelper, scrollContainerSelector) {
    const container = document.querySelector(scrollContainerSelector);
    if (!container) return null;

    let isThrottled = false;

    const onScroll = () => {
        if (isThrottled) return;
        isThrottled = true;

        requestAnimationFrame(() => {
            const scrollTop = container.scrollTop;
            const scrollHeight = container.scrollHeight;
            const clientHeight = container.clientHeight;

            let percentage = 0;
            if (scrollHeight > clientHeight) {
                percentage = Math.round((scrollTop / (scrollHeight - clientHeight)) * 100);
            }

            // Ensure bounds
            percentage = Math.max(0, Math.min(100, percentage));

            dotNetHelper.invokeMethodAsync('HandleScrollPercentChanged', percentage);
            isThrottled = false;
        });
    };

    container.addEventListener('scroll', onScroll, { passive: true });

    // Initial calculation after a brief layout delay
    setTimeout(onScroll, 100);

    return {
        dispose: () => {
            container.removeEventListener('scroll', onScroll);
        }
    };
}