滚动贴靠实时反馈

Web 功能： 滚动贴靠事件 滚动贴靠

概览

用户在与轮播界面或图库等界面元素互动时，希望获得即时的视觉反馈。传统的滚动贴靠仅在滚动手势完成且元素稳定后提供反馈。通过使用滚动贴靠事件（特别是 scrollsnapchanging），您可以在滚动操作期间提供实时反馈，在用户松开触控或鼠标之前突出显示待贴靠的目标。

实现

1. 您可以说scrollsnapchanging

将 scrollsnapchanging 的事件监听器附加到滚动容器。当浏览器确定可能会选择新的贴靠目标时，就会触发此事件。

const container = document.querySelector('#gallery');
const thumbnails = document.querySelectorAll('.thumbnail');
const items = document.querySelectorAll('.gallery-item');

container.addEventListener('scrollsnapchanging', (event) => {
  // Highlight pending snap target during scroll for real-time feedback.
  const pendingTarget = event.snapTargetInline;
  const index = [...items].indexOf(pendingTarget);

  if (index === -1 || !thumbnails[index]) return;

  // Use lightweight class toggle to avoid layout thrashing during rapid events.
  // Note: aria-current is NOT toggled here. It tracks the settled "current"
  // item, which is updated in the scrollsnapchange handler below.
  thumbnails.forEach((thumb) => thumb.classList.remove('pending'));
  thumbnails[index].classList.add('pending');
});

此示例使用 snapTargetInline，因为图库会水平滚动。如果滚动容器垂直滚动，请改用 snapTargetBlock。

2. 您可以说scrollsnapchange

如需在滚动动作完成且元素实际贴靠时最终确定状态，请监听 scrollsnapchange 事件。这是确定最终活跃状态所必需的。

container.addEventListener('scrollsnapchange', (event) => {
  // Promote pending state to active on scroll completion.
  const snappedTarget = event.snapTargetInline;
  const index = [...items].indexOf(snappedTarget);

  if (index === -1 || !thumbnails[index]) return;

  // Establish final active state and clean up pending.
  thumbnails.forEach((thumb) => {
    thumb.classList.remove('pending', 'active');
    thumb.removeAttribute('aria-current');
  });
  thumbnails[index].classList.add('active');
  thumbnails[index].setAttribute('aria-current', 'true');
});

3. 同步初始状态

网页加载时，浏览器可能会恢复滚动位置（例如，通过历史记录遍历或锚链接）。scrollsnapchange 和 scroll 事件都不会自动触发。运行一次性几何检查，使界面与初始滚动位置保持同步。

// Note: For item.offsetLeft to be relative to the container, 
// the container MUST be the offsetParent (e.g., `position: relative`).
const findClosestItemIndex = () => {
  // Center-distance assumes scroll-snap-align: center on items.
  // For start-aligned snap, compare scrollLeft to item.offsetLeft directly.
  const containerCenter = container.scrollLeft + container.clientWidth / 2;
  let closestIndex = 0;
  let minDistance = Infinity;

  items.forEach((item, index) => {
    const itemCenter = item.offsetLeft + item.offsetWidth / 2;
    const distance = Math.abs(containerCenter - itemCenter);
    if (distance < minDistance) {
      minDistance = distance;
      closestIndex = index;
    }
  });
  return closestIndex;
};

const initActiveItem = () => {
  const closestIndex = findClosestItemIndex();
  if (!thumbnails[closestIndex]) return;

  thumbnails.forEach((thumb) => {
    thumb.classList.remove('pending', 'active');
    thumb.removeAttribute('aria-current');
  });
  thumbnails[closestIndex].classList.add('active');
  thumbnails[closestIndex].setAttribute('aria-current', 'true');
};

if (document.readyState === 'complete') {
  initActiveItem();
} else {
  window.addEventListener('load', initActiveItem, { once: true });
}

后备策略

对于不支持 scrollsnapchanging 的浏览器，默认情况下，界面不会在滚动滑动手势期间提供即时反馈，并且关联的界面会与内容不同步。

强制性要求：为不支持此功能的浏览器提供后备方案，否则关联的界面将与内容不同步。

务必使用 scroll 事件监听器以及 requestAnimationFrame 和几何距离计算来模拟 scrollsnapchanging 的实时特性，以便在用户主动滚动时确定最接近的贴靠目标。

if ('onscrollsnapchanging' in Element.prototype) {
  // Use native scroll snap events
} else {
  // Fallback: use scroll + requestAnimationFrame for eager feedback
  // (assumes the same container, thumbnails, items defined in step 1)
  let scrollTimeout;
  let rafId = null;

  const promotePendingToActive = () => {
    const closestIndex = findClosestItemIndex();
    if (!thumbnails[closestIndex]) return;
    thumbnails.forEach((thumb) => {
      thumb.classList.remove('pending', 'active');
      thumb.removeAttribute('aria-current');
    });
    thumbnails[closestIndex].classList.add('active');
    thumbnails[closestIndex].setAttribute('aria-current', 'true');
  };

  container.addEventListener('scroll', () => {
    if (rafId) return;
    rafId = requestAnimationFrame(() => {
      rafId = null;
      const closestIndex = findClosestItemIndex();
      if (!thumbnails[closestIndex]) return;

      // DO NOT forget to clean up stale pending classes
      thumbnails.forEach((thumb) => thumb.classList.remove('pending'));
      thumbnails[closestIndex].classList.add('pending');
    });

    // Debounce fallback for browsers that don't support scrollend
    clearTimeout(scrollTimeout);
    scrollTimeout = setTimeout(promotePendingToActive, 100);
  }, { passive: true });

  // Fallback: use Baseline `scrollend` event to promote pending to active cleanly where supported
  container.addEventListener('scrollend', () => {
    clearTimeout(scrollTimeout);
    promotePendingToActive();
  });
}

几何 scroll + requestAnimationFrame 后备方案可密切模拟原生贴靠预测的行为，包括正确处理程序化滚动。由于 scrollIntoView 等函数在执行期间会自然触发 scroll 事件，因此界面将在整个滚动动画中保持流畅同步，而无需额外的自定义逻辑。