Add an image gallery shortcode

assets/js/gallery.js

@@ -0,0 +1,116 @@
+// Gallery shortcode: opens the full image in a lightbox (a modal <dialog>) with its alt text
+// and credit. Arrow buttons, arrow keys and swiping move between the images of the gallery;
+// Escape, the close button or a click beside the image close it.
+{
+  const button = (className, label, symbol) => {
+    const element = document.createElement('button');
+    element.type = 'button';
+    element.className = `lightbox__button ${className}`;
+    element.setAttribute('aria-label', label);
+    element.innerHTML = `<span aria-hidden="true">${symbol}</span>`;
+    return element;
+  };
+
+  const lightbox = (gallery) => {
+    const links = [...gallery.querySelectorAll('.gallery__link')];
+    const labels = gallery.dataset;
+    let index = 0;
+
+    const dialog = document.createElement('dialog');
+    dialog.className = 'lightbox';
+    dialog.setAttribute('aria-label', labels.labelDialog);
+    dialog.innerHTML = `
+      <figure class="lightbox__figure">
+        <img class="lightbox__image" alt="">
+        <figcaption class="lightbox__caption" aria-live="polite">
+          <span class="lightbox__alt"></span>
+          <span class="lightbox__credit"></span>
+          <span class="lightbox__counter"></span>
+        </figcaption>
+      </figure>`;
+    const close = button('lightbox__close', labels.labelClose, '×');
+    const previous = button('lightbox__previous', labels.labelPrevious, '‹');
+    const next = button('lightbox__next', labels.labelNext, '›');
+    dialog.append(close);
+    if (links.length > 1) dialog.append(previous, next);
+    document.body.append(dialog);
+
+    const image = dialog.querySelector('.lightbox__image');
+    const preload = (link) => {
+      new Image().src = link.href;
+    };
+
+    const show = (newIndex) => {
+      index = (newIndex + links.length) % links.length;
+      const link = links[index];
+      image.src = link.href;
+      image.width = link.dataset.width;
+      image.height = link.dataset.height;
+      image.alt = link.dataset.alt;
+      // The alt text is shown as the caption, so screen readers get it once, from the image.
+      dialog.querySelector('.lightbox__alt').textContent = link.dataset.alt;
+      dialog.querySelector('.lightbox__alt').setAttribute('aria-hidden', 'true');
+      dialog.querySelector('.lightbox__credit').textContent = link.dataset.credit ?? '';
+      dialog.querySelector('.lightbox__counter').textContent =
+        links.length > 1 ? `${index + 1} / ${links.length}` : '';
+      if (links.length > 1) {
+        preload(links[(index + 1) % links.length]);
+        preload(links[(index - 1 + links.length) % links.length]);
+      }
+    };
+
+    links.forEach((link, i) => {
+      link.addEventListener('click', (event) => {
+        // Let modified clicks open the image in a new tab or window as usual.
+        if (event.button !== 0 || event.metaKey || event.ctrlKey || event.shiftKey || event.altKey) return;
+        event.preventDefault();
+        show(i);
+        dialog.showModal();
+        close.focus();
+      });
+    });
+
+    // Closing goes through here, so that focus goes back to the thumbnail of the image that was
+    // shown (the browser would restore it to where it was before the lightbox opened).
+    const hide = () => {
+      dialog.close();
+      links[index].focus();
+    };
+
+    close.addEventListener('click', hide);
+    // Escape, or the back gesture on Android.
+    dialog.addEventListener('cancel', (event) => {
+      event.preventDefault();
+      hide();
+    });
+    previous.addEventListener('click', () => show(index - 1));
+    next.addEventListener('click', () => show(index + 1));
+
+    // A click on the dark area around the image closes the lightbox.
+    dialog.addEventListener('click', (event) => {
+      if (event.target === dialog || event.target.classList.contains('lightbox__figure')) hide();
+    });
+
+    dialog.addEventListener('keydown', (event) => {
+      if (links.length < 2) return;
+      if (event.key === 'ArrowLeft') show(index - 1);
+      else if (event.key === 'ArrowRight') show(index + 1);
+      else return;
+      event.preventDefault();
+    });
+
+    // Swiping left or right on touch screens.
+    let startX = null;
+    dialog.addEventListener('pointerdown', (event) => {
+      if (event.pointerType !== 'mouse') startX = event.clientX;
+    });
+    dialog.addEventListener('pointerup', (event) => {
+      if (startX === null || links.length < 2) return;
+      const distance = event.clientX - startX;
+      startX = null;
+      if (Math.abs(distance) > 50) show(index + (distance < 0 ? 1 : -1));
+    });
+  };
+
+  document.querySelectorAll('[data-gallery]').forEach(lightbox);
+}