PoC for preloading responsive images for blocks

assets/src/blocks/CarouselHeader/SlideBackground.js

@@ -1,11 +1,41 @@
+function parseWidths(srcset) {
+  const entries = srcset.split(',').map(entry => entry.trim());
+  const widths = entries.map(entry => {
+    const width = entry.split(' ')[1];
+    return width.replace('w', 'px');
+  });
+  return widths;
+}
+
+function createSizesAttribute(widths) {
+  return widths.map(width => {
+    return `(max-width: ${parseInt(width)}px) ${width}`;
+  }).join(', ');
+}
+
 export const SlideBackground = ({slide, decoding}) => {
   const {image_url, image_srcset, focal_points, image_alt} = slide;
+  const image_widths = parseWidths(image_srcset);
+  const image_sizes = createSizesAttribute(image_widths);
+
+  // Preload the images
+  if (image_srcset && image_sizes) {
+    const preloadLink = document.createElement('link');
+    preloadLink.rel = 'preload';
+    preloadLink.as = 'image';
+    preloadLink.href = image_url;
+    preloadLink.imagesrcset = image_srcset;
+    preloadLink.imagesizes = image_sizes.concat(', 100vw');
+    document.head.appendChild(preloadLink);
+  }
+
   return (
     <div className="background-holder">
       <img
         src={image_url}
         style={{objectPosition: `${(focal_points?.x || .5) * 100}% ${(focal_points?.y || .5) * 100}%`}}
         srcSet={image_srcset}
+        sizes={image_sizes}
         alt={image_alt}
         {...decoding ? {decoding: 'async'} : {}}
       />