refactor(core): optimize image loading

- JavaScript runs so fast that LQIP will never be detected
- Increase the running priority of image processing in posts

Enhancement for #1267

_javascript/modules/components/img-loading.js

@@ -2,30 +2,60 @@
  * Setting up image lazy loading and LQIP switching
  */
 
+const ATTR_DATA_SRC = 'data-src';
+const ATTR_DATA_LQIP = 'data-lqip';
+const C_SHIMMER = 'shimmer';
+const C_BLUR = 'blur';
+
+function handleImage() {
+  const $img = $(this);
+
+  if (this.hasAttribute(ATTR_DATA_LQIP) && this.complete) {
+    $img.parent().removeClass(C_BLUR);
+  } else {
+    $img.parent().removeClass(C_SHIMMER);
+  }
+}
+
+/* Switch LQIP with real image url */
+function switchLQIP(img) {
+  // Sometimes loaded from cache without 'data-src'
+  if (img.hasAttribute(ATTR_DATA_SRC)) {
+    const $img = $(img);
+    const dataSrc = $img.attr(ATTR_DATA_SRC);
+    $img.attr('src', encodeURI(dataSrc));
+  }
+}
+
 export function loadImg() {
-  const $images = $('main img[loading="lazy"]');
-  const $lqip = $('main img[data-lqip="true"]');
+  const $images = $('article img');
 
-  if ($images.length > 0) {
-    $images.on('load', function () {
-      /* Stop shimmer when image loaded */
-      $(this).parent().removeClass('shimmer');
-    });
+  if ($images.length) {
+    $images.on('load', handleImage);
+  }
 
-    $images.each(function () {
-      /* Images loaded from the browser cache do not trigger the 'load' event */
-      if ($(this).prop('complete')) {
-        $(this).parent().removeClass('shimmer');
+  /* Images loaded from the browser cache do not trigger the 'load' event */
+  $('article img[loading="lazy"]').each(function () {
+    if (this.complete) {
+      $(this).parent().removeClass(C_SHIMMER);
+    }
+  });
+
+  const $lqips = $(`article img[${ATTR_DATA_LQIP}="true"]`);
+
+  if ($lqips.length) {
+    const isHome = $('#post-list').length > 0;
+
+    $lqips.each(function () {
+      if (isHome) {
+        // JavaScript runs so fast that LQIPs in home page will never be detected
+        // Delay 50ms to ensure LQIPs visibility
+        setTimeout(() => {
+          switchLQIP(this);
+        }, 50);
+      } else {
+        switchLQIP(this);
       }
     });
   }
-
-  if ($lqip.length > 0) {
-    $lqip.each(function () {
-      /* Switch LQIP with real image url */
-      const dataSrc = $(this).attr('data-src');
-      $(this).attr('src', encodeURI(dataSrc));
-      $(this).removeAttr('data-src data-lqip');
-    });
-  }
 }