Resolve "Lazy load images on the Frontend"

6 files changed, 106 insertions, 3 deletions

diff --git a/app/assets/javascripts/copy_as_gfm.js b/app/assets/javascripts/copy_as_gfm.js
index ba9d9a3e1f7..54257531284 100644
--- a/app/assets/javascripts/copy_as_gfm.js
+++ b/app/assets/javascripts/copy_as_gfm.js
@@ -1,6 +1,7 @@
 /* eslint-disable class-methods-use-this, object-shorthand, no-unused-vars, no-use-before-define, no-new, max-len, no-restricted-syntax, guard-for-in, no-continue */
 
 import './lib/utils/common_utils';
+import { placeholderImage } from './lazy_loader';
 
 const gfmRules = {
   // The filters referenced in lib/banzai/pipeline/gfm_pipeline.rb convert
@@ -56,6 +57,11 @@ const gfmRules = {
       return text;
     },
   },
+  ImageLazyLoadFilter: {
+    'img'(el, text) {
+      return `![${el.getAttribute('alt')}](${el.getAttribute('src')})`;
+    },
+  },
   VideoLinkFilter: {
     '.video-container'(el) {
       const videoEl = el.querySelector('video');
@@ -163,7 +169,9 @@ const gfmRules = {
       return text.trim().split('\n').map(s => `> ${s}`.trim()).join('\n');
     },
     'img'(el) {
-      return `![${el.getAttribute('alt')}](${el.getAttribute('src')})`;
+      const imageSrc = el.src;
+      const imageUrl = imageSrc && imageSrc !== placeholderImage ? imageSrc : (el.dataset.src || '');
+      return `![${el.getAttribute('alt')}](${imageUrl})`;
     },
     'a.anchor'(el, text) {
       // Don't render a Markdown link for the anchor link inside a heading
diff --git a/app/assets/javascripts/lazy_loader.js b/app/assets/javascripts/lazy_loader.js
new file mode 100644
index 00000000000..3d64b121fa7
--- /dev/null
+++ b/app/assets/javascripts/lazy_loader.js
@@ -0,0 +1,76 @@
+/* eslint-disable one-export, one-var, one-var-declaration-per-line */
+
+import _ from 'underscore';
+
+export const placeholderImage = 'data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==';
+const SCROLL_THRESHOLD = 300;
+
+export default class LazyLoader {
+  constructor(options = {}) {
+    this.lazyImages = [];
+    this.observerNode = options.observerNode || '#content-body';
+
+    const throttledScrollCheck = _.throttle(() => this.scrollCheck(), 300);
+    const debouncedElementsInView = _.debounce(() => this.checkElementsInView(), 300);
+
+    window.addEventListener('scroll', throttledScrollCheck);
+    window.addEventListener('resize', debouncedElementsInView);
+
+    const scrollContainer = options.scrollContainer || window;
+    scrollContainer.addEventListener('load', () => this.loadCheck());
+  }
+  searchLazyImages() {
+    this.lazyImages = [].slice.call(document.querySelectorAll('.lazy'));
+    this.checkElementsInView();
+  }
+  startContentObserver() {
+    const contentNode = document.querySelector(this.observerNode) || document.querySelector('body');
+
+    if (contentNode) {
+      const observer = new MutationObserver(() => this.searchLazyImages());
+
+      observer.observe(contentNode, {
+        childList: true,
+        subtree: true,
+      });
+    }
+  }
+  loadCheck() {
+    this.searchLazyImages();
+    this.startContentObserver();
+  }
+  scrollCheck() {
+    requestAnimationFrame(() => this.checkElementsInView());
+  }
+  checkElementsInView() {
+    const scrollTop = pageYOffset;
+    const visHeight = scrollTop + innerHeight + SCROLL_THRESHOLD;
+    let imgBoundRect, imgTop, imgBound;
+
+    // Loading Images which are in the current viewport or close to them
+    this.lazyImages = this.lazyImages.filter((selectedImage) => {
+      if (selectedImage.getAttribute('data-src')) {
+        imgBoundRect = selectedImage.getBoundingClientRect();
+
+        imgTop = scrollTop + imgBoundRect.top;
+        imgBound = imgTop + imgBoundRect.height;
+
+        if (scrollTop < imgBound && visHeight > imgTop) {
+          LazyLoader.loadImage(selectedImage);
+          return false;
+        }
+
+        return true;
+      }
+      return false;
+    });
+  }
+  static loadImage(img) {
+    if (img.getAttribute('data-src')) {
+      img.setAttribute('src', img.getAttribute('data-src'));
+      img.removeAttribute('data-src');
+      img.classList.remove('lazy');
+      img.classList.add('js-lazy-loaded');
+    }
+  }
+}
diff --git a/app/assets/javascripts/main.js b/app/assets/javascripts/main.js
index 26c67fb721c..44b502cdab3 100644
--- a/app/assets/javascripts/main.js
+++ b/app/assets/javascripts/main.js
@@ -109,6 +109,7 @@ import './label_manager';
 import './labels';
 import './labels_select';
 import './layout_nav';
+import LazyLoader from './lazy_loader';
 import './line_highlighter';
 import './logo';
 import './member_expiration_date';
@@ -166,6 +167,11 @@ window.addEventListener('load', function onLoad() {
   gl.utils.handleLocationHash();
 }, false);
 
+gl.lazyLoader = new LazyLoader({
+  scrollContainer: window,
+  observerNode: '#content-body'
+});
+
 $(function () {
   var $body = $('body');
   var $document = $(document);
diff --git a/app/assets/stylesheets/framework/avatar.scss b/app/assets/stylesheets/framework/avatar.scss
index 06f7af33f94..0dfa7a31d31 100644
--- a/app/assets/stylesheets/framework/avatar.scss
+++ b/app/assets/stylesheets/framework/avatar.scss
@@ -35,6 +35,8 @@
   width: 40px;
   height: 40px;
   padding: 0;
+  background: $avatar-background;
+  overflow: hidden;
 
   &.avatar-inline {
     float: none;
diff --git a/app/assets/stylesheets/framework/typography.scss b/app/assets/stylesheets/framework/typography.scss
index 8a58c1ed567..befd8133be0 100644
--- a/app/assets/stylesheets/framework/typography.scss
+++ b/app/assets/stylesheets/framework/typography.scss
@@ -11,8 +11,17 @@
   }
 
   img {
-    max-width: 100%;
+    /*max-width: 100%;*/
     margin: 0 0 8px;
+    min-width: 200px;
+    min-height: 100px;
+    background-color: $gray-lightest;
+  }
+
+  img.js-lazy-loaded {
+    min-width: none;
+    min-height: none;
+    background-color: none;
   }
 
   p a:not(.no-attachment-icon) img {
diff --git a/app/assets/stylesheets/framework/variables.scss b/app/assets/stylesheets/framework/variables.scss
index 7016208f624..cf0a1ad57d0 100644
--- a/app/assets/stylesheets/framework/variables.scss
+++ b/app/assets/stylesheets/framework/variables.scss
@@ -379,7 +379,9 @@ $issue-boards-card-shadow: rgba(186, 186, 186, 0.5);
 * Avatar
 */
 $avatar_radius: 50%;
-$avatar-border: $border-color;
+$avatar-border: $gray-normal;
+$avatar-border-hover: $gray-darker;
+$avatar-background: $gray-lightest;
 $gl-avatar-size: 40px;
 
 /*