Implement lazy image loading (copied from freefall reader)

web/main.ts

@@ -1,5 +1,9 @@
 class VisitAnalytics {
     landmarksObserver: IntersectionObserver;
+    lazyloadObserver: IntersectionObserver;
+    lastWidth: number = 0;
+    resizeTimeout: number | null = null;
+    images: HTMLImageElement[] = [];
 
     constructor() {
         window.plausible = window.plausible || function () { (window.plausible.q = window.plausible.q || []).push(arguments); };
@@ -20,6 +24,14 @@ class VisitAnalytics {
                 threshold: 1.0
             }
         );
+        this.lazyloadObserver = new IntersectionObserver(
+            (entries, observer) => this.handleLazyload(entries, observer),
+            {
+                root: null,
+                rootMargin: "75% 0px 75% 0px",
+                threshold: 0.1
+            }
+        );
 
         {
             var all = document.querySelectorAll(".chapter-announce");
@@ -27,7 +39,18 @@ class VisitAnalytics {
             this.landmarksObserver.observe(document.querySelector("footer"));
         }
 
-        console.log("Intersection observer ready");
+        {
+            var all = document.querySelectorAll(".image-post");
+            all.forEach(elem => {
+                this.lazyloadObserver.observe(elem);
+                this.images.push(elem.querySelector(".post-image > img"));
+            });
+
+            this.resizeTimeout = setTimeout(() => this.resizeDebounced(), 200);
+            window.addEventListener("resize", (event) => this.handleResize(event));
+        }
+
+        console.log("Event handlers and observers ready");
     }
 
     handleLandmarks(entries: IntersectionObserverEntry[], observer: IntersectionObserver) {
@@ -42,5 +65,46 @@ class VisitAnalytics {
             }
         );
     }
+
+    resizeDebounced() {
+        clearTimeout(this.resizeTimeout);
+        this.resizeTimeout = null;
+
+        const elem = document.querySelector(".post-content");
+        // Note: depends on main.css
+        const elemWidth = Math.floor(elem.clientWidth * 0.95);
+
+        if (elemWidth != this.lastWidth) {
+            this.lastWidth = elemWidth;
+            this.images.forEach(img => {
+                const naturalHeight = +img.getAttribute("data-height");
+                const naturalWidth = +img.getAttribute("data-width");
+                img.height = Math.floor(naturalHeight * Math.min(1, elemWidth / naturalWidth));
+            })
+        }
+    }
+
+    handleResize(event) {
+        if (event && event.type == "resize") {
+            if (this.resizeTimeout != null)
+                clearTimeout(this.resizeTimeout);
+            this.resizeTimeout = setTimeout(() => this.resizeDebounced(), 200);
+        }
+    }
+
+    handleLazyload(entries: IntersectionObserverEntry[], observer: IntersectionObserver) {
+        entries.filter(e => e.isIntersecting).forEach(e => {
+                const imgElem = e.target.querySelector<HTMLImageElement>(`img`);
+
+                imgElem.addEventListener("load", () => {
+                    imgElem.classList.add("loaded");
+                });
+                imgElem.src = imgElem.getAttribute("data-src");
+
+                observer.unobserve(e.target);
+                console.log(`Lazyloaded ${ e.target.id ? e.target.id : e.target.tagName }`);
+            }
+        );
+    }
 }
 new VisitAnalytics();