Load JS view iframe only once in viewport (#1607)

assets/js/hooks/js_view.js

@@ -3,7 +3,12 @@ import {
   getAttributeOrThrow,
   parseInteger,
 } from "../lib/attribute";
-import { isElementHidden, randomId, randomToken } from "../lib/utils";
+import {
+  isElementHidden,
+  isElementVisibleInViewport,
+  randomId,
+  randomToken,
+} from "../lib/utils";
 import { globalPubSub } from "../lib/pub_sub";
 import {
   getChannel,
@@ -72,7 +77,7 @@ const JSView = {
 
     this.channel = getChannel(this.props.sessionId, this.props.clientId);
 
-    this.removeIframe = this.createIframe();
+    this.iframeActions = this.createIframe();
 
     // Setup child communication
     this.childReadyPromise = new Promise((resolve, reject) => {
@@ -89,7 +94,9 @@ const JSView = {
     this.hiddenInput.style.display = "none";
     this.el.appendChild(this.hiddenInput);
 
+    this.iframeActions.visibilityPromise.then(() => {
       this.loadIframe();
+    });
 
     // Channel events
 
@@ -154,7 +161,7 @@ const JSView = {
   destroyed() {
     window.removeEventListener("message", this._handleWindowMessage);
 
-    this.removeIframe();
+    this.iframeActions.remove();
 
     this.unsubscribeFromChannelEvents();
     this.channel.push("disconnect", { ref: this.props.ref });
@@ -248,11 +255,36 @@ const JSView = {
       );
     });
 
-    return () => {
+    // We detect when the placeholder enters viewport and becomes visible,
+    // based on that we can load the iframe contents lazily
+
+    let viewportIntersectionObserver = null;
+
+    const visibilityPromise = new Promise((resolve, reject) => {
+      if (isElementVisibleInViewport(this.iframePlaceholder)) {
+        resolve();
+      } else {
+        viewportIntersectionObserver = new IntersectionObserver((entries) => {
+          if (isElementVisibleInViewport(this.iframePlaceholder)) {
+            viewportIntersectionObserver.disconnect();
+            resolve();
+          }
+        });
+        viewportIntersectionObserver.observe(this.iframePlaceholder);
+      }
+    });
+
+    // Cleanup
+
+    const remove = () => {
       resizeObserver.disconnect();
       intersectionObserver.disconnect();
+      viewportIntersectionObserver && viewportIntersectionObserver.disconnect();
       this.iframe.remove();
+      this.iframePlaceholder.remove();
     };
+
+    return { visibilityPromise, remove };
   },
 
   repositionIframe() {

assets/js/lib/utils.js

@@ -21,6 +21,10 @@ export function isElementHidden(element) {
   return element.offsetParent === null;
 }
 
+export function isElementVisibleInViewport(element) {
+  return !isElementHidden(element) && isElementInViewport(element);
+}
+
 export function clamp(n, x, y) {
   return Math.min(Math.max(n, x), y);
 }