Add IntersectionObserverWrapper to cut down on re-renders (#3406)

app/javascript/mastodon/features/ui/util/intersection_observer_wrapper.js

@@ -0,0 +1,48 @@
+// Wrapper for IntersectionObserver in order to make working with it
+// a bit easier. We also follow this performance advice:
+// "If you need to observe multiple elements, it is both possible and
+// advised to observe multiple elements using the same IntersectionObserver
+// instance by calling observe() multiple times."
+// https://developers.google.com/web/updates/2016/04/intersectionobserver
+
+class IntersectionObserverWrapper {
+
+  callbacks = {};
+  observerBacklog = [];
+  observer = null;
+
+  connect (options) {
+    const onIntersection = (entries) => {
+      entries.forEach(entry => {
+        const id = entry.target.getAttribute('data-id');
+        if (this.callbacks[id]) {
+          this.callbacks[id](entry);
+        }
+      });
+    };
+
+    this.observer = new IntersectionObserver(onIntersection, options);
+    this.observerBacklog.forEach(([ id, node, callback ]) => {
+      this.observe(id, node, callback);
+    });
+    this.observerBacklog = null;
+  }
+
+  observe (id, node, callback) {
+    if (!this.observer) {
+      this.observerBacklog.push([ id, node, callback ]);
+    } else {
+      this.callbacks[id] = callback;
+      this.observer.observe(node);
+    }
+  }
+
+  disconnect () {
+    if (this.observer) {
+      this.observer.disconnect();
+    }
+  }
+
+}
+
+export default IntersectionObserverWrapper;