fix(infra): memory leak (#9013)

packages/frontend/component/src/utils/index.ts

@@ -1,3 +1,4 @@
+export * from './observe-intersection';
 export * from './observe-resize';
 export { startScopedViewTransition } from './view-transition';
 export * from './with-unit';

packages/frontend/component/src/utils/observe-intersection.ts

@@ -0,0 +1,77 @@
+type ObserveIntersection = {
+  callback: (entity: IntersectionObserverEntry) => void;
+  dispose: () => void;
+};
+
+let _intersectionObserver: IntersectionObserver | null = null;
+const elementsMap = new WeakMap<Element, Array<ObserveIntersection>>();
+
+// for debugging
+if (typeof window !== 'undefined') {
+  (window as any)._intersectionObserverElementsMap = elementsMap;
+}
+
+/**
+ * @internal get or initialize the IntersectionObserver instance
+ */
+const getIntersectionObserver = () =>
+  (_intersectionObserver ??= new IntersectionObserver(entries => {
+    entries.forEach(entry => {
+      const listeners = elementsMap.get(entry.target) ?? [];
+      listeners.forEach(({ callback }) => callback(entry));
+    });
+  }));
+
+/**
+ * @internal remove element's specific listener
+ */
+const removeListener = (element: Element, listener: ObserveIntersection) => {
+  if (!element) return;
+  const listeners = elementsMap.get(element) ?? [];
+  const observer = getIntersectionObserver();
+  // remove the listener from the element
+  if (listeners.includes(listener)) {
+    elementsMap.set(
+      element,
+      listeners.filter(l => l !== listener)
+    );
+  }
+  // if no more listeners, unobserve the element
+  if (elementsMap.get(element)?.length === 0) {
+    observer.unobserve(element);
+    elementsMap.delete(element);
+  }
+};
+
+/**
+ * A function to observe the intersection of an element use global IntersectionObserver.
+ *
+ * ```ts
+ * useEffect(() => {
+ *  const dispose1 = observeIntersection(elRef1.current, (entry) => {});
+ *  const dispose2 = observeIntersection(elRef2.current, (entry) => {});
+ *
+ *  return () => {
+ *   dispose1();
+ *   dispose2();
+ *  };
+ * }, [])
+ * ```
+ * @return A function to dispose the observer.
+ */
+export const observeIntersection = (
+  element: Element,
+  callback: ObserveIntersection['callback']
+) => {
+  const observer = getIntersectionObserver();
+  if (!elementsMap.has(element)) {
+    observer.observe(element);
+  }
+  const prevListeners = elementsMap.get(element) ?? [];
+  const listener = { callback, dispose: () => {} };
+  listener.dispose = () => removeListener(element, listener);
+
+  elementsMap.set(element, [...prevListeners, listener]);
+
+  return listener.dispose;
+};