feat(web): persist scroll position on navigation back to album (#11388)

Co-authored-by: Calum Dingwall <caburum@users.noreply.github.com>
Co-authored-by: Alex <alex.tran1502@gmail.com>

web/src/lib/actions/scroll-memory.ts

@@ -0,0 +1,87 @@
+import { navigating } from '$app/stores';
+import { AppRoute, SessionStorageKey } from '$lib/constants';
+import { handlePromiseError } from '$lib/utils';
+
+interface Options {
+  /**
+   * {@link AppRoute} for subpages that scroll state should be kept while visiting.
+   *
+   * This must be kept the same in all subpages of this route for the scroll memory clearer to work.
+   */
+  routeStartsWith: AppRoute;
+  /**
+   * Function to clear additional data/state before scrolling (ex infinite scroll).
+   */
+  beforeClear?: () => void;
+}
+
+interface PageOptions extends Options {
+  /**
+   * Function to save additional data/state before scrolling (ex infinite scroll).
+   */
+  beforeSave?: () => void;
+  /**
+   * Function to load additional data/state before scrolling (ex infinite scroll).
+   */
+  beforeScroll?: () => Promise<void>;
+}
+
+/**
+ * @param node The scroll slot element, typically from {@link UserPageLayout}
+ */
+export function scrollMemory(
+  node: HTMLElement,
+  { routeStartsWith, beforeSave, beforeClear, beforeScroll }: PageOptions,
+) {
+  const unsubscribeNavigating = navigating.subscribe((navigation) => {
+    const existingScroll = sessionStorage.getItem(SessionStorageKey.SCROLL_POSITION);
+    if (navigation?.to && !existingScroll) {
+      // Save current scroll information when going into a subpage.
+      if (navigation.to.url.pathname.startsWith(routeStartsWith)) {
+        beforeSave?.();
+        sessionStorage.setItem(SessionStorageKey.SCROLL_POSITION, node.scrollTop.toString());
+      } else {
+        beforeClear?.();
+        sessionStorage.removeItem(SessionStorageKey.SCROLL_POSITION);
+      }
+    }
+  });
+
+  handlePromiseError(
+    (async () => {
+      await beforeScroll?.();
+
+      const newScroll = sessionStorage.getItem(SessionStorageKey.SCROLL_POSITION);
+      if (newScroll) {
+        node.scroll({
+          top: Number.parseFloat(newScroll),
+          behavior: 'instant',
+        });
+      }
+      beforeClear?.();
+      sessionStorage.removeItem(SessionStorageKey.SCROLL_POSITION);
+    })(),
+  );
+
+  return {
+    destroy() {
+      unsubscribeNavigating();
+    },
+  };
+}
+
+export function scrollMemoryClearer(_node: HTMLElement, { routeStartsWith, beforeClear }: Options) {
+  const unsubscribeNavigating = navigating.subscribe((navigation) => {
+    // Forget scroll position from main page if going somewhere else.
+    if (navigation?.to && !navigation?.to.url.pathname.startsWith(routeStartsWith)) {
+      beforeClear?.();
+      sessionStorage.removeItem(SessionStorageKey.SCROLL_POSITION);
+    }
+  });
+
+  return {
+    destroy() {
+      unsubscribeNavigating();
+    },
+  };
+}