feat(web): nav/slideshow view transitions and crossfade

Change-Id: I0a37b417ee4c247dcc93d442c976eede6a6a6964

web/src/lib/utils/transition-utils.ts

@@ -23,3 +23,63 @@ export function startViewerTransition(
     },
   });
 }
+
+let activeOverlay: HTMLElement | undefined;
+
+export function removeCrossfadeOverlay() {
+  if (activeOverlay) {
+    activeOverlay.remove();
+    activeOverlay = undefined;
+  }
+}
+
+export async function crossfadeViewerContent(updateFn: () => void | Promise<void>, duration = 200) {
+  const viewerContent = document.querySelector<HTMLElement>('[data-viewer-content]');
+  if (!viewerContent) {
+    await updateFn();
+    return;
+  }
+
+  removeCrossfadeOverlay();
+
+  const clone = viewerContent.cloneNode(true) as HTMLElement;
+  Object.assign(clone.style, {
+    position: 'absolute',
+    inset: '0',
+    zIndex: '1',
+    pointerEvents: 'none',
+  });
+  delete clone.dataset.viewerContent;
+  if (!viewerContent.parentElement) {
+    await updateFn();
+    return;
+  }
+  viewerContent.parentElement.append(clone);
+  activeOverlay = clone;
+
+  const ready = eventManager.untilNext('ViewerOpenTransitionReady');
+  await updateFn();
+
+  try {
+    await ready;
+  } catch {
+    clone.remove();
+    if (activeOverlay === clone) {
+      activeOverlay = undefined;
+    }
+    return;
+  }
+
+  const fadeOut = clone.animate([{ opacity: 1 }, { opacity: 0 }], {
+    duration,
+    easing: 'cubic-bezier(0.4, 0, 1, 1)',
+    fill: 'forwards',
+  });
+
+  void fadeOut.finished.then(() => {
+    clone.remove();
+    if (activeOverlay === clone) {
+      activeOverlay = undefined;
+    }
+  });
+}