From cdd7f6158b26eae86e29b424b2c7859b3d1d3355 Mon Sep 17 00:00:00 2001
From: Marcel Mraz <marcel@excalidraw.com>
Date: Wed, 22 Jan 2025 22:16:33 +0100
Subject: [PATCH] Testing concurrent remote updates (wip)

---
 packages/excalidraw/components/App.tsx |  5 +++--
 packages/excalidraw/store.ts           |  2 ++
 packages/excalidraw/sync/client.ts     | 14 ++++++++++++--
 3 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/packages/excalidraw/components/App.tsx b/packages/excalidraw/components/App.tsx
index da395822aa..97747f0f89 100644
--- a/packages/excalidraw/components/App.tsx
+++ b/packages/excalidraw/components/App.tsx
@@ -3878,6 +3878,7 @@ class App extends React.Component<AppProps, AppState> {
       // flush all incoming updates immediately, so that they couldn't be batched with other updates, having different `storeAction`
       flushSync(() => {
         const { elements, appState, collaborators, storeAction } = sceneData;
+
         const nextElements = elements
           ? syncInvalidIndices(elements)
           : undefined;
@@ -3892,8 +3893,8 @@ class App extends React.Component<AppProps, AppState> {
 
           const nextCommittedElements = elements
             ? this.store.filterUncomittedElements(
-                this.scene.getElementsMapIncludingDeleted(), // Only used to detect uncomitted local elements
-                arrayToMap(nextElements ?? []), // We expect all (already reconciled) elements
+                this.scene.getElementsMapIncludingDeleted(), // only used to detect uncomitted local elements
+                arrayToMap(nextElements ?? []), // we expect all (already reconciled) elements
               )
             : prevCommittedElements;
 
diff --git a/packages/excalidraw/store.ts b/packages/excalidraw/store.ts
index f0bc82d54e..10a5821050 100644
--- a/packages/excalidraw/store.ts
+++ b/packages/excalidraw/store.ts
@@ -224,6 +224,8 @@ export class Store {
     const increment = new EphemeralStoreIncrement(change);
 
     // Notify listeners with the increment
+    // CFDO: consider having this async instead, possibly should also happen after the component updates;
+    // or get rid of filtering local in progress elements, switch to unidirectional store flow and keep it synchronous
     this.onStoreIncrementEmitter.trigger(increment);
 
     return nextSnapshot;
diff --git a/packages/excalidraw/sync/client.ts b/packages/excalidraw/sync/client.ts
index 2d393dd97f..a23d2768d5 100644
--- a/packages/excalidraw/sync/client.ts
+++ b/packages/excalidraw/sync/client.ts
@@ -17,6 +17,7 @@ import type { ExcalidrawElement, SceneElementsMap } from "../element/types";
 import type { CLIENT_MESSAGE_RAW, SERVER_DELTA, CHANGE } from "./protocol";
 import { debounce } from "../utils";
 import { randomId } from "../random";
+import { orderByFractionalIndex } from "../fractionalIndex";
 
 class SocketMessage implements CLIENT_MESSAGE_RAW {
   constructor(
@@ -388,13 +389,18 @@ export class SyncClient {
           !existingElement || // new element
           existingElement.version < relayedElement.version // updated element
         ) {
+          // CFDO: in theory could make the yet unsynced element (due to a bug) to move to the top
           nextElements.set(id, relayedElement);
           this.relayedElementsVersionsCache.set(id, relayedElement.version);
         }
       }
 
+      const orderedElements = orderByFractionalIndex(
+        Array.from(nextElements.values()),
+      );
+
       this.api.updateScene({
-        elements: Array.from(nextElements.values()),
+        elements: orderedElements,
         storeAction: StoreAction.UPDATE,
       });
     } catch (e) {
@@ -468,9 +474,13 @@ export class SyncClient {
         prevSnapshot = this.api.store.snapshot;
       }
 
+      const orderedElements = orderByFractionalIndex(
+        Array.from(nextElements.values()),
+      );
+
       // CFDO: might need to restore first due to potentially stale delta versions
       this.api.updateScene({
-        elements: Array.from(nextElements.values()),
+        elements: orderedElements,
         // even though the snapshot should be up-to-date already,
         // still some more updates might be triggered,
         // i.e. as a result from syncing invalid indices