feat: multiplayer undo / redo (#7348)

excalidraw-app/App.tsx

@@ -438,7 +438,7 @@ const ExcalidrawWrapper = () => {
             excalidrawAPI.updateScene({
               ...data.scene,
               ...restore(data.scene, null, null, { repairBindings: true }),
-              commitToHistory: true,
+              commitToStore: true,
             });
           }
         });

excalidraw-app/collab/Collab.tsx

@@ -356,7 +356,6 @@ class Collab extends PureComponent<CollabProps, CollabState> {
 
       this.excalidrawAPI.updateScene({
         elements,
-        commitToHistory: false,
       });
     }
   };
@@ -501,14 +500,12 @@ class Collab extends PureComponent<CollabProps, CollabState> {
         }
         return element;
       });
-      // remove deleted elements from elements array & history to ensure we don't
+      // remove deleted elements from elements array to ensure we don't
       // expose potentially sensitive user data in case user manually deletes
       // existing elements (or clears scene), which would otherwise be persisted
       // to database even if deleted before creating the room.
-      this.excalidrawAPI.history.clear();
       this.excalidrawAPI.updateScene({
         elements,
-        commitToHistory: true,
       });
 
       this.saveCollabRoomToFirebase(getSyncableElements(elements));
@@ -544,9 +541,7 @@ class Collab extends PureComponent<CollabProps, CollabState> {
               const remoteElements = decryptedData.payload.elements;
               const reconciledElements =
                 this._reconcileElements(remoteElements);
-              this.handleRemoteSceneUpdate(reconciledElements, {
-                init: true,
-              });
+              this.handleRemoteSceneUpdate(reconciledElements);
               // noop if already resolved via init from firebase
               scenePromise.resolve({
                 elements: reconciledElements,
@@ -745,19 +740,11 @@ class Collab extends PureComponent<CollabProps, CollabState> {
 
   private handleRemoteSceneUpdate = (
     elements: ReconciledExcalidrawElement[],
-    { init = false }: { init?: boolean } = {},
   ) => {
     this.excalidrawAPI.updateScene({
       elements,
-      commitToHistory: !!init,
     });
 
-    // We haven't yet implemented multiplayer undo functionality, so we clear the undo stack
-    // when we receive any messages from another peer. This UX can be pretty rough -- if you
-    // undo, a user makes a change, and then try to redo, your element(s) will be lost. However,
-    // right now we think this is the right tradeoff.
-    this.excalidrawAPI.history.clear();
-
     this.loadImageFiles();
   };
 

excalidraw-app/data/index.ts

@@ -269,7 +269,7 @@ export const loadScene = async (
     // in the scene database/localStorage, and instead fetch them async
     // from a different database
     files: data.files,
-    commitToHistory: false,
+    commitToStore: false,
   };
 };
 

excalidraw-app/tests/collab.test.tsx

@@ -1,13 +1,18 @@
 import { vi } from "vitest";
 import {
+  act,
   render,
   updateSceneData,
   waitFor,
 } from "../../packages/excalidraw/tests/test-utils";
 import ExcalidrawApp from "../App";
 import { API } from "../../packages/excalidraw/tests/helpers/api";
-import { createUndoAction } from "../../packages/excalidraw/actions/actionHistory";
 import { syncInvalidIndices } from "../../packages/excalidraw/fractionalIndex";
+import {
+  createRedoAction,
+  createUndoAction,
+} from "../../packages/excalidraw/actions/actionHistory";
+import { newElementWith } from "../../packages/excalidraw";
 
 const { h } = window;
 
@@ -58,39 +63,188 @@ vi.mock("socket.io-client", () => {
   };
 });
 
+/**
+ * These test would deserve to be extended by testing collab with (at least) two clients simultanouesly,
+ * while having access to both scenes, appstates stores, histories and etc.
+ * i.e. multiplayer history tests could be a good first candidate, as we could test both history stacks simultaneously.
+ */
 describe("collaboration", () => {
-  it("creating room should reset deleted elements", async () => {
+  it("should allow to undo / redo even on force-deleted elements", async () => {
     await render(<ExcalidrawApp />);
-    // To update the scene with deleted elements before starting collab
+    const rect1Props = {
+      type: "rectangle",
+      id: "A",
+      height: 200,
+      width: 100,
+    } as const;
+
+    const rect2Props = {
+      type: "rectangle",
+      id: "B",
+      width: 100,
+      height: 200,
+    } as const;
+
+    const rect1 = API.createElement({ ...rect1Props });
+    const rect2 = API.createElement({ ...rect2Props });
+
     updateSceneData({
-      elements: syncInvalidIndices([
-        API.createElement({ type: "rectangle", id: "A" }),
-        API.createElement({
-          type: "rectangle",
-          id: "B",
-          isDeleted: true,
-        }),
-      ]),
-    });
-    await waitFor(() => {
-      expect(h.elements).toEqual([
-        expect.objectContaining({ id: "A" }),
-        expect.objectContaining({ id: "B", isDeleted: true }),
-      ]);
-      expect(API.getStateHistory().length).toBe(1);
-    });
-    window.collab.startCollaboration(null);
-    await waitFor(() => {
-      expect(h.elements).toEqual([expect.objectContaining({ id: "A" })]);
-      expect(API.getStateHistory().length).toBe(1);
+      elements: syncInvalidIndices([rect1, rect2]),
+      commitToStore: true,
+    });
+
+    updateSceneData({
+      elements: syncInvalidIndices([
+        rect1,
+        newElementWith(h.elements[1], { isDeleted: true }),
+      ]),
+      commitToStore: true,
     });
 
-    const undoAction = createUndoAction(h.history);
-    // noop
-    h.app.actionManager.executeAction(undoAction);
     await waitFor(() => {
-      expect(h.elements).toEqual([expect.objectContaining({ id: "A" })]);
-      expect(API.getStateHistory().length).toBe(1);
+      expect(API.getUndoStack().length).toBe(2);
+      expect(API.getSnapshot()).toEqual([
+        expect.objectContaining(rect1Props),
+        expect.objectContaining({ ...rect2Props, isDeleted: true }),
+      ]);
+      expect(h.elements).toEqual([
+        expect.objectContaining(rect1Props),
+        expect.objectContaining({ ...rect2Props, isDeleted: true }),
+      ]);
+    });
+
+    // one form of force deletion happens when starting the collab, not to sync potentially sensitive data into the server
+    window.collab.startCollaboration(null);
+
+    await waitFor(() => {
+      expect(API.getUndoStack().length).toBe(2);
+      // we never delete from the local snapshot as it is used for correct diff calculation
+      expect(API.getSnapshot()).toEqual([
+        expect.objectContaining(rect1Props),
+        expect.objectContaining({ ...rect2Props, isDeleted: true }),
+      ]);
+      expect(h.elements).toEqual([expect.objectContaining(rect1Props)]);
+    });
+
+    const undoAction = createUndoAction(h.history, h.store);
+    act(() => h.app.actionManager.executeAction(undoAction));
+
+    // with explicit undo (as addition) we expect our item to be restored from the snapshot!
+    await waitFor(() => {
+      expect(API.getUndoStack().length).toBe(1);
+      expect(API.getSnapshot()).toEqual([
+        expect.objectContaining(rect1Props),
+        expect.objectContaining({ ...rect2Props, isDeleted: false }),
+      ]);
+      expect(h.elements).toEqual([
+        expect.objectContaining(rect1Props),
+        expect.objectContaining({ ...rect2Props, isDeleted: false }),
+      ]);
+    });
+
+    // simulate force deleting the element remotely
+    updateSceneData({
+      elements: syncInvalidIndices([rect1]),
+    });
+
+    await waitFor(() => {
+      expect(API.getUndoStack().length).toBe(1);
+      expect(API.getRedoStack().length).toBe(1);
+      expect(API.getSnapshot()).toEqual([
+        expect.objectContaining(rect1Props),
+        expect.objectContaining({ ...rect2Props, isDeleted: true }),
+      ]);
+      expect(h.elements).toEqual([expect.objectContaining(rect1Props)]);
+    });
+
+    const redoAction = createRedoAction(h.history, h.store);
+    act(() => h.app.actionManager.executeAction(redoAction));
+
+    // with explicit redo (as removal) we again restore the element from the snapshot!
+    await waitFor(() => {
+      expect(API.getUndoStack().length).toBe(2);
+      expect(API.getRedoStack().length).toBe(0);
+      expect(API.getSnapshot()).toEqual([
+        expect.objectContaining(rect1Props),
+        expect.objectContaining({ ...rect2Props, isDeleted: true }),
+      ]);
+      expect(h.elements).toEqual([
+        expect.objectContaining(rect1Props),
+        expect.objectContaining({ ...rect2Props, isDeleted: true }),
+      ]);
+    });
+
+    act(() => h.app.actionManager.executeAction(undoAction));
+
+    // simulate local update
+    updateSceneData({
+      elements: syncInvalidIndices([
+        h.elements[0],
+        newElementWith(h.elements[1], { x: 100 }),
+      ]),
+      commitToStore: true,
+    });
+
+    await waitFor(() => {
+      expect(API.getUndoStack().length).toBe(2);
+      expect(API.getRedoStack().length).toBe(0);
+      expect(API.getSnapshot()).toEqual([
+        expect.objectContaining(rect1Props),
+        expect.objectContaining({ ...rect2Props, isDeleted: false, x: 100 }),
+      ]);
+      expect(h.elements).toEqual([
+        expect.objectContaining(rect1Props),
+        expect.objectContaining({ ...rect2Props, isDeleted: false, x: 100 }),
+      ]);
+    });
+
+    act(() => h.app.actionManager.executeAction(undoAction));
+
+    // we expect to iterate the stack to the first visible change
+    await waitFor(() => {
+      expect(API.getUndoStack().length).toBe(1);
+      expect(API.getRedoStack().length).toBe(1);
+      expect(API.getSnapshot()).toEqual([
+        expect.objectContaining(rect1Props),
+        expect.objectContaining({ ...rect2Props, isDeleted: false, x: 0 }),
+      ]);
+      expect(h.elements).toEqual([
+        expect.objectContaining(rect1Props),
+        expect.objectContaining({ ...rect2Props, isDeleted: false, x: 0 }),
+      ]);
+    });
+
+    // simulate force deleting the element remotely
+    updateSceneData({
+      elements: syncInvalidIndices([rect1]),
+    });
+
+    // snapshot was correctly updated and marked the element as deleted
+    await waitFor(() => {
+      expect(API.getUndoStack().length).toBe(1);
+      expect(API.getRedoStack().length).toBe(1);
+      expect(API.getSnapshot()).toEqual([
+        expect.objectContaining(rect1Props),
+        expect.objectContaining({ ...rect2Props, isDeleted: true, x: 0 }),
+      ]);
+      expect(h.elements).toEqual([expect.objectContaining(rect1Props)]);
+    });
+
+    act(() => h.app.actionManager.executeAction(redoAction));
+
+    // with explicit redo (as update) we again restored the element from the snapshot!
+    await waitFor(() => {
+      expect(API.getUndoStack().length).toBe(2);
+      expect(API.getRedoStack().length).toBe(0);
+      expect(API.getSnapshot()).toEqual([
+        expect.objectContaining({ id: "A", isDeleted: false }),
+        expect.objectContaining({ id: "B", isDeleted: true, x: 100 }),
+      ]);
+      expect(h.history.isRedoStackEmpty).toBeTruthy();
+      expect(h.elements).toEqual([
+        expect.objectContaining({ id: "A", isDeleted: false }),
+        expect.objectContaining({ id: "B", isDeleted: true, x: 100 }),
+      ]);
     });
   });
 });