Factor out a (public) function to find a room's predecessor

spec/unit/room.spec.ts

@@ -33,6 +33,7 @@ import {
     IRelationsRequestOpts,
     IStateEventWithRoomId,
     JoinRule,
+    MatrixClient,
     MatrixEvent,
     MatrixEventEvent,
     PendingEventOrdering,
@@ -3225,4 +3226,52 @@ describe("Room", function () {
             expect(room.getBlacklistUnverifiedDevices()).toBe(false);
         });
     });
+
+    describe("findPredecessorRoomId", () => {
+        function roomCreateEvent(newRoomId: string, predecessorRoomId: string | null): MatrixEvent {
+            const content: {
+                creator: string;
+                ["m.federate"]: boolean;
+                room_version: string;
+                predecessor: { event_id: string; room_id: string } | undefined;
+            } = {
+                "creator": "@daryl:alexandria.example.com",
+                "predecessor": undefined,
+                "m.federate": true,
+                "room_version": "9",
+            };
+            if (predecessorRoomId) {
+                content.predecessor = {
+                    event_id: "spec_is_not_clear_what_id_this_is",
+                    room_id: predecessorRoomId,
+                };
+            }
+            return new MatrixEvent({
+                content,
+                event_id: `create_event_id_pred_${predecessorRoomId}`,
+                origin_server_ts: 1432735824653,
+                room_id: newRoomId,
+                sender: "@daryl:alexandria.example.com",
+                state_key: "",
+                type: "m.room.create",
+            });
+        }
+
+        it("Returns null if there is no create event", () => {
+            const room = new Room("roomid", null as unknown as MatrixClient, "@u:example.com");
+            expect(room.findPredecessorRoomId()).toBeNull();
+        });
+
+        it("Returns null if the create event has no predecessor", () => {
+            const room = new Room("roomid", null as unknown as MatrixClient, "@u:example.com");
+            room.addLiveEvents([roomCreateEvent("roomid", null)]);
+            expect(room.findPredecessorRoomId()).toBeNull();
+        });
+
+        it("Returns the predecessor ID if one is provided via create event", () => {
+            const room = new Room("roomid", null as unknown as MatrixClient, "@u:example.com");
+            room.addLiveEvents([roomCreateEvent("roomid", "replacedroomid")]);
+            expect(room.findPredecessorRoomId()).toBe("replacedroomid");
+        });
+    });
 });

src/client.ts

@@ -3770,13 +3770,9 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
 
         const replacedRooms = new Set();
         for (const r of allRooms) {
-            const createEvent = r.currentState.getStateEvents(EventType.RoomCreate, "");
-            // invites are included in this list and we don't know their create events yet
-            if (createEvent) {
-                const predecessor = createEvent.getContent()["predecessor"];
-                if (predecessor && predecessor["room_id"]) {
-                    replacedRooms.add(predecessor["room_id"]);
-                }
+            const predecessor = r.findPredecessorRoomId();
+            if (predecessor) {
+                replacedRooms.add(predecessor);
             }
         }
 

src/models/room.ts

@@ -2969,6 +2969,24 @@ export class Room extends ReadReceipt<RoomEmittedEvents, RoomEventHandlerMap> {
         return this.getType() === RoomType.ElementVideo;
     }
 
+    /**
+     * @returns the ID of the room that was this room's predecessor, or null if
+     *          this room has no predecessor.
+     */
+    public findPredecessorRoomId(): string | null {
+        const createEvent = this.currentState.getStateEvents(EventType.RoomCreate, "");
+        if (createEvent) {
+            const predecessor = createEvent.getContent()["predecessor"];
+            if (predecessor) {
+                const roomId = predecessor["room_id"];
+                if (roomId) {
+                    return roomId;
+                }
+            }
+        }
+        return null;
+    }
+
     private roomNameGenerator(state: RoomNameState): string {
         if (this.client.roomNameGenerator) {
             const name = this.client.roomNameGenerator(this.roomId, state);