RoomListViewModel: Track the index of the active room in the list (#29519)

* Introduce a hook to track active room

This hook simply keeps a state which tracks the index of the active room
in the list of rooms passed through props. This index will be recomputed
if the active rooms changes or if the list itself changed.

* Use hook in the view model

* Write tests

* Fix broken tests

test/unit-tests/components/viewmodels/roomlist/RoomListViewModel-test.tsx

@@ -21,6 +21,7 @@ import SettingsStore from "../../../../../src/settings/SettingsStore";
 import { hasCreateRoomRights, createRoom } from "../../../../../src/components/viewmodels/roomlist/utils";
 import dispatcher from "../../../../../src/dispatcher/dispatcher";
 import { Action } from "../../../../../src/dispatcher/actions";
+import { SdkContextClass } from "../../../../../src/contexts/SDKContext";
 
 jest.mock("../../../../../src/components/viewmodels/roomlist/utils", () => ({
     hasCreateRoomRights: jest.fn().mockReturnValue(false),
@@ -289,4 +290,48 @@ describe("RoomListViewModel", () => {
             expect(spy).toHaveBeenCalledWith(Action.CreateChat);
         });
     });
+
+    describe("active index", () => {
+        it("should recalculate active index when list of rooms change", () => {
+            const { rooms } = mockAndCreateRooms();
+            // Let's say that the first room is the active room initially
+            jest.spyOn(SdkContextClass.instance.roomViewStore, "getRoomId").mockImplementation(() => rooms[0].roomId);
+
+            const { result: vm } = renderHook(() => useRoomListViewModel());
+            expect(vm.current.activeIndex).toEqual(0);
+
+            // Let's say that a new room is added and that becomes active
+            const newRoom = mkStubRoom("bar:matrix.org", "Bar", undefined);
+            jest.spyOn(SdkContextClass.instance.roomViewStore, "getRoomId").mockImplementation(() => newRoom.roomId);
+            rooms.push(newRoom);
+            act(() => RoomListStoreV3.instance.emit(LISTS_UPDATE_EVENT));
+
+            // Now the active room should be the last room which we just added
+            expect(vm.current.activeIndex).toEqual(rooms.length - 1);
+        });
+
+        it("should recalculate active index when active room changes", () => {
+            const { rooms } = mockAndCreateRooms();
+            const { result: vm } = renderHook(() => useRoomListViewModel());
+
+            // No active room yet
+            expect(vm.current.activeIndex).toBeUndefined();
+
+            // Let's say that room at index 5 becomes active
+            const room = rooms[5];
+            act(() => {
+                dispatcher.dispatch(
+                    {
+                        action: Action.ActiveRoomChanged,
+                        oldRoomId: null,
+                        newRoomId: room.roomId,
+                    },
+                    true,
+                );
+            });
+
+            // We expect index 5 to be active now
+            expect(vm.current.activeIndex).toEqual(5);
+        });
+    });
 });