RoomListViewModel: Make the active room sticky in the list (#29551)

* Add new hook for sticky room

This hook takes the filtered, sorted rooms and returns a new list of
rooms such that the active room is kept in the same index even when the
list has changes.

* Use new hook in view model

* Add tests

* Use single * in comments

src/components/viewmodels/roomlist/RoomListViewModel.tsx

@@ -18,7 +18,7 @@ import SpaceStore from "../../../stores/spaces/SpaceStore";
 import dispatcher from "../../../dispatcher/dispatcher";
 import { Action } from "../../../dispatcher/actions";
 import { useMatrixClientContext } from "../../../contexts/MatrixClientContext";
-import { useIndexForActiveRoom } from "./useIndexForActiveRoom";
+import { useStickyRoomList } from "./useStickyRoomList";
 
 export interface RoomListViewState {
     /**
@@ -97,8 +97,14 @@ export interface RoomListViewState {
  */
 export function useRoomListViewModel(): RoomListViewState {
     const matrixClient = useMatrixClientContext();
-    const { primaryFilters, activePrimaryFilter, rooms, activateSecondaryFilter, activeSecondaryFilter } =
-        useFilteredRooms();
+    const {
+        primaryFilters,
+        activePrimaryFilter,
+        rooms: filteredRooms,
+        activateSecondaryFilter,
+        activeSecondaryFilter,
+    } = useFilteredRooms();
+    const { activeIndex, rooms } = useStickyRoomList(filteredRooms);
 
     const currentSpace = useEventEmitterState<Room | null>(
         SpaceStore.instance,
@@ -107,7 +113,6 @@ export function useRoomListViewModel(): RoomListViewState {
     );
     const canCreateRoom = hasCreateRoomRights(matrixClient, currentSpace);
 
-    const activeIndex = useIndexForActiveRoom(rooms);
     const { activeSortOption, sort } = useSorter();
     const { shouldShowMessagePreview, toggleMessagePreview } = useMessagePreviewToggle();
 

src/components/viewmodels/roomlist/useIndexForActiveRoom.tsx

@@ -1,44 +0,0 @@
-/*
- * Copyright 2025 New Vector Ltd.
- *
- * SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
- * Please see LICENSE files in the repository root for full details.
- */
-
-import { useCallback, useEffect, useState } from "react";
-
-import { SdkContextClass } from "../../../contexts/SDKContext";
-import { useDispatcher } from "../../../hooks/useDispatcher";
-import dispatcher from "../../../dispatcher/dispatcher";
-import { Action } from "../../../dispatcher/actions";
-import type { Room } from "matrix-js-sdk/src/matrix";
-
-/**
- * Tracks the index of the active room in the given array of rooms.
- * @param rooms list of rooms
- * @returns index of the active room or undefined otherwise.
- */
-export function useIndexForActiveRoom(rooms: Room[]): number | undefined {
-    const [index, setIndex] = useState<number | undefined>(undefined);
-
-    const calculateIndex = useCallback(
-        (newRoomId?: string) => {
-            const activeRoomId = newRoomId ?? SdkContextClass.instance.roomViewStore.getRoomId();
-            const index = rooms.findIndex((room) => room.roomId === activeRoomId);
-            setIndex(index === -1 ? undefined : index);
-        },
-        [rooms],
-    );
-
-    // Re-calculate the index when the active room has changed.
-    useDispatcher(dispatcher, (payload) => {
-        if (payload.action === Action.ActiveRoomChanged) calculateIndex(payload.newRoomId);
-    });
-
-    // Re-calculate the index when the list of rooms has changed.
-    useEffect(() => {
-        calculateIndex();
-    }, [calculateIndex, rooms]);
-
-    return index;
-}

src/components/viewmodels/roomlist/useStickyRoomList.tsx

@@ -0,0 +1,117 @@
+/*
+ * Copyright 2025 New Vector Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+import { useCallback, useEffect, useState } from "react";
+
+import { SdkContextClass } from "../../../contexts/SDKContext";
+import { useDispatcher } from "../../../hooks/useDispatcher";
+import dispatcher from "../../../dispatcher/dispatcher";
+import { Action } from "../../../dispatcher/actions";
+import type { Room } from "matrix-js-sdk/src/matrix";
+import type { Optional } from "matrix-events-sdk";
+
+function getIndexByRoomId(rooms: Room[], roomId: Optional<string>): number | undefined {
+    const index = rooms.findIndex((room) => room.roomId === roomId);
+    return index === -1 ? undefined : index;
+}
+
+function getRoomsWithStickyRoom(
+    rooms: Room[],
+    oldIndex: number | undefined,
+    newIndex: number | undefined,
+    isRoomChange: boolean,
+): { newRooms: Room[]; newIndex: number | undefined } {
+    const updated = { newIndex, newRooms: rooms };
+    if (isRoomChange) {
+        /*
+         * When opening another room, the index should obviously change.
+         */
+        return updated;
+    }
+    if (newIndex === undefined || oldIndex === undefined) {
+        /*
+         * If oldIndex is undefined, then there was no active room before.
+         * So nothing to do in regards to sticky room.
+         * Similarly, if newIndex is undefined, there's no active room anymore.
+         */
+        return updated;
+    }
+    if (newIndex === oldIndex) {
+        /*
+         * If the index hasn't changed, we have nothing to do.
+         */
+        return updated;
+    }
+    if (oldIndex > rooms.length - 1) {
+        /*
+         * If the old index falls out of the bounds of the rooms array
+         * (usually because rooms were removed), we can no longer place
+         * the active room in the same old index.
+         */
+        return updated;
+    }
+
+    /*
+     * Making the active room sticky is as simple as removing it from
+     * its new index and placing it in the old index.
+     */
+    const newRooms = [...rooms];
+    const [newRoom] = newRooms.splice(newIndex, 1);
+    newRooms.splice(oldIndex, 0, newRoom);
+
+    return { newIndex: oldIndex, newRooms };
+}
+
+interface StickyRoomListResult {
+    /**
+     * List of rooms with sticky active room.
+     */
+    rooms: Room[];
+    /**
+     * Index of the active room in the room list.
+     */
+    activeIndex: number | undefined;
+}
+
+/**
+ * - Provides a list of rooms such that the active room is sticky i.e the active room is kept
+ * in the same index even when the order of rooms in the list changes.
+ * - Provides the index of the active room.
+ * @param rooms list of rooms
+ * @see {@link StickyRoomListResult} details what this hook returns..
+ */
+export function useStickyRoomList(rooms: Room[]): StickyRoomListResult {
+    const [listState, setListState] = useState<{ index: number | undefined; roomsWithStickyRoom: Room[] }>({
+        index: undefined,
+        roomsWithStickyRoom: rooms,
+    });
+
+    const updateRoomsAndIndex = useCallback(
+        (newRoomId?: string, isRoomChange: boolean = false) => {
+            setListState((current) => {
+                const activeRoomId = newRoomId ?? SdkContextClass.instance.roomViewStore.getRoomId();
+                const newActiveIndex = getIndexByRoomId(rooms, activeRoomId);
+                const oldIndex = current.index;
+                const { newIndex, newRooms } = getRoomsWithStickyRoom(rooms, oldIndex, newActiveIndex, isRoomChange);
+                return { index: newIndex, roomsWithStickyRoom: newRooms };
+            });
+        },
+        [rooms],
+    );
+
+    // Re-calculate the index when the active room has changed.
+    useDispatcher(dispatcher, (payload) => {
+        if (payload.action === Action.ActiveRoomChanged) updateRoomsAndIndex(payload.newRoomId, true);
+    });
+
+    // Re-calculate the index when the list of rooms has changed.
+    useEffect(() => {
+        updateRoomsAndIndex();
+    }, [rooms, updateRoomsAndIndex]);
+
+    return { activeIndex: listState.index, rooms: listState.roomsWithStickyRoom };
+}

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

@@ -314,34 +314,51 @@ describe("RoomListViewModel", () => {
         });
     });
 
-    describe("active index", () => {
-        it("should recalculate active index when list of rooms change", () => {
+    describe("Sticky room and active index", () => {
+        function expectActiveRoom(vm: ReturnType<typeof useRoomListViewModel>, i: number, roomId: string) {
+            expect(vm.activeIndex).toEqual(i);
+            expect(vm.rooms[i].roomId).toEqual(roomId);
+        }
+
+        it("active room and active index are retained on order 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);
+
+            // Let's say that the room at index 5 is active
+            const roomId = rooms[5].roomId;
+            jest.spyOn(SdkContextClass.instance.roomViewStore, "getRoomId").mockImplementation(() => roomId);
 
             const { result: vm } = renderHook(() => useRoomListViewModel());
-            expect(vm.current.activeIndex).toEqual(0);
+            expect(vm.current.activeIndex).toEqual(5);
 
-            // 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);
+            // Let's say that room at index 9 moves to index 5
+            const room9 = rooms[9];
+            rooms.splice(9, 1);
+            rooms.splice(5, 0, room9);
             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);
+            // Active room index should still be 5
+            expectActiveRoom(vm.current, 5, roomId);
+
+            // Let's add 2 new rooms from index 0
+            const newRoom1 = mkStubRoom("bar1:matrix.org", "Bar 1", undefined);
+            const newRoom2 = mkStubRoom("bar2:matrix.org", "Bar 2", undefined);
+            rooms.unshift(newRoom1, newRoom2);
+            act(() => RoomListStoreV3.instance.emit(LISTS_UPDATE_EVENT));
+
+            // Active room index should still be 5
+            expectActiveRoom(vm.current, 5, roomId);
         });
 
-        it("should recalculate active index when active room changes", () => {
+        it("active room and active index are updated when another room is opened", () => {
             const { rooms } = mockAndCreateRooms();
+            const roomId = rooms[5].roomId;
+            jest.spyOn(SdkContextClass.instance.roomViewStore, "getRoomId").mockImplementation(() => roomId);
+
             const { result: vm } = renderHook(() => useRoomListViewModel());
+            expectActiveRoom(vm.current, 5, roomId);
 
-            // No active room yet
-            expect(vm.current.activeIndex).toBeUndefined();
-
-            // Let's say that room at index 5 becomes active
-            const room = rooms[5];
+            // Let's say that room at index 9 becomes active
+            const room = rooms[9];
             act(() => {
                 dispatcher.dispatch(
                     {
@@ -353,8 +370,76 @@ describe("RoomListViewModel", () => {
                 );
             });
 
-            // We expect index 5 to be active now
-            expect(vm.current.activeIndex).toEqual(5);
+            // Active room index should change to reflect new room
+            expectActiveRoom(vm.current, 9, room.roomId);
+        });
+
+        it("active room and active index are updated when active index spills out of rooms array bounds", () => {
+            const { rooms } = mockAndCreateRooms();
+            // Let's say that the room at index 5 is active
+            const roomId = rooms[5].roomId;
+            jest.spyOn(SdkContextClass.instance.roomViewStore, "getRoomId").mockImplementation(() => roomId);
+
+            const { result: vm } = renderHook(() => useRoomListViewModel());
+            expectActiveRoom(vm.current, 5, roomId);
+
+            // Let's say that we remove rooms from the start of the array
+            for (let i = 0; i < 4; ++i) {
+                // We should be able to do 4 deletions before we run out of rooms
+                rooms.splice(0, 1);
+                act(() => RoomListStoreV3.instance.emit(LISTS_UPDATE_EVENT));
+                expectActiveRoom(vm.current, 5, roomId);
+            }
+
+            // If we remove one more room from the start, there's not going to be enough rooms
+            // to maintain the active index.
+            rooms.splice(0, 1);
+            act(() => RoomListStoreV3.instance.emit(LISTS_UPDATE_EVENT));
+            expectActiveRoom(vm.current, 0, roomId);
+        });
+
+        it("active room and active index are retained when rooms that appear after the active room are deleted", () => {
+            const { rooms } = mockAndCreateRooms();
+            // Let's say that the room at index 5 is active
+            const roomId = rooms[5].roomId;
+            jest.spyOn(SdkContextClass.instance.roomViewStore, "getRoomId").mockImplementation(() => roomId);
+
+            const { result: vm } = renderHook(() => useRoomListViewModel());
+            expectActiveRoom(vm.current, 5, roomId);
+
+            // Let's say that we remove rooms from the start of the array
+            for (let i = 0; i < 4; ++i) {
+                // Deleting rooms after index 5 (active) should not update the active index
+                rooms.splice(6, 1);
+                act(() => RoomListStoreV3.instance.emit(LISTS_UPDATE_EVENT));
+                expectActiveRoom(vm.current, 5, roomId);
+            }
+        });
+
+        it("active room index becomes undefined when active room is deleted", () => {
+            const { rooms } = mockAndCreateRooms();
+            // Let's say that the room at index 5 is active
+            let roomId: string | undefined = rooms[5].roomId;
+            jest.spyOn(SdkContextClass.instance.roomViewStore, "getRoomId").mockImplementation(() => roomId);
+
+            const { result: vm } = renderHook(() => useRoomListViewModel());
+            expectActiveRoom(vm.current, 5, roomId);
+
+            // Let's remove the active room (i.e room at index 5)
+            rooms.splice(5, 1);
+            roomId = undefined;
+            act(() => RoomListStoreV3.instance.emit(LISTS_UPDATE_EVENT));
+            expect(vm.current.activeIndex).toBeUndefined();
+        });
+
+        it("active room index is initially undefined", () => {
+            mockAndCreateRooms();
+
+            // Let's say that there's no active room currently
+            jest.spyOn(SdkContextClass.instance.roomViewStore, "getRoomId").mockImplementation(() => undefined);
+
+            const { result: vm } = renderHook(() => useRoomListViewModel());
+            expect(vm.current.activeIndex).toEqual(undefined);
         });
     });
 });