Introduce a RoomScrollStateStore

to keep the place we're scrolled to in rooms. This mainly eleimates
the extra, superfluous onRoomViewStoreUpdate callback that
happened when the previous room saved back its scroll state.
Moving the scroll state to a separate store means we can have this
not emit events because nothing needs to know when the scroll state
changes.

src/components/structures/RoomView.js

@@ -47,6 +47,7 @@ import KeyCode from '../../KeyCode';
 import UserProvider from '../../autocomplete/UserProvider';
 
 import RoomViewStore from '../../stores/RoomViewStore';
+import RoomScrollStateStore from '../../stores/RoomScrollStateStore';
 
 let DEBUG = false;
 let debuglog = function() {};
@@ -163,7 +164,6 @@ module.exports = React.createClass({
             roomLoadError: RoomViewStore.getRoomLoadError(),
             joining: RoomViewStore.isJoining(),
             initialEventId: RoomViewStore.getInitialEventId(),
-            initialEventPixelOffset: RoomViewStore.getInitialEventPixelOffset(),
             isInitialEventHighlighted: RoomViewStore.isInitialEventHighlighted(),
             forwardingEvent: RoomViewStore.getForwardingEvent(),
             shouldPeek: RoomViewStore.shouldPeek(),
@@ -191,18 +191,32 @@ module.exports = React.createClass({
             newState.room = MatrixClientPeg.get().getRoom(newState.roomId);
         }
 
+        if (this.state.roomId !== newState.roomId) {
+            // Store the scroll state for the previous room so that we can return to this
+            // position when viewing this room in future.
+            if (this.state.roomId) {
+                RoomScrollStateStore.setScrollState(this.state.roomId, this._getScrollState());
+            }
+
+            // ...and get the scroll state for the new room
+
+            // If an event ID wasn't specified, default to the one saved for this room
+            // via update_scroll_state. Assume initialEventPixelOffset should be set.
+            if (!newState.initialEventId) {
+                const roomScrollState = RoomScrollStateStore.getScrollState(newState.roomId);
+                if (roomScrollState) {
+                    newState.initialEventId = roomScrollState.focussedEvent;
+                    newState.initialEventPixelOffset = roomScrollState.pixelOffset;
+                }
+            }
+        }
+
         // Clear the search results when clicking a search result (which changes the
         // currently scrolled to event, this.state.initialEventId).
         if (this.state.initialEventId !== newState.initialEventId) {
             newState.searchResults = null;
         }
 
-        // Store the scroll state for the previous room so that we can return to this
-        // position when viewing this room in future.
-        if (this.state.roomId !== newState.roomId) {
-            this._updateScrollMap(this.state.roomId);
-        }
-
         this.setState(newState, () => {
             // At this point, this.state.roomId could be null (e.g. the alias might not
             // have been resolved yet) so anything called here must handle this case.
@@ -340,7 +354,7 @@ module.exports = React.createClass({
         this.unmounted = true;
 
         // update the scroll map before we get unmounted
-        this._updateScrollMap(this.state.roomId);
+        RoomScrollStateStore.setScrollState(this.state.roomId, this._getScrollState());
 
         if (this.refs.roomView) {
             // disconnect the D&D event listeners from the room view. This
@@ -617,18 +631,6 @@ module.exports = React.createClass({
         });
     },
 
-    _updateScrollMap(roomId) {
-        // No point updating scroll state if the room ID hasn't been resolved yet
-        if (!roomId) {
-            return;
-        }
-        dis.dispatch({
-            action: 'update_scroll_state',
-            room_id: roomId,
-            scroll_state: this._getScrollState(),
-        });
-    },
-
     onRoom: function(room) {
         if (!room || room.roomId !== this.state.roomId) {
             return;

src/stores/RoomScrollStateStore.js

@@ -0,0 +1,37 @@
+/*
+Copyright 2017 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+/**
+ * Stores where the user has scrolled to in each room
+ */
+class RoomScrollStateStore {
+    constructor() {
+        this._scrollStateMap = {};
+    }
+
+    getScrollState(roomId) {
+        return this._scrollStateMap[roomId];
+    }
+
+    setScrollState(roomId, scrollState) {
+        this._scrollStateMap[roomId] = scrollState;
+    }
+}
+
+if (global.mx_RoomScrollStateStore === undefined) {
+    global.mx_RoomScrollStateStore = new RoomScrollStateStore();
+}
+export default global.mx_RoomScrollStateStore;

src/stores/RoomViewStore.js

@@ -30,8 +30,6 @@ const INITIAL_STATE = {
 
     // The event to scroll to when the room is first viewed
     initialEventId: null,
-    // The offset to display the initial event at (see scrollStateMap)
-    initialEventPixelOffset: null,
     // Whether to highlight the initial event
     isInitialEventHighlighted: false,
 
@@ -41,20 +39,6 @@ const INITIAL_STATE = {
     roomLoading: false,
     // Any error that has occurred during loading
     roomLoadError: null,
-    // A map from room id to scroll state.
-    //
-    // If there is no special scroll state (ie, we are following the live
-    // timeline), the scroll state is null. Otherwise, it is an object with
-    // the following properties:
-    //
-    //    focussedEvent: the ID of the 'focussed' event. Typically this is
-    //        the last event fully visible in the viewport, though if we
-    //        have done an explicit scroll to an explicit event, it will be
-    //        that event.
-    //
-    //    pixelOffset: the number of pixels the window is scrolled down
-    //        from the focussedEvent.
-    scrollStateMap: {},
 
     forwardingEvent: null,
 };
@@ -115,9 +99,6 @@ class RoomViewStore extends Store {
             case 'on_logged_out':
                 this.reset();
                 break;
-            case 'update_scroll_state':
-                this._updateScrollState(payload);
-                break;
             case 'forward_event':
                 this._setState({
                     forwardingEvent: payload.event,
@@ -132,7 +113,6 @@ class RoomViewStore extends Store {
                 roomId: payload.room_id,
                 roomAlias: payload.room_alias,
                 initialEventId: payload.event_id,
-                initialEventPixelOffset: undefined,
                 isInitialEventHighlighted: payload.highlighted,
                 forwardingEvent: null,
                 roomLoading: false,
@@ -145,16 +125,6 @@ class RoomViewStore extends Store {
                 newState.joining = false;
             }
 
-            // If an event ID wasn't specified, default to the one saved for this room
-            // via update_scroll_state. Assume initialEventPixelOffset should be set.
-            if (!newState.initialEventId) {
-                const roomScrollState = this._state.scrollStateMap[payload.room_id];
-                if (roomScrollState) {
-                    newState.initialEventId = roomScrollState.focussedEvent;
-                    newState.initialEventPixelOffset = roomScrollState.pixelOffset;
-                }
-            }
-
             if (this._state.forwardingEvent) {
                 dis.dispatch({
                     action: 'send_event',