Implement support for watching for changes in settings

This implements a dream of one day being able to listen for changes in a settings to react to them, regardless of which device actually changed the setting. The use case for this kind of thing is extremely limited, but when it is needed it should be more than powerful enough.

src/settings/handlers/RoomDeviceSettingsHandler.js

@@ -1,5 +1,6 @@
 /*
 Copyright 2017 Travis Ralston
+Copyright 2019 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.
@@ -16,12 +17,19 @@ limitations under the License.
 
 import Promise from 'bluebird';
 import SettingsHandler from "./SettingsHandler";
+import {WatchManager} from "../WatchManager";
 
 /**
  * Gets and sets settings at the "room-device" level for the current device in a particular
  * room.
  */
 export default class RoomDeviceSettingsHandler extends SettingsHandler {
+    constructor() {
+        super();
+
+        this._watchers = new WatchManager();
+    }
+
     getValue(settingName, roomId) {
         // Special case blacklist setting to use legacy values
         if (settingName === "blacklistUnverifiedDevices") {
@@ -44,6 +52,7 @@ export default class RoomDeviceSettingsHandler extends SettingsHandler {
             if (!value["blacklistUnverifiedDevicesPerRoom"]) value["blacklistUnverifiedDevicesPerRoom"] = {};
             value["blacklistUnverifiedDevicesPerRoom"][roomId] = newValue;
             localStorage.setItem("mx_local_settings", JSON.stringify(value));
+            this._watchers.notifyUpdate(settingName, roomId, newValue);
             return Promise.resolve();
         }
 
@@ -54,6 +63,7 @@ export default class RoomDeviceSettingsHandler extends SettingsHandler {
             localStorage.setItem(this._getKey(settingName, roomId), newValue);
         }
 
+        this._watchers.notifyUpdate(settingName, roomId, newValue);
         return Promise.resolve();
     }
 
@@ -65,6 +75,14 @@ export default class RoomDeviceSettingsHandler extends SettingsHandler {
         return localStorage !== undefined && localStorage !== null;
     }
 
+    watchSetting(settingName, roomId, cb) {
+        this._watchers.watchSetting(settingName, roomId, cb);
+    }
+
+    unwatchSetting(cb) {
+        this._watchers.unwatchSetting(cb);
+    }
+
     _read(key) {
         const rawValue = localStorage.getItem(key);
         if (!rawValue) return null;