Basic font settings.

Include a default value getter in the store in order to make the
default value easy to work with.

src/settings/Settings.js

@@ -29,6 +29,7 @@ import ThemeController from './controllers/ThemeController';
 import PushToMatrixClientController from './controllers/PushToMatrixClientController';
 import ReloadOnChangeController from "./controllers/ReloadOnChangeController";
 import {RIGHT_PANEL_PHASES} from "../stores/RightPanelStorePhases";
+import FontSizeController from './controllers/FontSizeController';
 
 // These are just a bunch of helper arrays to avoid copy/pasting a bunch of times
 const LEVELS_ROOM_SETTINGS = ['device', 'room-device', 'room-account', 'account', 'config'];
@@ -94,6 +95,12 @@ export const SETTINGS = {
     //     // not use this for new settings.
     //     invertedSettingName: "my-negative-setting",
     // },
+    "font_size": {
+        displayName: _td("Font size"),
+        supportedLevels: LEVELS_ACCOUNT_SETTINGS,
+        default: 16,
+        controller: new FontSizeController(),
+    },
     "feature_pinning": {
         isFeature: true,
         displayName: _td("Message Pinning"),

src/settings/SettingsStore.js

@@ -370,6 +370,20 @@ export default class SettingsStore {
         return SettingsStore._getFinalValue(setting, level, roomId, null, null);
     }
 
+    /**
+     * Gets the default value of a setting.
+     * @param {string} settingName The name of the setting to read the value of.
+     * @return {*} The value, or null if not found
+     */
+    static getDefaultValue(settingName, roomId = null, excludeDefault = false) {
+        // Verify that the setting is actually a setting
+        if (!SETTINGS[settingName]) {
+            throw new Error("Setting '" + settingName + "' does not appear to be a setting.");
+        }
+
+        return SETTINGS[settingName].default;
+    }
+
     static _getFinalValue(setting, level, roomId, calculatedValue, calculatedAtLevel) {
         let resultingValue = calculatedValue;
 

src/settings/controllers/FontSizeController.js

@@ -0,0 +1,33 @@
+/*
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+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.
+*/
+
+import SettingController from "./SettingController";
+import dis from "../../dispatcher";
+
+export default class FontSizeController extends SettingController {
+
+    constructor() {
+        super()
+    }
+
+    onChange(level, roomId, newValue) {
+        // Dispatch font size change so that everything open responds to the change.
+        dis.dispatch({
+            action: "update-font-size",
+            size: newValue,
+        });
+    }
+}