allow a setting controller to validate and revert a change (asynchronously)

src/components/structures/UserSettings.js

@@ -844,8 +844,8 @@ module.exports = React.createClass({
         SettingsStore.getLabsFeatures().forEach((featureId) => {
             // TODO: this ought to be a separate component so that we don't need
             // to rebind the onChange each time we render
-            const onChange = (e) => {
-                SettingsStore.setFeatureEnabled(featureId, e.target.checked);
+            const onChange = async (e) => {
+                await SettingsStore.setFeatureEnabled(featureId, e.target.checked);
                 this.forceUpdate();
             };
 
@@ -855,7 +855,7 @@ module.exports = React.createClass({
                         type="checkbox"
                         id={featureId}
                         name={featureId}
-                        defaultChecked={SettingsStore.isFeatureEnabled(featureId)}
+                        checked={SettingsStore.isFeatureEnabled(featureId)}
                         onChange={onChange}
                     />
                     <label htmlFor={featureId}>{ SettingsStore.getDisplayName(featureId) }</label>

src/settings/SettingsStore.js

@@ -260,7 +260,7 @@ export default class SettingsStore {
      * @param {*} value The new value of the setting, may be null.
      * @return {Promise} Resolves when the setting has been changed.
      */
-    static setValue(settingName, roomId, level, value) {
+    static async setValue(settingName, roomId, level, value) {
         // Verify that the setting is actually a setting
         if (!SETTINGS[settingName]) {
             throw new Error("Setting '" + settingName + "' does not appear to be a setting.");
@@ -275,11 +275,21 @@ export default class SettingsStore {
             throw new Error("User cannot set " + settingName + " at " + level + " in " + roomId);
         }
 
-        return handler.setValue(settingName, roomId, value).then(() => {
-            const controller = SETTINGS[settingName].controller;
-            if (!controller) return;
+        const controller = SETTINGS[settingName].controller;
+        if (controller) {
+            const changeAllowed = await controller.canChangeTo(level, roomId, value);
+            if (!changeAllowed) {
+                return false;
+            }
+        }
+
+        await handler.setValue(settingName, roomId, value);
+
+        if (controller) {
             controller.onChange(level, roomId, value);
-        });
+        }
+
+        return true;
     }
 
     /**

src/settings/controllers/SettingController.js

@@ -39,6 +39,10 @@ export default class SettingController {
         return null; // no override
     }
 
+    canChangeTo(level, roomId, newValue) {
+        return Promise.resolve(true);
+    }
+
     /**
      * Called when the setting value has been changed.
      * @param {string} level The level at which the setting has been modified.