Add launcher prefetch setting

Summary: Allows users to override the GK status of prefetching. It is implemented as a Tri-State. "Unset" means that the local Flipper config won't take precedent over the GK setting and will leave it unchanged. If a user interacts with the setting, it gets persisted into the config and will from then-on override the GK, meaning that the user has an opt-in mechanism and if we open the GK to more people, they effectively have an opt-out.

Reviewed By: jknoxville

Differential Revision: D18008259

fbshipit-source-id: bdfde9a8b9acf43aa60c84800a7979a29a4e9364

src/chrome/SettingsSheet.tsx

@@ -18,6 +18,7 @@ import ToggledSection from './settings/ToggledSection';
 import {FilePathConfigField, ConfigText} from './settings/configFields';
 import isEqual from 'lodash.isequal';
 import restartFlipper from '../utils/restartFlipper';
+import LauncherSettingsPanel from '../fb-stubs/LauncherSettingsPanel';
 
 const Container = styled(FlexColumn)({
   padding: 20,
@@ -102,6 +103,17 @@ class SettingsSheet extends Component<Props, State> {
             frozen
           />
         </ToggledSection>
+        <LauncherSettingsPanel
+          enabledInConfig={this.state.updatedSettings.enablePrefetching}
+          onChange={v => {
+            this.setState({
+              updatedSettings: {
+                ...this.state.updatedSettings,
+                enablePrefetching: v,
+              },
+            });
+          }}
+        />
         <br />
         <FlexRow>
           <Spacer />

src/fb-stubs/LauncherSettingsPanel.tsx

@@ -0,0 +1,17 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+import {Tristate} from 'src/reducers/settings';
+
+export default function(_props: {
+  enabledInConfig: Tristate;
+  onChange: (v: Tristate) => void;
+}) {
+  return null;
+}

src/fb-stubs/Prefetcher.tsx

@@ -8,3 +8,4 @@
  */
 
 export default async function setupPrefetcher() {}
+export const shouldInstallPrefetcher = () => false;

src/reducers/settings.tsx

@@ -9,9 +9,21 @@
 
 import {Actions} from './index';
 
+export enum Tristate {
+  True,
+  False,
+  Unset,
+}
+
 export type Settings = {
   androidHome: string;
   enableAndroid: boolean;
+  /**
+   * If unset, this will assume the value of the GK setting.
+   * Note that this setting has no effect in the open source version
+   * of Flipper.
+   */
+  enablePrefetching: Tristate;
 };
 
 export type Action =
@@ -24,6 +36,7 @@ export type Action =
 const initialState: Settings = {
   androidHome: '/opt/android_sdk',
   enableAndroid: true,
+  enablePrefetching: Tristate.Unset,
 };
 
 export default function reducer(