Types for BackHandler

Summary: Improve types for BackHandler

Reviewed By: benjaffe

Differential Revision: D9838175

fbshipit-source-id: 839a5a695b468974aadb7bc0da1a23ef844fdd7f

Libraries/Utilities/BackHandler.android.js

@@ -4,8 +4,8 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @format
  * @flow
+ * @format
  */
 
 'use strict';
@@ -15,10 +15,7 @@ const RCTDeviceEventEmitter = require('RCTDeviceEventEmitter');
 
 const DEVICE_BACK_EVENT = 'hardwareBackPress';
 
-type BackPressEventName = $Enum<{
-  backPress: string,
-  hardwareBackPress: string,
-}>;
+type BackPressEventName = 'backPress' | 'hardwareBackPress';
 
 const _backPressSubscriptions = [];
 
@@ -62,7 +59,18 @@ RCTDeviceEventEmitter.addListener(DEVICE_BACK_EVENT, function() {
  * });
  * ```
  */
-const BackHandler = {
+type TBackHandler = {|
+  +exitApp: () => void,
+  +addEventListener: (
+    eventName: BackPressEventName,
+    handler: Function,
+  ) => {remove: () => void},
+  +removeEventListener: (
+    eventName: BackPressEventName,
+    handler: Function,
+  ) => void,
+|};
+const BackHandler: TBackHandler = {
   exitApp: function(): void {
     DeviceEventManager.invokeDefaultBackPressHandler();
   },