Add spec for AppState (facebook#24880)

Summary:
part of facebook#24875.

## Changelog

[General] [Added] - Add TM spec for AppState
Pull Request resolved: facebook#24880

Differential Revision: D15379482

Pulled By: fkgozali

fbshipit-source-id: a76a145a77388cd2ba5cb97d17570162b38f1089

Libraries/AppState/AppState.js

@@ -12,8 +12,7 @@
 
 const EventEmitter = require('../vendor/emitter/EventEmitter');
 const NativeEventEmitter = require('../EventEmitter/NativeEventEmitter');
-const NativeModules = require('../BatchedBridge/NativeModules');
-const RCTAppState = NativeModules.AppState;
+import NativeAppState from './NativeAppState';
 
 const logError = require('../Utilities/logError');
 const invariant = require('invariant');
@@ -30,15 +29,15 @@ class AppState extends NativeEventEmitter {
   isAvailable: boolean;
 
   constructor() {
-    super(RCTAppState);
+    super(NativeAppState);
 
     this.isAvailable = true;
     this._eventHandlers = {
       change: new Map(),
       memoryWarning: new Map(),
     };
 
-    this.currentState = RCTAppState.initialAppState;
+    this.currentState = NativeAppState.getConstants().initialAppState;
 
     let eventUpdated = false;
 
@@ -54,7 +53,7 @@ class AppState extends NativeEventEmitter {
     // TODO: see above - this request just populates the value of `currentState`
     // when the module is first initialized. Would be better to get rid of the
     // prop and expose `getCurrentAppState` method directly.
-    RCTAppState.getCurrentAppState(appStateData => {
+    NativeAppState.getCurrentAppState(appStateData => {
       // It's possible that the state will have changed here & listeners need to be notified
       if (!eventUpdated && this.currentState !== appStateData.app_state) {
         this.currentState = appStateData.app_state;
@@ -152,7 +151,7 @@ class MissingNativeAppStateShim extends EventEmitter {
 // This module depends on the native `RCTAppState` module. If you don't include it,
 // `AppState.isAvailable` will return `false`, and any method calls will throw.
 // We reassign the class variable to keep the autodoc generator happy.
-if (RCTAppState) {
+if (NativeAppState) {
   AppState = new AppState();
 } else {
   AppState = new MissingNativeAppStateShim();

Libraries/AppState/NativeAppState.js

@@ -0,0 +1,30 @@
+/**
+ * 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.
+ *
+ * @flow
+ * @format
+ */
+
+'use strict';
+
+import type {TurboModule} from 'RCTExport';
+import * as TurboModuleRegistry from 'TurboModuleRegistry';
+
+export interface Spec extends TurboModule {
+  +getConstants: () => {|
+    initialAppState: string,
+  |};
+  +getCurrentAppState: (
+    success: (appState: {|app_state: string|}) => void,
+    failure: (error: Object) => void,
+  ) => void;
+
+  // Events
+  +addListener: (eventName: string) => void;
+  +removeListeners: (count: number) => void;
+}
+
+export default TurboModuleRegistry.getEnforcing<Spec>('AppState');