[fix] AppState addEventListener returns subscription object

Close necolas#2129

Author: netshade

packages/docs/src/pages/docs/apis/app-state.md

@@ -36,7 +36,7 @@ Returns the current state of the app.
 
 ### Static methods
 
-{% call macro.prop('addEventListener', '(type: ?string, listener: (boolean) => void) => void') %}
+{% call macro.prop('addEventListener', '(type: ?string, listener: (boolean) => void) => ?EmitterSubscription') %}
 Add a listener to `AppState` changes. Listen to the `"change"` event type. The handler is called with the app state value.
 {% endcall %}
 

packages/examples/pages/app-state/index.js

@@ -18,9 +18,9 @@ export default function AppStatePage() {
       }));
     };
 
-    AppState.addEventListener('change', handleChange);
+    const subscription = AppState.addEventListener('change', handleChange);
     return () => {
-      AppState.removeEventListener('change', handleChange);
+      subscription.remove();
     };
   }, []);
 

packages/react-native-web/src/exports/AppState/__tests__/index-test.js

@@ -5,22 +5,22 @@ import AppState from '..';
 describe('apis/AppState', () => {
   const handler = () => {};
 
-  afterEach(() => {
-    try {
-      AppState.removeEventListener('change', handler);
-    } catch (e) {}
-  });
-
   describe('addEventListener', () => {
     test('throws if the provided "eventType" is not supported', () => {
       expect(() => AppState.addEventListener('foo', handler)).toThrow();
-      expect(() => AppState.addEventListener('change', handler)).not.toThrow();
+      expect(() => AppState.addEventListener('change', handler).remove()).not.toThrow();
     });
   });
 
   describe('removeEventListener', () => {
-    test('throws if the handler is not registered', () => {
-      expect(() => AppState.removeEventListener('change', handler)).toThrow();
+    beforeEach(() => {
+      // removeEventListener logs a deprecation warning, ignore
+      jest.spyOn(console, 'error');
+      console.error.mockImplementation(() => {});
+    });
+
+    afterEach(() => {
+      console.error.mockRestore();
     });
 
     test('throws if the provided "eventType" is not supported', () => {

packages/react-native-web/src/exports/AppState/index.js

@@ -9,8 +9,8 @@
  */
 
 import { canUseDOM } from 'fbjs/lib/ExecutionEnvironment';
-import findIndex from 'array-find-index';
 import invariant from 'fbjs/lib/invariant';
+import EventEmitter from '../../vendor/react-native/emitter/_EventEmitter';
 
 // Android 4.4 browser
 const isPrefixed =
@@ -25,7 +25,7 @@ const AppStates = {
   ACTIVE: 'active'
 };
 
-const listeners = [];
+let changeEmitter = null;
 
 export default class AppState {
   static isAvailable = canUseDOM && document[VISIBILITY_STATE_PROPERTY];
@@ -53,9 +53,19 @@ export default class AppState {
         type
       );
       if (type === 'change') {
-        const callback = () => handler(AppState.currentState);
-        listeners.push([handler, callback]);
-        document.addEventListener(VISIBILITY_CHANGE_EVENT, callback, false);
+        if (!changeEmitter) {
+          changeEmitter = new EventEmitter();
+          document.addEventListener(
+            VISIBILITY_CHANGE_EVENT,
+            () => {
+              if (changeEmitter) {
+                changeEmitter.emit('change', AppState.currentState);
+              }
+            },
+            false
+          );
+        }
+        return changeEmitter.addListener(type, handler);
       }
     }
   }
@@ -67,15 +77,8 @@ export default class AppState {
         'Trying to remove listener for unknown event: "%s"',
         type
       );
-      if (type === 'change') {
-        const listenerIndex = findIndex(listeners, (pair) => pair[0] === handler);
-        invariant(
-          listenerIndex !== -1,
-          'Trying to remove AppState listener for unregistered handler'
-        );
-        const callback = listeners[listenerIndex][1];
-        document.removeEventListener(VISIBILITY_CHANGE_EVENT, callback, false);
-        listeners.splice(listenerIndex, 1);
+      if (type === 'change' && changeEmitter) {
+        changeEmitter.removeListener(handler);
       }
     }
   }