Feature to listen on window focus events (#25039)

Summary:
Addressed issue: https://github.com/facebook/react-native/issues/24149

On Android, activity's lifecycle events are not triggered when the user pulls down the Status Bar (opening Notification Drawer). In order to know that, you need to override [onWindowFocusChanged method](https://developer.android.com/reference/android/app/Activity.html#onWindowFocusChanged(boolean)).

## Changelog

[Android] [Added] - Adds a new listener for `onWindowFocusChanged`
[JavaScript] [Added] - New event, `focusChanged`, to listen on focus gain/loss
Pull Request resolved: https://github.com/facebook/react-native/pull/25039

Differential Revision: D15644954

Pulled By: cpojer

fbshipit-source-id: 823acffc4287bec4bf56e9f5ffcac65c01cf13d3

Libraries/AppState/AppState.js

@@ -25,6 +25,7 @@ const invariant = require('invariant');
  */
 class AppState extends NativeEventEmitter {
   _eventHandlers: Object;
+  _supportedEvents = ['change', 'memoryWarning', 'blur', 'focus'];
   currentState: ?string;
   isAvailable: boolean;
 
@@ -32,10 +33,10 @@ class AppState extends NativeEventEmitter {
     super(NativeAppState);
 
     this.isAvailable = true;
-    this._eventHandlers = {
-      change: new Map(),
-      memoryWarning: new Map(),
-    };
+    this._eventHandlers = this._supportedEvents.reduce((handlers, key) => {
+      handlers[key] = new Map();
+      return handlers;
+    }, {});
 
     this.currentState = NativeAppState.getConstants().initialAppState;
 
@@ -75,22 +76,43 @@ class AppState extends NativeEventEmitter {
    */
   addEventListener(type: string, handler: Function) {
     invariant(
-      ['change', 'memoryWarning'].indexOf(type) !== -1,
+      this._supportedEvents.indexOf(type) !== -1,
       'Trying to subscribe to unknown event: "%s"',
       type,
     );
-    if (type === 'change') {
-      this._eventHandlers[type].set(
-        handler,
-        this.addListener('appStateDidChange', appStateData => {
-          handler(appStateData.app_state);
-        }),
-      );
-    } else if (type === 'memoryWarning') {
-      this._eventHandlers[type].set(
-        handler,
-        this.addListener('memoryWarning', handler),
-      );
+
+    switch (type) {
+      case 'change': {
+        this._eventHandlers[type].set(
+          handler,
+          this.addListener('appStateDidChange', appStateData => {
+            handler(appStateData.app_state);
+          }),
+        );
+        break;
+      }
+      case 'memoryWarning': {
+        this._eventHandlers[type].set(
+          handler,
+          this.addListener('memoryWarning', handler),
+        );
+        break;
+      }
+
+      case 'blur':
+      case 'focus': {
+        this._eventHandlers[type].set(
+          handler,
+          this.addListener('appStateFocusChange', hasFocus => {
+            if (type === 'blur' && !hasFocus) {
+              handler();
+            }
+            if (type === 'focus' && hasFocus) {
+              handler();
+            }
+          }),
+        );
+      }
     }
   }
 
@@ -101,7 +123,7 @@ class AppState extends NativeEventEmitter {
    */
   removeEventListener(type: string, handler: Function) {
     invariant(
-      ['change', 'memoryWarning'].indexOf(type) !== -1,
+      this._supportedEvents.indexOf(type) !== -1,
       'Trying to remove listener for unknown event: "%s"',
       type,
     );

ReactAndroid/src/main/java/com/facebook/react/ReactActivity.java

@@ -125,6 +125,12 @@ public abstract class ReactActivity extends AppCompatActivity
     mDelegate.onRequestPermissionsResult(requestCode, permissions, grantResults);
   }
 
+  @Override
+  public void onWindowFocusChanged(boolean hasFocus) {
+    super.onWindowFocusChanged(hasFocus);
+    mDelegate.onWindowFocusChanged(hasFocus);
+  }
+
   protected final ReactNativeHost getReactNativeHost() {
     return mDelegate.getReactNativeHost();
   }

ReactAndroid/src/main/java/com/facebook/react/ReactActivityDelegate.java

@@ -182,6 +182,12 @@ public class ReactActivityDelegate {
     return false;
   }
 
+  public void onWindowFocusChanged(boolean hasFocus) {
+    if (getReactNativeHost().hasInstance()) {
+      getReactNativeHost().getReactInstanceManager().onWindowFocusChange(hasFocus);
+    }
+  }
+
   @TargetApi(Build.VERSION_CODES.M)
   public void requestPermissions(
     String[] permissions,

ReactAndroid/src/main/java/com/facebook/react/ReactInstanceManager.java

@@ -703,6 +703,15 @@ public class ReactInstanceManager {
     }
   }
 
+  @ThreadConfined(UI)
+  public void onWindowFocusChange(boolean hasFocus) {
+    UiThreadUtil.assertOnUiThread();
+    ReactContext currentContext = getCurrentReactContext();
+    if (currentContext != null) {
+      currentContext.onWindowFocusChange(hasFocus);
+    }
+  }
+
   @ThreadConfined(UI)
   public void showDevOptionsDialog() {
     UiThreadUtil.assertOnUiThread();

ReactAndroid/src/main/java/com/facebook/react/bridge/ReactContext.java

@@ -36,6 +36,8 @@ public class ReactContext extends ContextWrapper {
       new CopyOnWriteArraySet<>();
   private final CopyOnWriteArraySet<ActivityEventListener> mActivityEventListeners =
       new CopyOnWriteArraySet<>();
+  private final CopyOnWriteArraySet<WindowFocusChangeListener> mWindowFocusEventListeners =
+    new CopyOnWriteArraySet<>();
 
   private LifecycleState mLifecycleState = LifecycleState.BEFORE_CREATE;
 
@@ -196,6 +198,14 @@ public class ReactContext extends ContextWrapper {
     mActivityEventListeners.remove(listener);
   }
 
+  public void addWindowFocusChangeListener(WindowFocusChangeListener listener) {
+    mWindowFocusEventListeners.add(listener);
+  }
+
+  public void removeWindowFocusChangeListener(WindowFocusChangeListener listener) {
+    mWindowFocusEventListeners.remove(listener);
+  }
+
   /**
    * Should be called by the hosting Fragment in {@link Fragment#onResume}
    */
@@ -281,6 +291,17 @@ public class ReactContext extends ContextWrapper {
     }
   }
 
+  public void onWindowFocusChange(boolean hasFocus) {
+    UiThreadUtil.assertOnUiThread();
+    for (WindowFocusChangeListener listener : mWindowFocusEventListeners) {
+      try {
+        listener.onWindowFocusChange(hasFocus);
+      } catch (RuntimeException e) {
+        handleException(e);
+      }
+    }
+  }
+
   public void assertOnUiQueueThread() {
     Assertions.assertNotNull(mUiMessageQueueThread).assertIsOnThread();
   }

ReactAndroid/src/main/java/com/facebook/react/bridge/WindowFocusChangeListener.java

@@ -0,0 +1,15 @@
+// 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.
+
+
+package com.facebook.react.bridge;
+
+
+/*
+ * Listener for receiving window focus events.
+ */
+public interface WindowFocusChangeListener {
+  void onWindowFocusChange(boolean hasFocus);
+}

ReactAndroid/src/main/java/com/facebook/react/modules/appstate/AppStateModule.java

@@ -13,6 +13,7 @@ import com.facebook.react.bridge.LifecycleEventListener;
 import com.facebook.react.bridge.ReactApplicationContext;
 import com.facebook.react.bridge.ReactContextBaseJavaModule;
 import com.facebook.react.bridge.ReactMethod;
+import com.facebook.react.bridge.WindowFocusChangeListener;
 import com.facebook.react.bridge.WritableMap;
 import com.facebook.react.common.LifecycleState;
 import com.facebook.react.module.annotations.ReactModule;
@@ -23,7 +24,7 @@ import java.util.Map;
 
 @ReactModule(name = AppStateModule.NAME)
 public class AppStateModule extends ReactContextBaseJavaModule
-        implements LifecycleEventListener {
+        implements LifecycleEventListener, WindowFocusChangeListener {
 
   protected static final String NAME = "AppState";
 
@@ -37,6 +38,7 @@ public class AppStateModule extends ReactContextBaseJavaModule
   public AppStateModule(ReactApplicationContext reactContext) {
     super(reactContext);
     reactContext.addLifecycleEventListener(this);
+    reactContext.addWindowFocusChangeListener(this);
     mAppState = (reactContext.getLifecycleState() == LifecycleState.RESUMED ?
             APP_STATE_ACTIVE : APP_STATE_BACKGROUND);
   }
@@ -76,6 +78,12 @@ public class AppStateModule extends ReactContextBaseJavaModule
     // catalyst instance is going to be immediately dropped, and all JS calls with it.
   }
 
+  @Override
+  public void onWindowFocusChange(boolean hasFocus) {
+    getReactApplicationContext().getJSModule(RCTDeviceEventEmitter.class)
+      .emit("appStateFocusChange", hasFocus);
+  }
+
   private WritableMap createAppStateEventMap() {
     WritableMap appState = Arguments.createMap();
     appState.putString("app_state", mAppState);