Only store completionHandlers on iOS native side when user adds listener

Note the following cases. This commit is catering for case 3:
1. User is listening for onNotificationDisplayed and is manually calling
the completionHandler
- manually called completionHandler is removed
- automatically called completionHandler is guarded against on iOS
native side

2. User is listening for onNotificationDisplayed and is not calling the
completionHandler:
- automatically called completionHandler is removed

3. User is not listening for onNotificationDisplayed
- On rn side we can only automatically call completionHandler if the
user _is_ listening. This means we need to detect if the user is
listening or not.

ios/RNFirebase/notifications/RNFirebaseNotifications.m

@@ -17,6 +17,7 @@
 @end
 
 @implementation RNFirebaseNotifications {
+    BOOL isUserHandlingOnNotificationDisplayed;
     NSMutableDictionary<NSString *, void (^)(UIBackgroundFetchResult)> *completionHandlers;
 }
 
@@ -56,8 +57,6 @@ RCT_EXPORT_MODULE();
 
     // Set static instance for use from AppDelegate
     theRNFirebaseNotifications = self;
-
-    completionHandlers = [[NSMutableDictionary alloc] init];
 }
 
 // PRE-BRIDGE-EVENTS: Consider enabling this to allow events built up before the bridge is built to be sent to the JS side
@@ -99,6 +98,21 @@ RCT_EXPORT_MODULE();
     }
 }
 
+RCT_EXPORT_METHOD(startHandlingNotificationDisplayed) {
+    isUserHandlingOnNotificationDisplayed = YES;
+    completionHandlers = [[NSMutableDictionary alloc] init];
+}
+
+RCT_EXPORT_METHOD(stopHandlingNotificationDisplayed) {
+    isUserHandlingOnNotificationDisplayed = NO;
+    NSArray *allHandlers = completionHandlers.allValues;
+    for (void (^ completionHandler)(UIBackgroundFetchResult) in allHandlers) {
+        completionHandler(UIBackgroundFetchResultNoData);
+    }
+    [completionHandlers removeAllObjects];
+    completionHandlers = nil;
+}
+
 RCT_EXPORT_METHOD(complete:(NSString*)handlerKey fetchResult:(NSString *)rnFetchResult) {
     UIBackgroundFetchResult fetchResult = UIBackgroundFetchResultNoData;
     if ([@"noData" isEqualToString:rnFetchResult]) {
@@ -117,6 +131,15 @@ RCT_EXPORT_METHOD(complete:(NSString*)handlerKey fetchResult:(NSString *)rnFetch
     }
 }
 
+- (void)executeOrStoreCompletionHandler:(void (^ _Nonnull)(UIBackgroundFetchResult))completionHandler notification:(NSDictionary *)notification {
+    if(isUserHandlingOnNotificationDisplayed) {
+        NSString *handlerKey = notification[@"notificationId"];
+        completionHandlers[handlerKey] = completionHandler;
+    } else {
+        completionHandler(UIBackgroundFetchResultNoData);
+    }
+}
+
 // Listen for background messages
 - (void)didReceiveRemoteNotification:(NSDictionary *)userInfo
               fetchCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler {
@@ -124,9 +147,12 @@ RCT_EXPORT_METHOD(complete:(NSString*)handlerKey fetchResult:(NSString *)rnFetch
     // Pass them over to the RNFirebaseMessaging handler instead
     if (userInfo[@"aps"] && ((NSDictionary*)userInfo[@"aps"]).count == 1 && userInfo[@"aps"][@"content-available"]) {
         [[RNFirebaseMessaging instance] didReceiveRemoteNotification:userInfo];
+        completionHandler(UIBackgroundFetchResultNoData);
         return;
     }
 
+    NSDictionary *notification = [self parseUserInfo:userInfo];
+
     NSString *event;
     if (RCTSharedApplication().applicationState == UIApplicationStateBackground) {
         event = NOTIFICATIONS_NOTIFICATION_DISPLAYED;
@@ -141,10 +167,10 @@ RCT_EXPORT_METHOD(complete:(NSString*)handlerKey fetchResult:(NSString *)rnFetch
         // - foreground notifications also go through willPresentNotification
         // - background notification presses also go through didReceiveNotificationResponse
         // This prevents duplicate messages from hitting the JS app
+        completionHandler(UIBackgroundFetchResultNoData);
         return;
     }
 
-    NSDictionary *notification = [self parseUserInfo:userInfo];
     // For onOpened events, we set the default action name as iOS 8/9 has no concept of actions
     if (event == NOTIFICATIONS_NOTIFICATION_OPENED) {
         notification = @{
@@ -153,8 +179,7 @@ RCT_EXPORT_METHOD(complete:(NSString*)handlerKey fetchResult:(NSString *)rnFetch
                          };
     }
 
-    NSString *handlerKey = notification[@"notificationId"];
+    [self executeOrStoreCompletionHandler:completionHandler notification:notification];
-    completionHandlers[handlerKey] = completionHandler;
 
     [self sendJSEvent:self name:event body:notification];
 }

lib/modules/notifications/index.js

@@ -54,6 +54,11 @@ const NATIVE_EVENTS = [
 export const MODULE_NAME = 'RNFirebaseNotifications';
 export const NAMESPACE = 'notifications';
 
+const hasListeners = (eventType: string): boolean => {
+  const listeners = SharedEventEmitter.listeners(eventType);
+  return listeners && listeners.length;
+};
+
 // iOS 8/9 scheduling
 // fireDate: Date;
 // timeZone: TimeZone;
@@ -255,11 +260,23 @@ export default class Notifications extends ModuleBase {
     }
 
     getLogger(this).info('Creating onNotificationDisplayed listener');
-    SharedEventEmitter.addListener('onNotificationDisplayed', listener);
+    SharedEventEmitter.addListener(
+      'onNotificationDisplayed',
+      listener
+    );
+    if (hasListeners('onNotificationDisplayed')) {
+      getNativeModule(this).startHandlingNotificationDisplayed();
+    }
 
     return () => {
       getLogger(this).info('Removing onNotificationDisplayed listener');
-      SharedEventEmitter.removeListener('onNotificationDisplayed', listener);
+      SharedEventEmitter.removeListener(
+        'onNotificationDisplayed',
+        listener
+      );
+      if (!hasListeners('onNotificationDisplayed')) {
+        getNativeModule(this).stopHandlingNotificationDisplayed();
+      }
     };
   }