perf(messaging, ios): Improve time to delivery of background messages on iOS (#5547)

* Defer callback event ‘messaging_message_received_background’ until the RN bridge is initialized and the javascript code has called setBackgroundMessageHandler
* Timeout tasks waiting on the client to register with setBackgroundMessageHandler

Author: Brian Meek

packages/messaging/ios/RNFBMessaging/RNFBMessaging+AppDelegate.h

@@ -25,11 +25,16 @@ NS_ASSUME_NONNULL_BEGIN
 
 @property _Nullable RCTPromiseRejectBlock registerPromiseRejecter;
 @property _Nullable RCTPromiseResolveBlock registerPromiseResolver;
+@property (nonatomic, strong) NSCondition *conditionBackgroundMessageHandlerSet;
+@property (nonatomic) BOOL backgroundMessageHandlerSet;
+
 
 + (_Nonnull instancetype)sharedInstance;
 
 - (void)observe;
 
+- (void)signalBackgroundMessageHandlerSet;
+
 - (void)setPromiseResolve:(RCTPromiseResolveBlock)resolve andPromiseReject:(RCTPromiseRejectBlock)reject;
 
 - (void)application:(UIApplication *)application didRegisterForRemoteNotificationsWithDeviceToken:(NSData *)deviceToken;

packages/messaging/ios/RNFBMessaging/RNFBMessaging+AppDelegate.m

@@ -33,6 +33,8 @@ + (instancetype)sharedInstance {
   __strong static RNFBMessagingAppDelegate *sharedInstance;
   dispatch_once(&once, ^{
     sharedInstance = [[RNFBMessagingAppDelegate alloc] init];
+    sharedInstance.conditionBackgroundMessageHandlerSet = [[NSCondition alloc] init];
+    sharedInstance.backgroundMessageHandlerSet = NO;
   });
   return sharedInstance;
 }
@@ -66,6 +68,16 @@ - (void)observe {
   });
 }
 
+// used to signal that a javascript handler for background messages is set
+- (void)signalBackgroundMessageHandlerSet {
+  RNFBMessagingAppDelegate *sharedInstance = [RNFBMessagingAppDelegate sharedInstance];
+  [sharedInstance.conditionBackgroundMessageHandlerSet lock];
+  DLog(@"signalBackgroundMessageHandlerSet sharedInstance.backgroundMessageHandlerSet was %@", sharedInstance.backgroundMessageHandlerSet ? @"YES" : @"NO");
+  sharedInstance.backgroundMessageHandlerSet = YES;
+  [sharedInstance.conditionBackgroundMessageHandlerSet broadcast];
+  [sharedInstance.conditionBackgroundMessageHandlerSet unlock];  
+}
+
 // used to temporarily store a promise instance to resolve calls to `registerForRemoteNotifications`
 - (void)setPromiseResolve:(RCTPromiseResolveBlock)resolve andPromiseReject:(RCTPromiseRejectBlock)reject {
   _registerPromiseResolver = resolve;
@@ -102,6 +114,7 @@ - (void)application:(UIApplication *)application didFailToRegisterForRemoteNotif
 - (void)application:(UIApplication *)application didReceiveRemoteNotification:(NSDictionary *)userInfo fetchCompletionHandler:(void (^)(UIBackgroundFetchResult result))completionHandler {
   #if __has_include(<FirebaseAuth/FirebaseAuth.h>)
   if ([[FIRAuth auth] canHandleNotification:userInfo]) {
+    DLog(@"didReceiveRemoteNotification Firebase Auth handeled the notification");
     completionHandler(UIBackgroundFetchResultNoData);
     return;
   }
@@ -110,6 +123,8 @@ - (void)application:(UIApplication *)application didReceiveRemoteNotification:(N
   [[NSNotificationCenter defaultCenter] postNotificationName:@"RNFBMessagingDidReceiveRemoteNotification" object:userInfo];
 
   if (userInfo[@"gcm.message_id"]) {
+    DLog(@"didReceiveRemoteNotification gcm.message_id was present %@", userInfo);
+
     if ([UIApplication sharedApplication].applicationState == UIApplicationStateBackground) {
       // If app is in background state, register background task to guarantee async queues aren't frozen.
       UIBackgroundTaskIdentifier __block backgroundTaskId = [application beginBackgroundTaskWithExpirationHandler:^{
@@ -129,13 +144,46 @@ - (void)application:(UIApplication *)application didReceiveRemoteNotification:(N
         }
       });
 
-      // TODO investigate later - RN bridge gets invalidated at start when in background and a new bridge created - losing all events
-      // TODO   so we just delay sending the event for a few seconds as a workaround
-      // TODO   most likely Remote Debugging causing bridge to be invalidated
-      dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t) (8 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
-        [[RNFBRCTEventEmitter shared] sendEventWithName:@"messaging_message_received_background" body:[RNFBMessagingSerializer remoteMessageUserInfoToDict:userInfo]];
-      });
-    } else {
+      RNFBMessagingAppDelegate *sharedInstance = [RNFBMessagingAppDelegate sharedInstance];
+      [sharedInstance.conditionBackgroundMessageHandlerSet lock];
+      @try {
+        DLog(@"didReceiveRemoteNotification sharedInstance.backgroundMessageHandlerSet = %@", sharedInstance.backgroundMessageHandlerSet ? @"YES" : @"NO");
+        if (sharedInstance.backgroundMessageHandlerSet) {
+          // Normal path, backgroundMessageHandlerSet has already been set, queue the notification for immediate delivery
+          dispatch_async(dispatch_get_main_queue(), ^{
+            [[RNFBRCTEventEmitter shared] sendEventWithName:@"messaging_message_received_background" body:[RNFBMessagingSerializer remoteMessageUserInfoToDict:userInfo]];
+          });
+          DLog(@"didReceiveRemoteNotification without waiting for backgroundMessageHandlerSet to be set");
+        } else {
+          // This spin needs to be on a background/concurrent queue to await the setup of backgroundMessageHandlerSet and not block the main thread
+          dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
+            // Reaquire the lock in this new closure
+            [sharedInstance.conditionBackgroundMessageHandlerSet lock];
+            @try {
+              // Spin/wait until backgroundMessageHandlerSet
+              // NB it is possible while this closure was being scheduled that backgroundMessageHandlerSet is already set and this loop is skipped
+              while (!sharedInstance.backgroundMessageHandlerSet) {
+                DLog(@"didReceiveRemoteNotification waiting for sharedInstance.backgroundMessageHandlerSet %@", sharedInstance.backgroundMessageHandlerSet ? @"YES" : @"NO");
+                if(![sharedInstance.conditionBackgroundMessageHandlerSet waitUntilDate:[NSDate dateWithTimeIntervalSinceNow:25]]) {
+                  // If after 25 seconds the client hasn't called backgroundMessageHandlerSet, give up on this notification
+                  ELog(@"didReceiveRemoteNotification timed out waiting for sharedInstance.backgroundMessageHandlerSet");
+                  return;
+                }
+              }
+              dispatch_async(dispatch_get_main_queue(), ^{
+                [[RNFBRCTEventEmitter shared] sendEventWithName:@"messaging_message_received_background" body:[RNFBMessagingSerializer remoteMessageUserInfoToDict:userInfo]];
+              });
+              DLog(@"didReceiveRemoteNotification after waiting for backgroundMessageHandlerSet");
+            } @finally {
+              [sharedInstance.conditionBackgroundMessageHandlerSet unlock];
+            }
+          });
+        }
+      } @finally {
+        [sharedInstance.conditionBackgroundMessageHandlerSet unlock];
+      }
+  } else {
+      DLog(@"didReceiveRemoteNotification while app was in foreground");
       [[RNFBRCTEventEmitter shared] sendEventWithName:@"messaging_message_received" body:[RNFBMessagingSerializer remoteMessageUserInfoToDict:userInfo]];
       completionHandler(UIBackgroundFetchResultNoData);
     }

packages/messaging/ios/RNFBMessaging/RNFBMessagingModule.m

@@ -89,6 +89,15 @@ - (NSDictionary *)constantsToExport {
   return resolve([NSNull null]);
 }
 
+RCT_EXPORT_METHOD(signalBackgroundMessageHandlerSet) {
+  DLog(@"signalBackgroundMessageHandlerSet called");
+  @try {
+    [[RNFBMessagingAppDelegate sharedInstance] signalBackgroundMessageHandlerSet];
+  } @catch (NSException *exception) {
+    ELog(@"signalBackgroundMessageHandlerSet failed");
+  }
+}
+
 RCT_EXPORT_METHOD(getToken:
   (RCTPromiseResolveBlock) resolve
     :(RCTPromiseRejectBlock) reject

packages/messaging/lib/index.js

@@ -288,7 +288,10 @@ class FirebaseMessagingModule extends FirebaseModule {
   }
 
   /**
-   * @platform android
+   * Set a handler that will be called when a message is received while the app is in the background.
+   * Should be called before the app is registered in `AppRegistry`, for example in `index.js`.
+   * An app is considered to be in the background if no active window is displayed.
+   * @param handler called with an argument of type messaging.RemoteMessage that must be async and return a Promise
    */
   setBackgroundMessageHandler(handler) {
     if (!isFunction(handler)) {
@@ -298,6 +301,9 @@ class FirebaseMessagingModule extends FirebaseModule {
     }
 
     backgroundMessageHandler = handler;
+    if (isIOS) {
+      this.native.signalBackgroundMessageHandlerSet();
+    }
   }
 
   sendMessage(remoteMessage) {