fix(messaging, ios): prevent getInitialMessage from being null at the start of the app

packages/firebase_messaging/firebase_messaging/example/lib/main.dart

@@ -9,14 +9,14 @@ import 'package:firebase_core/firebase_core.dart';
 import 'package:firebase_messaging/firebase_messaging.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
-import 'package:http/http.dart' as http;
 import 'package:flutter_local_notifications/flutter_local_notifications.dart';
+import 'package:http/http.dart' as http;
 
+import 'firebase_options.dart';
 import 'message.dart';
 import 'message_list.dart';
 import 'permissions.dart';
 import 'token_monitor.dart';
-import 'firebase_options.dart';
 
 /// Working example of FirebaseMessaging.
 /// Please use this in order to verify messages are working in foreground, background & terminated state.
@@ -171,11 +171,22 @@ class Application extends StatefulWidget {
 
 class _Application extends State<Application> {
   String? _token;
+  String? initialMessage;
+  bool _resolved = false;
 
   @override
   void initState() {
     super.initState();
 
+    FirebaseMessaging.instance.getInitialMessage().then(
+          (value) => setState(
+            () {
+              _resolved = true;
+              initialMessage = value?.data.toString();
+            },
+          ),
+        );
+
     FirebaseMessaging.onMessage.listen(showFlutterNotification);
 
     FirebaseMessaging.onMessageOpenedApp.listen((RemoteMessage message) {
@@ -289,13 +300,25 @@ class _Application extends State<Application> {
         child: Column(
           children: [
             MetaCard('Permissions', Permissions()),
+            MetaCard(
+              'Initial Message',
+              Column(
+                children: [
+                  Text(_resolved ? 'Resolved' : 'Resolving'),
+                  Text(initialMessage ?? 'None'),
+                ],
+              ),
+            ),
             MetaCard(
               'FCM Token',
               TokenMonitor((token) {
                 _token = token;
                 return token == null
                     ? const CircularProgressIndicator()
-                    : Text(token, style: const TextStyle(fontSize: 12));
+                    : SelectableText(
+                        token,
+                        style: const TextStyle(fontSize: 12),
+                      );
               }),
             ),
             ElevatedButton(

packages/firebase_messaging/firebase_messaging/ios/Classes/FLTFirebaseMessagingPlugin.m

@@ -22,6 +22,12 @@ @implementation FLTFirebaseMessagingPlugin {
   NSObject<FlutterPluginRegistrar> *_registrar;
   NSData *_apnsToken;
   NSDictionary *_initialNotification;
+
+  // Used to track if everything as been initialized before answering
+  // to the initialNotification request
+  BOOL _initialNotificationGathered;
+  FLTFirebaseMethodCallResult *_initialNotificationResult;
+
   NSString *_initialNoticationID;
   NSString *_notificationOpenedAppID;
 
@@ -43,6 +49,7 @@ - (instancetype)initWithFlutterMethodChannel:(FlutterMethodChannel *)channel
                    andFlutterPluginRegistrar:(NSObject<FlutterPluginRegistrar> *)registrar {
   self = [super init];
   if (self) {
+    _initialNotificationGathered = NO;
     _channel = channel;
     _registrar = registrar;
     // Application
@@ -109,7 +116,9 @@ - (void)handleMethodCall:(FlutterMethodCall *)call result:(FlutterResult)flutter
   [self ensureAPNSTokenSetting];
 
   if ([@"Messaging#getInitialMessage" isEqualToString:call.method]) {
-    methodCallResult.success([self copyInitialNotification]);
+    _initialNotificationResult = methodCallResult;
+    [self initialNotificationCallback];
+
   } else if ([@"Messaging#deleteToken" isEqualToString:call.method]) {
     [self messagingDeleteToken:call.arguments withMethodCallResult:methodCallResult];
   } else if ([@"Messaging#getAPNSToken" isEqualToString:call.method]) {
@@ -207,6 +216,8 @@ - (void)application_onDidFinishLaunchingNotification:(nonnull NSNotification *)n
         [FLTFirebaseMessagingPlugin remoteMessageUserInfoToDict:remoteNotification];
     _initialNoticationID = remoteNotification[@"gcm.message_id"];
   }
+  _initialNotificationGathered = YES;
+  [self initialNotificationCallback];
 
 #if TARGET_OS_OSX
   // For macOS we use swizzling to intercept as addApplicationDelegate does not exist on the macOS
@@ -330,7 +341,7 @@ - (void)userNotificationCenter:(UNUserNotificationCenter *)center
   }
 }
 
-// Called when a use interacts with a notification.
+// Called when a user interacts with a notification.
 - (void)userNotificationCenter:(UNUserNotificationCenter *)center
     didReceiveNotificationResponse:(UNNotificationResponse *)response
              withCompletionHandler:(void (^)(void))completionHandler
@@ -1013,6 +1024,13 @@ - (nullable NSDictionary *)copyInitialNotification {
   return nil;
 }
 
+- (void)initialNotificationCallback {
+  if (_initialNotificationGathered && _initialNotificationResult != nil) {
+    _initialNotificationResult.success([self copyInitialNotification]);
+    _initialNotificationResult = nil;
+  }
+}
+
 - (NSDictionary *)NSDictionaryForNSError:(NSError *)error {
   NSString *code = @"unknown";
   NSString *message = @"An unknown error has occurred.";