Add flutter code for usage together with firebase_messaging

docs/sdks/flutter/troubleshooting.md

@@ -7,6 +7,100 @@ title: Troubleshooting
 
 Please make sure you are not using the `firebase_messaging` dependency together with our SDK. The Android Firebase setup should be done directly in the Android project by following the official guide from the Firebase docs.
 
+If you want to use our SDK together with `firebase_messaging`, although we do not recommend it, you can use the following workaround:
+
+1. Add this to your `android/app/src/main/AndroidManifest.xml` file inside the `<application>` tag:
+```
+<service
+    android:name=".FirebaseMessagingServiceProxy"
+    android:exported="false">
+    <intent-filter>
+        <action android:name="com.google.firebase.MESSAGING_EVENT" />
+    </intent-filter>
+</service>
+``
+
+2. Create the `FirebaseMessagingServiceProxy` class in your Android project:
+```
+package com.example;
+
+import androidx.annotation.NonNull;
+
+import com.cleverpush.service.CleverPushFcmListenerService;
+import io.flutter.plugins.firebase.messaging.FlutterFirebaseMessagingService;
+import com.google.firebase.messaging.FirebaseMessagingService;
+import com.google.firebase.messaging.RemoteMessage;
+
+import java.lang.reflect.Field;
+import java.util.ArrayList;
+import java.util.List;
+
+public class FirebaseMessagingServiceProxy extends FirebaseMessagingService {
+    private final List<FirebaseMessagingService> messagingServices = new ArrayList<>(2);
+
+    public FirebaseMessagingServiceProxy() {
+        messagingServices.add(new CleverPushFcmListenerService());
+        messagingServices.add(new FlutterFirebaseMessagingService());
+    }
+
+    @Override
+    public void onNewToken(@NonNull String token) {
+        delegate(service -> {
+            injectContext(service);
+            service.onNewToken(token);
+        });
+    }
+
+    @Override
+    public void onMessageReceived(@NonNull RemoteMessage remoteMessage) {
+        delegate(service -> {
+            injectContext(service);
+            service.onMessageReceived(remoteMessage);
+        });
+    }
+
+    private void delegate(CPAction<FirebaseMessagingService> action) {
+        for (FirebaseMessagingService service : messagingServices) {
+            action.run(service);
+        }
+    }
+
+    private void injectContext(FirebaseMessagingService service) {
+        setField(service, "mBase", this);
+    }
+
+    private boolean setField(Object targetObject, String fieldName, Object fieldValue) {
+        Field field;
+        try {
+            field = targetObject.getClass().getDeclaredField(fieldName);
+        } catch (NoSuchFieldException e) {
+            field = null;
+        }
+        Class superClass = targetObject.getClass().getSuperclass();
+        while (field == null && superClass != null) {
+            try {
+                field = superClass.getDeclaredField(fieldName);
+            } catch (NoSuchFieldException e) {
+                superClass = superClass.getSuperclass();
+            }
+        }
+        if (field == null) {
+            return false;
+        }
+        field.setAccessible(true);
+        try {
+            field.set(targetObject, fieldValue);
+            return true;
+        } catch (IllegalAccessException e) {
+            return false;
+        }
+    }
+
+    interface CPAction<T> {
+        void run(T t);
+    }
+}
+```
 
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