feat: Network Scene Checker Component (#1271)

* Create Network Scene Checker Component

* Move objects to subscene in client

* Remove OnStartClient

* Rewritten with static dictionary

* Updated doc

* renamed image

* Replaced image

* fixed image name

* Changed to short-circuit return in Update

* Changed to using string for key since additive scenes may be unloaded

* Added null check

* Removed commented line

* Update Assets/Mirror/Components/NetworkSceneChecker.cs

Co-Authored-By: vis2k <info@noobtuts.com>

* Moved initializer to declaration

* Removed [Scene, SerializeField]

Co-authored-by: vis2k <info@noobtuts.com>

Assets/Mirror/Components/NetworkSceneChecker.cs

@@ -0,0 +1,115 @@
+using UnityEngine;
+using System.Collections.Generic;
+using UnityEngine.SceneManagement;
+
+namespace Mirror
+{
+    /// <summary>
+    /// Component that controls visibility of networked objects between scenes.
+    /// <para>Any object with this component on it will only be visible to other objects in the same scene</para>
+    /// <para>This would be used when the server has multiple additive subscenes loaded to isolate players to their respective subscenes</para>
+    /// </summary>
+    [AddComponentMenu("Network/NetworkSceneChecker")]
+    [RequireComponent(typeof(NetworkIdentity))]
+    [HelpURL("https://mirror-networking.com/docs/Components/NetworkSceneChecker.html")]
+    public class NetworkSceneChecker : NetworkBehaviour
+    {
+        /// <summary>
+        /// Flag to force this object to be hidden from all observers.
+        /// <para>If this object is a player object, it will not be hidden for that client.</para>
+        /// </summary>
+        [Tooltip("Enable to force this object to be hidden from all observers.")]
+        public bool forceHidden;
+
+        static Dictionary<string, HashSet<NetworkIdentity>> sceneCheckerObjects = new Dictionary<string, HashSet<NetworkIdentity>>();
+
+        string currentScene;
+
+        [ServerCallback]
+        void Awake()
+        {
+            currentScene = gameObject.scene.name;
+        }
+
+        public override void OnStartServer()
+        {
+            if (!sceneCheckerObjects.ContainsKey(currentScene))
+                sceneCheckerObjects.Add(currentScene, new HashSet<NetworkIdentity>());
+
+            sceneCheckerObjects[currentScene].Add(netIdentity);
+        }
+
+        [ServerCallback]
+        void Update()
+        {
+            if (currentScene == gameObject.scene.name)
+                return;
+
+            // This object is in a new scene so observers in the prior scene
+            // and the new scene need to rebuild their respective observers lists.
+
+            // Remove this object from the hashset of the scene it just left
+            sceneCheckerObjects[currentScene].Remove(netIdentity);
+
+            // RebuildObservers of all NetworkIdentity's in the scene this object just left
+            RebuildSceneObservers();
+
+            // Set this to the new scene this object just entered
+            currentScene = gameObject.scene.name;
+
+            // Make sure this new scene is in the dictionary
+            if (!sceneCheckerObjects.ContainsKey(currentScene))
+                sceneCheckerObjects.Add(currentScene, new HashSet<NetworkIdentity>());
+
+            // Add this object to the hashset of the new scene
+            sceneCheckerObjects[currentScene].Add(netIdentity);
+
+            // RebuildObservers of all NetworkIdentity's in the scene this object just entered
+            RebuildSceneObservers();
+        }
+
+        void RebuildSceneObservers()
+        {
+            foreach (NetworkIdentity networkIdentity in sceneCheckerObjects[currentScene])
+                if (networkIdentity != null)
+                    networkIdentity.RebuildObservers(false);
+        }
+
+        public override bool OnCheckObserver(NetworkConnection conn)
+        {
+            if (forceHidden)
+                return false;
+
+            return conn.identity.gameObject.scene == gameObject.scene;
+        }
+
+        // Always return true when overriding OnRebuildObservers so that
+        // Mirror knows not to use the built in rebuild method.
+        public override bool OnRebuildObservers(HashSet<NetworkConnection> observers, bool initialize)
+        {
+            // If forceHidden then return true without adding any observers.
+            if (forceHidden)
+                return true;
+
+            // Add everything in the hashset for this object's current scene
+            foreach (NetworkIdentity networkIdentity in sceneCheckerObjects[currentScene])
+                if (networkIdentity != null && networkIdentity.connectionToClient != null)
+                    observers.Add(networkIdentity.connectionToClient);
+
+            return true;
+        }
+
+        /// <summary>
+        /// Called when hiding and showing objects on the host.
+        /// On regular clients, objects simply spawn/despawn.
+        /// On host, objects need to remain in scene because the host is also the server.
+        /// In that case, we simply hide/show meshes for the host player.
+        /// </summary>
+        /// <param name="visible"></param>
+        public override void OnSetHostVisibility(bool visible)
+        {
+            foreach (Renderer rend in GetComponentsInChildren<Renderer>())
+                rend.enabled = visible;
+        }
+    }
+}

doc/Components/NetworkSceneChecker.md

@@ -0,0 +1,16 @@
+# NetworkSceneChecker
+
+The Network Scene Checker component controls the visibility of game objects for network clients, based on which scene they're in.
+
+![Network Scene Checker component](NetworkSceneChecker.png)
+
+-   **Force Hidden**  
+    Tick this checkbox to hide this object from all players.
+
+With the Network Scene Checker, a game running on a client doesn’t have information about game objects that are not visible. This has two main benefits: it reduces the amount of data sent across the network, and it makes your game more secure against hacking.
+
+This component would typically be used when the server has several subscenes loaded and needs to isolate networked objects to the subscene they're in.
+
+A game object with a Network Scene Checker component must also have a Network Identity component. When you create a Network Scene Checker component on a game object, Mirror also creates a Network Identity component on that game object if it does not already have one.
+
+Scene objects with a Network Scene Checker component are disabled when they're not in the same scene, and spawned objects are destroyed when they're not in the same scene.

doc/Components/toc.yml

@@ -10,6 +10,8 @@
   href: NetworkManagerHUD.md
 - name: NetworkProximityChecker
   href: NetworkProximityChecker.md
+- name: NetworkSceneChecker
+  href: NetworkSceneChecker.md
 - name: NetworkRoomManager
   href: NetworkRoomManager.md
 - name: NetworkRoomPlayer