fix: Headless / Dedicated Server detection now works on Unity 2019/2020 without dedicated server build support too (#3643)

Assets/Mirror/Components/Discovery/NetworkDiscoveryBase.cs

@@ -85,9 +85,10 @@ public virtual void Start()
                 transport = Transport.active;
 
             // Server mode? then start advertising
-#if UNITY_SERVER
+            if (Utils.IsHeadless())
+            {
                 AdvertiseServer();
-#endif
+            }
         }
 
         public static long RandomLong()

Assets/Mirror/Components/NetworkRoomManager.cs

@@ -251,10 +251,11 @@ public override void OnServerDisconnect(NetworkConnectionToClient conn)
             OnRoomServerDisconnect(conn);
             base.OnServerDisconnect(conn);
 
-#if UNITY_SERVER
+            if (Utils.IsHeadless())
+            {
                 if (numPlayers < 1)
                     StopServer();
-#endif
+            }
         }
 
         // Sequential index used in round-robin deployment of players into instances and score positioning

Assets/Mirror/Core/NetworkManager.cs

@@ -221,11 +221,10 @@ public virtual void Start()
             //
             // (tick rate is applied in StartServer!)
             //
-#if UNITY_SERVER
             // don't auto start in editor where we have a UI, only in builds.
             // otherwise if we switch to 'Dedicated Server' target and press
             // Play, it would auto start the server every time.
-            if (!Application.isEditor)
+            if (Utils.IsHeadless() && !Application.isEditor)
             {
                 if (autoStartServerBuild)
                 {
@@ -237,7 +236,6 @@ public virtual void Start()
                     StartClient();
                 }
             }
-#endif
         }
 
         // make sure to call base.Update() when overwriting
@@ -686,10 +684,11 @@ public virtual void OnApplicationQuit()
         // useful for headless benchmark clients.
         public virtual void ConfigureHeadlessFrameRate()
         {
-#if UNITY_SERVER
+            if (Utils.IsHeadless())
+            {
                 Application.targetFrameRate = sendRate;
                 // Debug.Log($"Server Tick Rate set to {Application.targetFrameRate} Hz.");
-#endif
+            }
         }
 
         bool InitializeSingleton()

Assets/Mirror/Core/NetworkServer.cs

@@ -120,7 +120,9 @@ public static void Listen(int maxConns)
 
                 if (Transport.active is PortTransport portTransport)
                 {
-#if UNITY_SERVER && !UNITY_EDITOR
+                    if (Utils.IsHeadless())
+                    {
+#if !UNITY_EDITOR
                         Console.ForegroundColor = ConsoleColor.Green;
                         Console.WriteLine($"Server listening on port {portTransport.Port}");
                         Console.ResetColor();
@@ -128,6 +130,7 @@ public static void Listen(int maxConns)
                         Debug.Log($"Server listening on port {portTransport.Port}");
 #endif
                     }
+                }
                 else
                     Debug.Log("Server started listening");
             }

Assets/Mirror/Core/Tools/Utils.cs

@@ -2,6 +2,7 @@
 using System.Runtime.CompilerServices;
 using System.Security.Cryptography;
 using UnityEngine;
+using UnityEngine.Rendering;
 using UnityEngine.SceneManagement;
 
 namespace Mirror
@@ -32,6 +33,15 @@ public static class Channels
 
     public static class Utils
     {
+        // detect headless / dedicated server mode
+        // in Unity 2019/2020, we need to check if there is no graphics device.
+        // in Unity 2021+, we could check for the #if UNITY_SERVER define.
+        // checking graphics device is safest.
+        // => it works on all Unity versions.
+        // => it keeps all the code active instead of ifdefing it out.
+        public static bool IsHeadless() =>
+            SystemInfo.graphicsDeviceType == GraphicsDeviceType.Null;
+
         public static uint GetTrueRandomUInt()
         {
             // use Crypto RNG to avoid having time based duplicates

Assets/Mirror/Examples/Room/Scripts/NetworkRoomManagerExt.cs

@@ -76,11 +76,14 @@ Setting showStartButton false when the button is pressed hides it in the game sc
         public override void OnRoomServerPlayersReady()
         {
             // calling the base method calls ServerChangeScene as soon as all players are in Ready state.
-#if UNITY_SERVER
+            if (Utils.IsHeadless())
+            {
                 base.OnRoomServerPlayersReady();
-#else
+            }
+            else
+            {
                 showStartButton = true;
-#endif
+            }
         }
 
         public override void OnGUI()

Assets/Mirror/Transports/Multiplex/MultiplexTransport.cs

@@ -285,13 +285,16 @@ public override void ServerStart()
 
                 if (transport is PortTransport portTransport)
                 {
-#if UNITY_SERVER
+                    if (Utils.IsHeadless())
+                    {
                         Console.ForegroundColor = ConsoleColor.Green;
                         Console.WriteLine($"Server listening on port {portTransport.Port}");
                         Console.ResetColor();
-#else
+                    }
+                    else
+                    {
                         Debug.Log($"Server listening on port {portTransport.Port}");
-#endif
+                    }
                 }
             }
         }