fix #1407: Check EnterPlayModeSettings and disable if needed for 2019.3 support (#1415)

* fix #1407: Check EnterPlayModeSettings and disable if needed for 2019.3 support

* add reference

Assets/Mirror/Editor/EnterPlayModeSettingsCheck.cs

@@ -0,0 +1,46 @@
+// Unity 2019.3 has an experimental 'disable domain reload on play'
+// feature. keeping any global state between sessions will break
+// Mirror and most of our user's projects. don't allow it for now.
+// https://blogs.unity3d.com/2019/11/05/enter-play-mode-faster-in-unity-2019-3/
+using UnityEditor;
+using UnityEngine;
+
+namespace Mirror
+{
+    public class EnterPlayModeSettingsCheck : MonoBehaviour
+    {
+#if UNITY_2019_3_OR_NEWER
+        [InitializeOnLoadMethod]
+        static void OnInitializeOnLoad()
+        {
+            // check immediately on load
+            CheckPlayModeOptions();
+
+            // check each time we press play. OnLoad is only called once and
+            // wouldn't detect editor-time setting changes
+            EditorApplication.playModeStateChanged += OnPlayModeStateChanged;
+        }
+
+        static void OnPlayModeStateChanged(PlayModeStateChange state)
+        {
+            // only check when entering play mode. no need to show it again
+            // when exiting.
+            if (state == PlayModeStateChange.EnteredPlayMode)
+            {
+                CheckPlayModeOptions();
+            }
+        }
+
+        static void CheckPlayModeOptions()
+        {
+            // enabling the checkbox is enough. it controls all the other
+            // settings.
+            if (EditorSettings.enterPlayModeOptionsEnabled)
+            {
+                Debug.LogError("Enter Play Mode Options are not supported by Mirror. Please disable 'ProjectSettings->Editor->Enter Play Mode Settings (Experimental)'.");
+                EditorApplication.isPlaying = false;
+            }
+        }
+#endif
+    }
+}

Assets/Mirror/Editor/EnterPlayModeSettingsCheck.cs.meta

@@ -0,0 +1,11 @@
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