Add OnPreShutdown and test

Author: jonathanhertz

com.unity.netcode.gameobjects/Runtime/Core/NetworkManager.cs

@@ -477,6 +477,12 @@ public struct ConnectionApprovalRequest
         /// </summary>
         public event Action OnClientStarted = null;
 
+        /// <summary>
+        /// Subscribe to this static event to get notifications before a <see cref="NetworkManager"/> instance is being destroyed.
+        /// This is useful if you want to use the state of anything the NetworkManager cleans up during its shutdown.
+        /// </summary>
+        public event Action OnPreShutdown = null;
+
         /// <summary>
         /// This callback is invoked once the local server is stopped.
         /// </summary>
@@ -1198,6 +1204,8 @@ internal void ShutdownInternal()
                 NetworkLog.LogInfo(nameof(ShutdownInternal));
             }
 
+            OnPreShutdown?.Invoke();
+
             this.UnregisterAllNetworkUpdates();
 
             // Everything is shutdown in the order of their dependencies

com.unity.netcode.gameobjects/Tests/Runtime/NetworkManagerEventsTests.cs

@@ -237,6 +237,46 @@ public IEnumerator OnClientAndServerStartedCalledWhenHostStarts()
             Assert.AreEqual(2, callbacksInvoked, "either OnServerStarted or OnClientStarted wasn't invoked");
         }
 
+        [UnityTest]
+        public IEnumerator OnPreShutdownCalledWhenShuttingDown()
+        {
+            bool preShutdownInvoked = false;
+            bool shutdownInvoked = false;
+            var gameObject = new GameObject(nameof(OnPreShutdownCalledWhenShuttingDown));
+            m_ServerManager = gameObject.AddComponent<NetworkManager>();
+
+            // Set dummy transport that does nothing
+            var transport = gameObject.AddComponent<DummyTransport>();
+            m_ServerManager.NetworkConfig = new NetworkConfig() { NetworkTransport = transport };
+
+            Action onPreShutdown = () =>
+            {
+                preShutdownInvoked = true;
+                Assert.IsFalse(shutdownInvoked, "OnPreShutdown was invoked after OnServerStopped");
+            };
+
+            Action<bool> onServerStopped = (bool wasAlsoClient) =>
+            {
+                shutdownInvoked = true;
+                Assert.IsTrue(preShutdownInvoked, "OnPreShutdown wasn't invoked before OnServerStopped");
+            };
+
+            // Start server to cause initialization process
+            Assert.True(m_ServerManager.StartServer());
+            Assert.True(m_ServerManager.IsListening);
+
+            m_ServerManager.OnPreShutdown += onPreShutdown;
+            m_ServerManager.OnServerStopped += onServerStopped;
+            m_ServerManager.Shutdown();
+            Object.DestroyImmediate(gameObject);
+
+            yield return WaitUntilManagerShutsdown();
+
+            Assert.False(m_ServerManager.IsListening);
+            Assert.True(preShutdownInvoked, "OnPreShutdown wasn't invoked");
+            Assert.True(shutdownInvoked, "OnServerStopped wasn't invoked");
+        }
+
         private IEnumerator WaitUntilManagerShutsdown()
         {
             /* Need two updates to actually shut down. First one to see the transport failing, which