Add unregisterSystem functionality

source/bubel/ecs/manager.d

@@ -172,6 +172,8 @@ export struct EntityManager
 
         foreach (ref system; systems)
         {
+            if (system.isAlive() == false)
+                continue;
             if (system.m_empty)
             {
                 if (system.m_update)
@@ -236,6 +238,8 @@ export struct EntityManager
 
         foreach (ref system; systems)
         {
+            if (system.isAlive() == false)
+                continue;
             foreach (caller; system.m_event_callers)
             {
                 event_callers[caller.id]++;
@@ -252,6 +256,8 @@ export struct EntityManager
 
         foreach (ref system; systems)
         {
+            if (system.isAlive() == false)
+                continue;
             foreach (caller; system.m_event_callers)
             {
                 events[caller.id].callers[event_callers[caller.id]].callback = caller.callback;
@@ -328,6 +334,21 @@ export struct EntityManager
         allocator.freeMemory();
     }
 
+
+    /************************************************************************************************************************
+    Unregister given system form EntityManager.
+    */
+    void unregisterSystem(Sys)()
+    {
+        assert(register_state,  "unregisterSystem must be called between beginRegister() and endRegister().");
+        ushort system_id = becsID!Sys;
+        System* system = getSystem(system_id);
+        assert(system, "System was not registered");
+        assert(system.isAlive, "System already unregistered");
+        system.destroy();
+        *system = System.init;
+    }
+
     /************************************************************************************************************************
     Same as "void registerSystem(Sys)(int priority, int pass = 0)" but use pass name instead of id.
     */
@@ -1887,6 +1908,8 @@ export struct EntityManager
 
             foreach (i, ref system; systems)
             {
+                if (system.isAlive() == false)
+                    continue;
                 if (system.m_empty)
                     continue;
                 if (system.m_update is null)
@@ -3095,6 +3118,8 @@ export struct EntityManager
 
         foreach (ref system; systems)
         {
+            if (system.isAlive() == false)
+                continue;
             if (system.enabled && system.m_begin)
                 system.m_execute = (cast(bool function(void*)) system.m_begin)(
                         system.m_system_pointer);
@@ -3109,6 +3134,10 @@ export struct EntityManager
 
         foreach (ref system; systems)
         {
+            if (system.isAlive() == false)
+            {
+                continue;
+            }
             if (system.enabled && system.m_end)
                 (cast(void function(void*)) system.m_end)(system.m_system_pointer);
         }

source/bubel/ecs/system.d

@@ -89,6 +89,14 @@ struct System
         return cast(const(char)[]) m_name;
     }
 
+    /************************************************************************************************************************
+    Return false if system was unregistered, true otherwise.
+    */
+    export bool isAlive() nothrow @nogc
+    {
+        return m_system_pointer != null;
+    }
+
 package:
 
     void destroy()