-added new listener callback: onChangeEntity. Callback is called when entity optional components are changed. There is no info about which components are changed (added/removed), but should be in the future).This is probably slowest callback.

source/ecs/manager.d

@@ -95,7 +95,7 @@ class EntityManager
         {
             if (system.m_update is null)
             {
-                if (system.m_add_entity || system.m_remove_entity)
+                if (system.m_add_entity || system.m_remove_entity || system.m_change_entity)
                 {
                     foreach (info; &entities_infos.byValue)
                     {
@@ -761,6 +761,7 @@ class EntityManager
 
         mixin(catchEntityFunc("m_add_entity", "onAddEntity"));
         mixin(catchEntityFunc("m_remove_entity", "onRemoveEntity"));
+        mixin(catchEntityFunc("m_change_entity", "onChangeEntity"));
 
         system.m_system_pointer = cast(void*) Mallocator.instance.make!Sys;
         system.m_priority = priority;
@@ -1210,10 +1211,9 @@ class EntityManager
 
             foreach (i, ref system; systems)
             {
-                //if(system.m_add_entity || system.m_remove_entity)info.systems[system.id] = true;
                 if (system.m_update is null)
                 {
-                    if (system.m_add_entity || system.m_remove_entity)
+                    if (system.m_add_entity || system.m_remove_entity || system.m_change_entity)
                         connectListenerToEntityInfo(*info, cast(uint) i);
                     continue;
                 }
@@ -1239,13 +1239,21 @@ class EntityManager
             Mallocator.instance.dispose(info.remove_listeners);
             info.remove_listeners = null;
         }
+        if (info.change_listeners)
+        {
+            Mallocator.instance.dispose(info.change_listeners);
+            info.change_listeners = null;
+        }
         //allocate local data
         ushort[] tmp_add = (cast(ushort*) alloca(systems.length * ushort.sizeof))[0
             .. systems.length];
         ushort[] tmp_rem = (cast(ushort*) alloca(systems.length * ushort.sizeof))[0
             .. systems.length];
+        ushort[] tmp_ch = (cast(ushort*) alloca(systems.length * ushort.sizeof))[0 .. systems
+            .length];
         int add_len = 0;
         int rem_len = 0;
+        int ch_len = 0;
         //assign listeners to lists
         foreach (i; 0 .. systems.length)
         {
@@ -1290,6 +1298,25 @@ class EntityManager
                     //assign listener
                     tmp_rem[j] = cast(ushort) i;
                 }
+                //onChangeEntity listener
+                if (system.m_change_entity)
+                {
+                    //find listener position by priority
+                    int j;
+                    for (j = 0; j < ch_len; j++)
+                    {
+                        if (systems[i].priority > systems[tmp_ch[j]].priority)
+                            break;
+                    }
+                    ch_len++;
+                    //move elements after new listener
+                    for (int k = ch_len; k > j; k--)
+                    {
+                        tmp_ch[k] = tmp_ch[k - 1];
+                    }
+                    //assign listener
+                    tmp_ch[j] = cast(ushort) i;
+                }
             }
         }
 
@@ -1304,6 +1331,12 @@ class EntityManager
             info.remove_listeners = Mallocator.instance.makeArray!ushort(rem_len);
             memcpy(info.remove_listeners.ptr, tmp_rem.ptr, rem_len * ushort.sizeof);
         }
+
+        if (ch_len)
+        {
+            info.change_listeners = Mallocator.instance.makeArray!ushort(ch_len);
+            memcpy(info.change_listeners.ptr, tmp_ch.ptr, ch_len * ushort.sizeof);
+        }
     }
 
     export void connectListenerToEntityInfo(ref EntityInfo entity, uint system_id) nothrow @nogc
@@ -1486,6 +1519,18 @@ class EntityManager
             }
         }
 
+        if (new_info.change_listeners)
+        {
+            foreach (listener; new_info.change_listeners)
+            {
+                if (info.systems[listener])
+                {
+                    callChangeEntityListener(&systems[listener], new_info, new_block,
+                            new_block.entities_count, new_block.entities_count + 1, del_ids);
+                }
+            }
+        }
+
         new_block.entities_count++;
 
         removeEntityNoID(entity, block);
@@ -1660,6 +1705,18 @@ class EntityManager
             }
         }
 
+        if (new_info.change_listeners)
+        {
+            foreach (listener; new_info.change_listeners)
+            {
+                if (info.systems[listener])
+                {
+                    callChangeEntityListener(&systems[listener], new_info, new_block,
+                            new_block.entities_count, new_block.entities_count + 1, new_ids);
+                }
+            }
+        }
+
         new_block.entities_count++;
         removeEntityNoID(entity, block);
     }
@@ -2026,6 +2083,43 @@ class EntityManager
         (cast(void function(ref ListenerCallData) nothrow @nogc) system.m_remove_entity)(data);
     }
 
+    private void callChangeEntityListener(System* system, EntityInfo* info,
+            EntitiesBlock* block, int begin, int end, ushort[] ch_ids) @nogc nothrow
+    {
+        int i = 0;
+        int j = 0;
+        bool is_ = false;
+        while (1)
+        {
+            if (ch_ids[i] == system.m_optional_components[j])
+            {
+                is_ = true;
+                break;
+            }
+            else if (ch_ids[i] > system.m_optional_components[j])
+            {
+                j++;
+                if (j >= system.m_optional_components.length)
+                    break;
+            }
+            else
+            {
+                i++;
+                if (i >= ch_ids.length)
+                    break;
+            }
+        }
+        if (!is_)
+            return;
+
+        ListenerCallData data;
+        data.system = system;
+        data.block = block;
+        data.begin = begin;
+        data.end = end;
+        (cast(void function(ref ListenerCallData) nothrow @nogc) system.m_change_entity)(data);
+    }
+
     private void updateBlocks()
     {
         foreach (ref thread; threads)
@@ -2393,6 +2487,8 @@ class EntityManager
         ushort[] add_listeners;
         ///systems which are listening for removed entities
         ushort[] remove_listeners;
+        ///systems which are listening for changed entities (changed in term of contained components)
+        ushort[] change_listeners;
 
         ///pointer to first block/page
         EntitiesBlock* first_block;

source/ecs/system.d

@@ -137,6 +137,7 @@ package:
 
     void* m_add_entity;
     void* m_remove_entity;
+    void* m_change_entity;
 
     //void function(ref EntityManager.CallData data) m_initialize;
     //void function(ref EntityManager.CallData data) m_deinitilize;