Add onAddToWorld and onRemoveFromWorld callbacks to systems. Also, adding or removing systems to a world twice raises an error.

spec/tiny_spec.lua

@@ -120,6 +120,11 @@ describe('tiny-ecs:', function()
             timePassed = 0
         end)
 
+        after_each(function()
+            world:clearSystems()
+            world:refresh()
+        end)
+
         it("Create World", function()
             assert.equals(world:getEntityCount(), 3)
             assert.equals(world:getSystemCount(), 2)
@@ -196,7 +201,7 @@ describe('tiny-ecs:', function()
 
         it("Add Systems Multiple Times", function()
             world:update(1)
-            world:add(moveSystem, oneTimeSystem)
+            assert.has_error(function() world:add(moveSystem, oneTimeSystem) end, "System already belongs to a World.")
             world:update(2)
             assert.equals(2, world:getSystemCount())
             assert.equals(3, world:getEntityCount())
@@ -206,7 +211,7 @@ describe('tiny-ecs:', function()
             world:update(1)
             world:remove(moveSystem)
             world:update(2)
-            world:remove(moveSystem)
+            assert.has_error(function() world:remove(moveSystem) end, "System does not belong to this World.")
             world:update(2)
             assert.equals(1, world:getSystemCount())
             assert.equals(3, world:getEntityCount())

tiny.lua

@@ -476,7 +476,6 @@ function tiny_manageSystems(world)
     for i = 1, #s2r do
         local system = s2r[i]
         local index = system.index
-        if system.world == world then
         local onRemove = system.onRemove
         if onRemove then
             local entityList = system.entities
@@ -488,6 +487,9 @@ function tiny_manageSystems(world)
         for j = index, #systems do
             systems[j].index = j
         end
+        local onRemoveFromWorld = system.onRemoveFromWorld
+        if onRemoveFromWorld then
+            onRemoveFromWorld(system, world)
         end
         s2r[i] = nil
 
@@ -514,6 +516,10 @@ function tiny_manageSystems(world)
             local index = #systems + 1
             system.index = index
             systems[index] = system
+            local onAddToWorld = system.onAddToWorld
+            if onAddToWorld then
+                onAddToWorld(system, world)
+            end
 
             -- Try to add Entities
             local onAdd = system.onAdd