Added Instance:onEntityAdded and :onEntityRemoved

lib/instance.lua

@@ -32,6 +32,8 @@ function Instance:addEntity(e)
       error("bad argument #1 to 'Instance:addEntity' (Entity expected, got "..type(e)..")", 2)
    end
 
+   self:onEntityAdded(e)
+
    e.instances:add(self)
    self.entities:add(e)
    self:checkEntity(e)
@@ -83,6 +85,8 @@ function Instance:flush()
          for i = 1, self.systems.size do
             self.systems:get(i):__remove(e)
          end
+
+         self:onEntityRemoved(e)
       end
    end
 
@@ -238,6 +242,16 @@ function Instance:clear()
    return self
 end
 
+--- Default callback for adding an Entity.
+-- @param e The Entity that was added
+function Instance:onEntityAdded(e)
+end
+
+--- Default callback for removing an Entity.
+-- @param e The Entity that was removed
+function Instance:onEntityRemoved(e)
+end
+
 return setmetatable(Instance, {
    __call = function(_, ...) return Instance.new(...) end,
 })