--- Children system.
-- Allows to build a hierarchy between entities.
--
-- An entity's parent entity is stored in its `parent` component.
-- An entity's children are stored in its `children` component (list of children entities).
--
-- You can set theses values before adding the entity to the world; when you add the entity it will add itself
-- to its parent children list and add all its children to the world.
--
-- If you remove an entity from the world, it will also remove all its children from the world.

return {
	name = "children",
	filter = true,
	onAdd = :(e)
		if not e.children then e.children = {} end
		if e.parent then -- add to parent
			let parentchildren = e.parent.children
			table.insert(parentchildren, e)
		end
		for _, o in ipairs(e.children) do -- add predefined children
			o.parent = e
			@world:add(o)
		end
	end,
	onRemove = :(e)
		for i=#e.children, 1, -1 do -- remove children
			@world:remove(e.children[i])
		end
		if e.parent then -- remove from parent
			let parentchildren = e.parent.children
			for i=#parentchildren, 1, -1 do
				if parentchildren[i] == e then
					table.remove(parentchildren, i)
					break
				end
			end
		end
	end
}