--- Children system
-- Allow to build a hierarchy between entities.
-- Children are stored directly in the .children entity table: they are added when their parent is added, and removed when it is removed from the world.

return {
	name = "children",
	filter = true,
	default = {
		parent = nil, -- reference to parent entity, if any
		-- ... list of children
	},
	methods = {
		--- Add a new entity to the world, using this entity as a parent.
		add = :(c, o)
			if not o.children then
				o.children = {}
			end
			o.children.parent = c.entity
			table.insert(c, o)
			@world:add(o)
		end,
		--- Remove an entity from the world and from this entity's children.
		remove = :(c, o)
			@world:remove(o)
			for i=#c, 1, -1 do
				if c[i] == o then
					table.remove(c, i)
					break
				end
			end
			o.children.parent = nil
		end
	},
	onAdd = :(c)
		for _, o in ipairs(c) do
			o.children.parent = c.entity
			@world:add(o)
		end
	end,
	onRemove = :(c)
		for _, o in ipairs(c) do
			@world:remove(o)
			o.children.parent = nil
		end
		if c.parent then
			c.parent.children:remove(c.entity)
		end
	end
}