Implement basic node scan of observers

mods/ITEMS/REDSTONE/mcl_observers/init.lua

@@ -9,13 +9,42 @@ local get_rules_flat = function(node)
 	return rules
 end
 
+-- Scan the node in front of the observer
+-- and update the observer state if needed.
+-- TODO: Also scan metadata changes.
+-- TODO: Ignore some node changes.
+local observer_scan = function(pos)
+	local node = minetest.get_node(pos)
+	local front = vector.add(pos, minetest.facedir_to_dir(node.param2))
+	local frontnode = minetest.get_node(front)
+	local meta = minetest.get_meta(pos)
+	local oldnode = meta:get_string("node_name")
+	local oldparam2 = meta:get_string("node_param2")
+	local meta_needs_updating = false
+	if oldnode ~= "" then
+		if not (frontnode.name == oldnode and frontnode.param2) then
+			-- Node state changed! Activate observer
+			minetest.set_node(pos, {name = "mcl_observers:observer_on", param2 = node.param2})
+			mesecon.receptor_on(pos)
+			meta_needs_updating = true
+		end
+	else
+		meta_needs_updating = true
+	end
+	if meta_needs_updating then
+		meta:set_string("node_name", frontnode.name)
+		meta:set_string("node_param2", frontnode.param2)
+	end
+	return frontnode
+end
+
 mesecon.register_node("mcl_observers:observer",
 {
 	is_ground_content = false,
 	sounds = mcl_sounds.node_sound_stone_defaults(),
 	paramtype2 = "facedir",
 	-- TODO: Add to craft guide and creative inventory when it's useful
-	groups = {pickaxey=1, not_in_craft_guide=1, not_in_creative_inventory=1 },
+	groups = {pickaxey=1, material_stone=1, not_in_craft_guide=1, not_in_creative_inventory=1 },
 	on_rotate = false,
 	_mcl_blast_resistance = 17.5,
 	_mcl_hardness = 3.5,
@@ -27,11 +56,13 @@ mesecon.register_node("mcl_observers:observer",
 		"mcl_observers_observer_side.png", "mcl_observers_observer_side.png",
 		"mcl_observers_observer_front.png", "mcl_observers_observer_back.png",
 	},
-	-- TODO: Detect node and state changes
 	mesecons = { receptor = {
 		state = mesecon.state.off,
 		rules = get_rules_flat,
 	}},
+	on_construct = function(pos)
+		observer_scan(pos)
+	end,
 
 	-- DEBUG code to manually turn on an observer by rightclick.
 	-- TODO: Remove this when observers are complete.
@@ -66,6 +97,17 @@ mesecon.register_node("mcl_observers:observer",
 }
 )
 
+-- Regularily check the observer nodes.
+-- TODO: This is rather slow and clunky. Find a more efficient way to do this.
+minetest.register_abm({
+	nodenames = {"mcl_observers:observer_off"},
+	interval = 1,
+	chance = 1,
+	action = function(pos, node)
+		observer_scan(pos)
+	end,
+})
+
 
 minetest.register_craft({
 	output = "mcl_observers:observer_off",