Add binoculars mod with binoculars item to allow zoom

Uses the new player object property 'can zoom'.
In survival mode, use of zoom requires the binoculars item in
inventory.
Zoom is automatically allowed in creative mode and for players with
the 'creative' privilege.
The binoculars.update_player_property() function is global so can be
redefined by a mod for alternative behaviour.

mods/binoculars/init.lua

@@ -0,0 +1,69 @@
+-- Mod global namespace
+
+binoculars = {}
+
+
+-- Cache creative mode setting
+
+local creative_mode_cache = minetest.settings:get_bool("creative_mode")
+
+
+-- Update player property
+-- Global to allow overriding
+
+function binoculars.update_player_property(player)
+	local creative_enabled =
+		(creative and creative.is_enabled_for(player:get_player_name())) or
+		creative_mode_cache
+
+	if creative_enabled or
+			player:get_inventory():contains_item("main", "binoculars:binoculars") then
+		player:set_properties({can_zoom = true})
+	else
+		player:set_properties({can_zoom = false})
+	end
+end
+
+
+-- Set player property 'on joinplayer'
+
+minetest.register_on_joinplayer(function(player)
+	binoculars.update_player_property(player)
+end)
+
+
+-- Cyclic update of player property
+
+local function cyclic_update()
+	for _, player in ipairs(minetest.get_connected_players()) do
+		binoculars.update_player_property(player)
+	end
+	minetest.after(4.7, cyclic_update)
+end
+
+minetest.after(4.7, cyclic_update)
+
+
+-- Binoculars item
+
+minetest.register_craftitem("binoculars:binoculars", {
+	description = "Binoculars",
+	inventory_image = "binoculars_binoculars.png",
+	stack_max = 1,
+
+	on_use = function(itemstack, user, pointed_thing)
+		binoculars.update_player_property(user)
+	end,
+})
+
+
+-- Crafting
+
+minetest.register_craft({
+	output = "binoculars:binoculars",
+	recipe = {
+		{"default:obsidian_glass", "", "default:obsidian_glass"},
+		{"default:bronze_ingot", "default:bronze_ingot", "default:bronze_ingot"},
+		{"default:obsidian_glass", "", "default:obsidian_glass"},
+	}
+})