minetest_modding_book/_en/map/abms.md

title	layout	root	idx	description	redirect_from
Active Block Modifiers	default	../..	3.2	Learn how to make ABMs to change blocks.	/en/chapters/abms.html

Introduction

An Active Block Modifier (ABM) allows you to run code on certain nodes at specific intervals.

Please be warned, ABMs which are too frequent or act on many nodes cause massive amounts of lag. Use them sparingly.

• Example: Growing Alien Grass
• Your Turn

Example: Growing Alien Grass

Alien grass, for the purposes of this chapter, is a type of grass which has a chance to appear near water.

To create it, you first need to register the grass, then write an ABM.

minetest.register_node("aliens:grass", {
    description = "Alien Grass",
    light_source = 3, -- The node radiates light. Values can be from 1 to 15
    tiles = {"aliens_grass.png"},
    groups = {choppy=1},
    on_use = minetest.item_eat(20)
})

minetest.register_abm({
    nodenames = {"default:dirt_with_grass"},
    neighbors = {"default:water_source", "default:water_flowing"},
    interval = 10.0, -- Run every 10 seconds
    chance = 50, -- Select every 1 in 50 nodes
    action = function(pos, node, active_object_count, active_object_count_wider)
        minetest.set_node({x = pos.x, y = pos.y + 1, z = pos.z}, {name = "aliens:grass"})
    end
})

This ABM runs every ten seconds. There is a 1 in 5 chance of the ABM running on each node that has the correct name and the correct neighbours. If the ABM runs on a node, an alien grass node is placed above it. Please be warned, this will delete any node previously located in that position. To prevent this you should include a check using minetest.get_node to make sure there is space for the grass.

Specifying a neighbour is optional. If you specify multiple neighbours, only one of them needs to be present to meet the requirements.

Specifying chance is also optional. If you don't specify the chance, the ABM will always run when the other conditions are met.

Your Turn

• Midas touch: Make water turn to gold blocks with a 1 in 100 chance, every 5 seconds.
• Decay: Make wood turn into dirt when water is a neighbour.
• Burnin': Make every air node catch on fire. (Tip: "air" and "fire:basic_flame"). Warning: expect the game to crash.