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name: defold-core description: > Build and debug the core of a Defold game in Lua: game.project settings, collections, game objects and components, script lifecycle and self state, go.property, input focus and bindings, URLs and message passing, factories, collection factories and collection proxies, plus building and smoke-running with Bob. Use when a project has game.project or .collection, .go, .script, .gui_script or .input_binding files, or the request mentions Defold, msg.post, on_message, on_input, acquire_input_focus, factory.create, collection proxy, bob.jar, or a Defold build error.

Defold Core

Structure, wire and verify a Defold game: editor resource files, the Lua script lifecycle, addressing and messages, runtime spawning and world loading, and a Bob build-and-run check. Examples target Defold 1.14.0; they were built with Bob 1.14.0 and run on the 1.14.0 headless engine. An existing project keeps its Defold version (see Version).

When to use

• Use when adding or fixing game objects, collections, scripts, input, messages, spawning or level loading in a Defold project (game.project in the root, .collection, .go, .script, .gui_script files), or when Bob reports a missing resource.

When not to use: plain Lua or LÖVE projects (love2d-core); GUI layout, sprites, tilemaps and physics depth beyond the pointers in the references; engine-agnostic design handled by the related skills below.

Workflow

1. Inspect first. Read game.project ([bootstrap] main_collection, [input] game_binding, [project] dependencies), the main collection, and the scripts near the request. Follow the project's folder layout, id naming and message names; do not reorganize. Note which Defold/Bob version CI or the team uses.
2. Edit the right file type. Game objects (.go) hold components; collections (.collection) place game objects and other collections; logic goes in .script (game objects), .gui_script (GUI) or shared .lua modules. Editor files are protobuf text: keep their structure, and copy shapes from references/file-formats.md instead of guessing fields.
3. Check references. Run python scripts/check_defold_project.py <project-dir> (path relative to this skill folder; Python 3.10+, no Defold install needed). Fix every ERROR (missing or wrong-case /paths, a game.project path without the compiled c suffix, a require whose file does not exist). Warnings cover files nothing uses yet, unfetched library dependencies and runtime-loaded files. Re-run until it exits 0.
4. Build with Bob. java -jar bob.jar --root . --variant headless --archive resolve build needs JDK 25 and a bob.jar matching the project's Defold version. Exit 1 plus an ERROR: line means a broken reference or a Lua syntax error (with file and line); references/file-formats.md shows how to trace an error that names a generated file.
5. Run it. A build does not prove the game works. Run a headless bundle under a timeout (references/build-and-run.md) and read the log: ERROR:SCRIPT: lines are Lua runtime errors, ERROR:GAMEOBJECT: lines bad addresses or socket clashes. Require a line your test prints when it finishes; the exit code alone proves nothing. The headless engine has no keyboard or mouse, so check input changes in the editor or a desktop build, and say what you could not verify.

Patterns

Project and resource references

game.project is an INI file in the project root. Resource paths everywhere are root-absolute (/main/player.go). In game.project they name the compiled file, so the source path gets a trailing c:

[bootstrap]
main_collection = /main/main.collectionc

[input]
game_binding = /input/game.input_bindingc

Bob builds only what is reachable from game.project; /builtins/... ships with the engine.

Collections, game objects, components

A collection places instances and can override script properties per instance; a game object lists file components and embedded components (escaped data: strings). Full verified files are in references/file-formats.md.

# main.collection (excerpt): place player.go and override its script's speed
instances {
  id: "player"
  prototype: "/main/player.go"
  component_properties {
    id: "player"
    properties { id: "speed" value: "250.0" type: PROPERTY_TYPE_NUMBER }
  }
}

Script lifecycle, self and properties

go.property("speed", 200)                   -- top level only, literal default; editable per instance

local LEFT = hash("left")                   -- hash ids once, not every frame
local RIGHT = hash("right")
local FIRE = hash("fire")

function init(self)
	msg.post(".", "acquire_input_focus")    -- no focus, no on_input calls
	self.held = {}                          -- per-instance state lives on self
end

function update(self, dt)
	local dir = (self.held[RIGHT] and 1 or 0) - (self.held[LEFT] and 1 or 0)
	if dir ~= 0 then
		local pos = go.get_position()
		pos.x = pos.x + dir * self.speed * dt    -- dt-scaled: same speed at any frame rate
		go.set_position(pos)
	end
end

function on_input(self, action_id, action)
	if action_id == LEFT or action_id == RIGHT then
		if action.pressed then self.held[action_id] = true
		elseif action.released then self.held[action_id] = nil end
		return true                         -- consumed: listeners lower on the stack skip it
	elseif action_id == FIRE and action.pressed then
		factory.create("#bulletfactory", go.get_position(), nil,
			{ velocity = vmath.vector3(0, 600, 0) })
		return true
	end
end

function final(self)
	msg.post(".", "release_input_focus")
end

Callbacks: init, final, fixed_update, update, late_update (after every update, before render), on_message, on_input, on_reload. Update order between game objects is unspecified. fixed_update runs 0..N times per frame at [engine] fixed_update_frequency; set [physics] use_fixed_timestep = 1 too so physics steps at the same rate, and put physics-driven code there. self is userdata: store fields on it, but pairs(self) and pprint(self) do not work. Property types: number, boolean, hash, msg.url, vmath.vector3/vector4, vmath.quat, resources such as resource.atlas("/main/main.atlas"), and (since 1.13.2) strings.

Input bindings and the input stack

.input_binding maps devices to action names (key_trigger, mouse_trigger, plus gamepad, touch and text triggers). Every component that acquired focus sits on an input stack; the last to acquire is on top and gets input first, and returning true from on_input stops it there, which is how pause menus block gameplay input. Key actions report pressed and released; track held keys yourself as above.

Addressing and messages

A URL is [socket:][path][#fragment]: "." is this game object, "#" this component, "#sprite" a sibling component, "enemy#ai" a relative object, "/manager#controller" an absolute one, "main:/loader#loader" an object in another world. Collections create naming scopes, so a string id is resolved relative to the sender's collection; a pre-hashed id is always absolute.

msg.post("/manager#controller", "enemy_spotted", { id = go.get_id() })

function on_message(self, message_id, message, sender)
	if message_id == hash("enemy_spotted") then
		print("from", sender, "id", message.id)
	end
end

Messages are queued and dispatched later in the frame; the table is capped at 2 KB. A bad address logs ERROR:GAMEOBJECT: Instance '/x' could not be found when dispatching message.

Factories and collection factories

-- factory component "#bulletfactory" -> one game object; properties override go.property values
local id = factory.create("#bulletfactory", pos, nil, { velocity = vmath.vector3(0, 600, 0) })

-- collectionfactory component "#wavefactory" -> table of collection ids -> new instance ids
local ids = collectionfactory.create("#wavefactory", vmath.vector3(480, 500, 0))
local leader = ids[hash("/leader")]         -- e.g. hash: [/collection0/leader]
go.delete(ids, true)                        -- later: delete them and their children

Position and rotation default to the spawning object's. go.delete() takes effect at the end of the frame. Per-instance properties for a collection factory are keyed by id: { [hash("/leader")] = { hp = 5 } }.

Collection proxies (load another world)

-- excerpt from the loader script; the collection's "#levelproxy" component is a collectionproxy
function init(self)
	msg.post(".", "acquire_input_focus")        -- else the loaded world gets no input
	msg.post("#levelproxy", "async_load")       -- or "load"
end

function on_message(self, message_id, message, sender)
	if message_id == hash("proxy_loaded") then
		msg.post(sender, "init")
		msg.post(sender, "enable")
	elseif message_id == hash("level_done") then
		msg.post("#levelproxy", "unload")       -- disables and finalizes first
	elseif message_id == hash("proxy_unloaded") then
		-- safe to load the next level here
	end
end

The loaded world's socket is the name: of the loaded collection file, so scripts in it reply with msg.post("main:/loader#loader", ...) and its own URLs read level1:/level#level. set_time_step { factor = 0, mode = 0 } pauses a proxy world.

Version

game.project has no engine version key. Use the Defold editor or bob.jar version the project is built with (CI often pins a release sha1), and library.defold_min_version when the project is a library. java -jar bob.jar --version prints the version and sha1. Keep that version; do not migrate unless asked.

Pitfalls

• Bob: the bootstrap.main_collection '/main/main.collectio' can't be found → the setting lacks the compiled suffix and Bob stripped the last letter. Write /main/main.collectionc (same for input.game_binding → .input_bindingc).
• Bob: ERROR: build/default/_generated_….factory:0: the prototype '/main/x.go' can't be found → an embedded component in some .go points at a missing file. Search the sources for the quoted path (/main/x.go), not the generated name, or run the checker.
• on_input never fires → the object never sent acquire_input_focus, the action name differs from the binding, input.game_binding points elsewhere, or a listener higher on the stack returns true.
• Input dead inside a proxy-loaded level → the game object holding the collection proxy did not acquire input focus.
• already a socket with the same name and the proxy never sends proxy_loaded → the loaded collection's name: equals an already loaded world (often both are "main" or "default"). Give every proxied collection a unique name.
• fixed_update never runs → [engine] fixed_update_frequency is 0 (default 60). For physics also set [physics] use_fixed_timestep = 1, as the manual requires both.
• module 'modules.util' not found at run time although Bob succeeded → a require used slashes (require "modules/util"). Use the dotted path from the project root, require "modules.util" for /modules/util.lua.
• tostring(id) prints hash: [<unknown:…>] → hashes keep their source text only in debug builds. Compare hashes, never their strings.
• A second engine logs Failed to initialize Remotery, or a test run never ends → the debug engine opens an engine service (port 8001), a log server and a profiler port, and waits forever without sys.exit. Run one engine at a time and always use a timeout.

References

• references/file-formats.md — verified minimal game.project, .collection, .go, .atlas, .input_binding and script files, and how to trace build errors back to a source file; read before writing editor files by hand.
• references/build-and-run.md — JDK 25 and bob.jar, Bob commands, platforms and