Node: Base building block
├── Scene: Reusable node tree (saved as .tscn)
├── Resource: Data container (saved as .tres)
├── Signal: Event communication
└── Group: Node categorization

class_name Player
extends CharacterBody2D
 
# Signals
signal health_changed(new_health: int)
signal died
 
# Exports (Inspector-editable)
@export var speed: float = 200.0
@export var max_health: int = 100
@export_range(0, 1) var damage_reduction: float = 0.0
@export_group("Combat")
@export var attack_damage: int = 10
@export var attack_cooldown: float = 0.5
 
# Onready (initialized when ready)
@onready var sprite: Sprite2D = $Sprite2D
@onready var animation: AnimationPlayer = $AnimationPlayer
@onready var hitbox: Area2D = $Hitbox
 
# Private variables (convention: underscore prefix)
var _health: int
var _can_attack: bool = true
 
func _ready() -> void:
    _health = max_health
 
func _physics_process(delta: float) -> void:
    var direction := Input.get_vector("left", "right", "up", "down")
    velocity = direction * speed
    move_and_slide()
 
func take_damage(amount: int) -> void:
    var actual_damage := int(amount * (1.0 - damage_reduction))
    _health = max(_health - actual_damage, 0)
    health_changed.emit(_health)
 
    if _health <= 0:
        died.emit()

# state_machine.gd
class_name StateMachine
extends Node
 
signal state_changed(from_state: StringName, to_state: StringName)
 
@export var initial_state: State
 
var current_state: State
var states: Dictionary = {}
 
func _ready() -> void:
    # Register all State children
    for child in get_children():
        if child is State:
            states[child.name] = child
            child.state_machine = self
            child.process_mode = Node.PROCESS_MODE_DISABLED
 
    # Start initial state
    if initial_state:
        current_state = initial_state
        current_state.process_mode = Node.PROCESS_MODE_INHERIT
        current_state.enter()
 
func _process(delta: float) -> void:
    if current_state:
        current_state.update(delta)
 
func _physics_process(delta: float) -> void:
    if current_state:
        current_state.physics_update(delta)
 
func _unhandled_input(event: InputEvent) -> void:
    if current_state:
        current_state.handle_input(event)
 
func transition_to(state_name: StringName, msg: Dictionary = {}) -> void:
    if not states.has(state_name):
        push_error("State '%s' not found" % state_name)
        return
 
    var previous_state := current_state
    previous_state.exit()
    previous_state.process_mode = Node.PROCESS_MODE_DISABLED
 
    current_state = states[state_name]
    current_state.process_mode = Node.PROCESS_MODE_INHERIT
    current_state.enter(msg)
 
    state_changed.emit(previous_state.name, current_state.name)

# state.gd
class_name State
extends Node
 
var state_machine: StateMachine
 
func enter(_msg: Dictionary = {}) -> void:
    pass
 
func exit() -> void:
    pass
 
func update(_delta: float) -> void:
    pass
 
func physics_update(_delta: float) -> void:
    pass
 
func handle_input(_event: InputEvent) -> void:
    pass

# player_idle.gd
class_name PlayerIdle
extends State
 
@export var player: Player
 
func enter(_msg: Dictionary = {}) -> void:
    player.animation.play("idle")
 
func physics_update(_delta: float) -> void:
    var direction := Input.get_vector("left", "right", "up", "down")
 
    if direction != Vector2.ZERO:
        state_machine.transition_to("Move")
 
func handle_input(event: InputEvent) -> void:
    if event.is_action_pressed("attack"):
        state_machine.transition_to("Attack")
    elif event.is_action_pressed("jump"):
        state_machine.transition_to("Jump")

# game_manager.gd (Add to Project Settings > Autoload)
extends Node
 
signal game_started
signal game_paused(is_paused: bool)
signal game_over(won: bool)
signal score_changed(new_score: int)
 
enum GameState { MENU, PLAYING, PAUSED, GAME_OVER }
 
var state: GameState = GameState.MENU
var score: int = 0:
    set(value):
        score = value
        score_changed.emit(score)
 
var high_score: int = 0
 
func _ready() -> void:
    process_mode = Node.PROCESS_MODE_ALWAYS
    _load_high_score()
 
func _input(event: InputEvent) -> void:
    if event.is_action_pressed("pause") and state == GameState.PLAYING:
        toggle_pause()
 
func start_game() -> void:
    score = 0
    state = GameState.PLAYING
    game_started.emit()
 
func toggle_pause() -> void:
    var is_paused := state != GameState.PAUSED
 
    if is_paused:
        state = GameState.PAUSED
        get_tree().paused = true
    else:
        state = GameState.PLAYING
        get_tree().paused = false
 
    game_paused.emit(is_paused)
 
func end_game(won: bool) -> void:
    state = GameState.GAME_OVER
 
    if score > high_score:
        high_score = score
        _save_high_score()
 
    game_over.emit(won)
 
func add_score(points: int) -> void:
    score += points
 
func _load_high_score() -> void:
    if FileAccess.file_exists("user://high_score.save"):
        var file := FileAccess.open("user://high_score.save", FileAccess.READ)
        high_score = file.get_32()
 
func _save_high_score() -> void:
    var file := FileAccess.open("user://high_score.save", FileAccess.WRITE)
    file.store_32(high_score)

# event_bus.gd (Global signal bus)
extends Node
 
# Player events
signal player_spawned(player: Node2D)
signal player_died(player: Node2D)
signal player_health_changed(health: int, max_health: int)
 
# Enemy events
signal enemy_spawned(enemy: Node2D)
signal enemy_died(enemy: Node2D, position: Vector2)
 
# Item events
signal item_collected(item_type: StringName, value: int)
signal powerup_activated(powerup_type: StringName)
 
# Level events
signal level_started(level_number: int)
signal level_completed(level_number: int, time: float)
signal checkpoint_reached(checkpoint_id: int)

# weapon_data.gd
class_name WeaponData
extends Resource
 
@export var name: StringName
@export var damage: int
@export var attack_speed: float
@export var range: float
@export_multiline var description: String
@export var icon: Texture2D
@export var projectile_scene: PackedScene
@export var sound_attack: AudioStream

# character_stats.gd
class_name CharacterStats
extends Resource
 
signal stat_changed(stat_name: StringName, new_value: float)
 
@export var max_health: float = 100.0
@export var attack: float = 10.0
@export var defense: float = 5.0
@export var speed: float = 200.0
 
# Runtime values (not saved)
var _current_health: float
 
func _init() -> void:
    _current_health = max_health
 
func get_current_health() -> float:
    return _current_health
 
func take_damage(amount: float) -> float:
    var actual_damage := maxf(amount - defense, 1.0)
    _current_health = maxf(_current_health - actual_damage, 0.0)
    stat_changed.emit("health", _current_health)
    return actual_damage
 
func heal(amount: float) -> void:
    _current_health = minf(_current_health + amount, max_health)
    stat_changed.emit("health", _current_health)
 
func duplicate_for_runtime() -> CharacterStats:
    var copy := duplicate() as CharacterStats
    copy._current_health = copy.max_health
    return copy

# Using resources
class_name Character
extends CharacterBody2D
 
@export var base_stats: CharacterStats
@export var weapon: WeaponData
 
var stats: CharacterStats
 
func _ready() -> void:
    # Create runtime copy to avoid modifying the resource
    stats = base_stats.duplicate_for_runtime()
    stats.stat_changed.connect(_on_stat_changed)
 
func attack() -> void:
    if weapon:
        print("Attacking with %s for %d damage" % [weapon.name, weapon.damage])
 
func _on_stat_changed(stat_name: StringName, value: float) -> void:
    if stat_name == "health" and value <= 0:
        die()

# object_pool.gd
class_name ObjectPool
extends Node
 
@export var pooled_scene: PackedScene
@export var initial_size: int = 10
@export var can_grow: bool = true
 
var _available: Array[Node] = []
var _in_use: Array[Node] = []
 
func _ready() -> void:
    _initialize_pool()
 
func _initialize_pool() -> void:
    for i in initial_size:
        _create_instance()
 
func _create_instance() -> Node:
    var instance := pooled_scene.instantiate()
    instance.process_mode = Node.PROCESS_MODE_DISABLED
    instance.visible = false
    add_child(instance)
    _available.append(instance)
 
    # Connect return signal if exists
    if instance.has_signal("returned_to_pool"):
        instance.returned_to_pool.connect(_return_to_pool.bind(instance))
 
    return instance
 
func get_instance() -> Node:
    var instance: Node
 
    if _available.is_empty():
        if can_grow:
            instance = _create_instance()
            _available.erase(instance)
        else:
            push_warning("Pool exhausted and cannot grow")
            return null
    else:
        instance = _available.pop_back()
 
    instance.process_mode = Node.PROCESS_MODE_INHERIT
    instance.visible = true
    _in_use.append(instance)
 
    if instance.has_method("on_spawn"):
        instance.on_spawn()
 
    return instance
 
func _return_to_pool(instance: Node) -> void:
    if not instance in _in_use:
        return
 
    _in_use.erase(instance)
 
    if instance.has_method("on_despawn"):
        instance.on_despawn()
 
    instance.process_mode = Node.PROCESS_MODE_DISABLED
    instance.visible = false
    _available.append(instance)
 
func return_all() -> void:
    for instance in _in_use.duplicate():
        _return_to_pool(instance)

# pooled_bullet.gd
class_name PooledBullet
extends Area2D
 
signal returned_to_pool
 
@export var speed: float = 500.0
@export var lifetime: float = 5.0
 
var direction: Vector2
var _timer: float
 
func on_spawn() -> void:
    _timer = lifetime
 
func on_despawn() -> void:
    direction = Vector2.ZERO
 
func initialize(pos: Vector2, dir: Vector2) -> void:
    global_position = pos
    direction = dir.normalized()
    rotation = direction.angle()
 
func _physics_process(delta: float) -> void:
    position += direction * speed * delta
 
    _timer -= delta
    if _timer <= 0:
        returned_to_pool.emit()
 
func _on_body_entered(body: Node2D) -> void:
    if body.has_method("take_damage"):
        body.take_damage(10)
    returned_to_pool.emit()

# health_component.gd
class_name HealthComponent
extends Node
 
signal health_changed(current: int, maximum: int)
signal damaged(amount: int, source: Node)
signal healed(amount: int)
signal died
 
@export var max_health: int = 100
@export var invincibility_time: float = 0.0
 
var current_health: int:
    set(value):
        var old := current_health
        current_health = clampi(value, 0, max_health)
        if current_health != old:
            health_changed.emit(current_health, max_health)
 
var _invincible: bool = false
 
func _ready() -> void:
    current_health = max_health
 
func take_damage(amount: int, source: Node = null) -> int:
    if _invincible or current_health <= 0:
        return 0
 
    var actual := mini(amount, current_health)
    current_health -= actual
    damaged.emit(actual, source)
 
    if current_health <= 0:
        died.emit()
    elif invincibility_time > 0:
        _start_invincibility()
 
    return actual
 
func heal(amount: int) -> int:
    var actual := mini(amount, max_health - current_health)
    current_health += actual
    if actual > 0:
        healed.emit(actual)
    return actual
 
func _start_invincibility() -> void:
    _invincible = true
    await get_tree().create_timer(invincibility_time).timeout
    _invincible = false

# hitbox_component.gd
class_name HitboxComponent
extends Area2D
 
signal hit(hurtbox: HurtboxComponent)
 
@export var damage: int = 10
@export var knockback_force: float = 200.0
 
var owner_node: Node
 
func _ready() -> void:
    owner_node = get_parent()
    area_entered.connect(_on_area_entered)
 
func _on_area_entered(area: Area2D) -> void:
    if area is HurtboxComponent:
        var hurtbox := area as HurtboxComponent
        if hurtbox.owner_node != owner_node:
            hit.emit(hurtbox)
            hurtbox.receive_hit(self)

# hurtbox_component.gd
class_name HurtboxComponent
extends Area2D
 
signal hurt(hitbox: HitboxComponent)
 
@export var health_component: HealthComponent
 
var owner_node: Node
 
func _ready() -> void:
    owner_node = get_parent()
 
func receive_hit(hitbox: HitboxComponent) -> void:
    hurt.emit(hitbox)
 
    if health_component:
        health_component.take_damage(hitbox.damage, hitbox.owner_node)

# scene_manager.gd (Autoload)
extends Node
 
signal scene_loading_started(scene_path: String)
signal scene_loading_progress(progress: float)
signal scene_loaded(scene: Node)
signal transition_started
signal transition_finished
 
@export var transition_scene: PackedScene
@export var loading_scene: PackedScene
 
var _current_scene: Node
var _transition: CanvasLayer
var _loader: ResourceLoader
 
func _ready() -> void:
    _current_scene = get_tree().current_scene
 
    if transition_scene:
        _transition = transition_scene.instantiate()
        add_child(_transition)
        _transition.visible = false
 
func change_scene(scene_path: String, with_transition: bool = true) -> void:
    if with_transition:
        await _play_transition_out()
 
    _load_scene(scene_path)
 
func change_scene_packed(scene: PackedScene, with_transition: bool = true) -> void:
    if with_transition:
        await _play_transition_out()
 
    _swap_scene(scene.instantiate())
 
func _load_scene(path: String) -> void:
    scene_loading_started.emit(path)
 
    # Check if already loaded
    if ResourceLoader.has_cached(path):
        var scene := load(path) as PackedScene
        _swap_scene(scene.instantiate())
        return
 
    # Async loading
    ResourceLoader.load_threaded_request(path)
 
    while true:
        var progress := []
        var status := ResourceLoader.load_threaded_get_status(path, progress)
 
        match status:
            ResourceLoader.THREAD_LOAD_IN_PROGRESS:
                scene_loading_progress.emit(progress[0])
                await get_tree().process_frame
            ResourceLoader.THREAD_LOAD_LOADED:
                var scene := ResourceLoader.load_threaded_get(path) as PackedScene
                _swap_scene(scene.instantiate())
                return
            _:
                push_error("Failed to load scene: %s" % path)
                return
 
func _swap_scene(new_scene: Node) -> void:
    if _current_scene:
        _current_scene.queue_free()
 
    _current_scene = new_scene
    get_tree().root.add_child(_current_scene)
    get_tree().current_scene = _current_scene
 
    scene_loaded.emit(_current_scene)
    await _play_transition_in()
 
func _play_transition_out() -> void:
    if not _transition:
        return
 
    transition_started.emit()
    _transition.visible = true
 
    if _transition.has_method("transition_out"):
        await _transition.transition_out()
    else:
        await get_tree().create_timer(0.3).timeout
 
func _play_transition_in() -> void:
    if not _transition:
        transition_finished.emit()
        return
 
    if _transition.has_method("transition_in"):
        await _transition.transition_in()
    else:
        await get_tree().create_timer(0.3).timeout
 
    _transition.visible = false
    transition_finished.emit()

# save_manager.gd (Autoload)
extends Node
 
const SAVE_PATH := "user://savegame.save"
const ENCRYPTION_KEY := "your_secret_key_here"
 
signal save_completed
signal load_completed
signal save_error(message: String)
 
func save_game(data: Dictionary) -> void:
    var file := FileAccess.open_encrypted_with_pass(
        SAVE_PATH,
        FileAccess.WRITE,
        ENCRYPTION_KEY
    )
 
    if file == null:
        save_error.emit("Could not open save file")
        return
 
    var json := JSON.stringify(data)
    file.store_string(json)
    file.close()
 
    save_completed.emit()
 
func load_game() -> Dictionary:
    if not FileAccess.file_exists(SAVE_PATH):
        return {}
 
    var file := FileAccess.open_encrypted_with_pass(
        SAVE_PATH,
        FileAccess.READ,
        ENCRYPTION_KEY
    )
 
    if file == null:
        save_error.emit("Could not open save file")
        return {}
 
    var json := file.get_as_text()
    file.close()
 
    var parsed := JSON.parse_string(json)
    if parsed == null:
        save_error.emit("Could not parse save data")
        return {}
 
    load_completed.emit()
    return parsed
 
func delete_save() -> void:
    if FileAccess.file_exists(SAVE_PATH):
        DirAccess.remove_absolute(SAVE_PATH)
 
func has_save() -> bool:
    return FileAccess.file_exists(SAVE_PATH)

# saveable.gd (Attach to saveable nodes)
class_name Saveable
extends Node
 
@export var save_id: String
 
func _ready() -> void:
    if save_id.is_empty():
        save_id = str(get_path())
 
func get_save_data() -> Dictionary:
    var parent := get_parent()
    var data := {"id": save_id}
 
    if parent is Node2D:
        data["position"] = {"x": parent.position.x, "y": parent.position.y}
 
    if parent.has_method("get_custom_save_data"):
        data.merge(parent.get_custom_save_data())
 
    return data
 
func load_save_data(data: Dictionary) -> void:
    var parent := get_parent()
 
    if data.has("position") and parent is Node2D:
        parent.position = Vector2(data.position.x, data.position.y)
 
    if parent.has_method("load_custom_save_data"):
        parent.load_custom_save_data(data)

# 1. Cache node references
@onready var sprite := $Sprite2D  # Good
# $Sprite2D in _process()  # Bad - repeated lookup
 
# 2. Use object pooling for frequent spawning
# See Pattern 4
 
# 3. Avoid allocations in hot paths
var _reusable_array: Array = []
 
func _process(_delta: float) -> void:
    _reusable_array.clear()  # Reuse instead of creating new
 
# 4. Use static typing
func calculate(value: float) -> float:  # Good
    return value * 2.0
 
# 5. Disable processing when not needed
func _on_off_screen() -> void:
    set_process(false)
    set_physics_process(false)