รูปแบบการใช้ Signal ใน Godot 4 — คู่มือฉบับสมบูรณ์

# Simple signal with no parameters
signal game_over

# Signal with typed parameters
signal health_changed(new_health: int)

# Signal with multiple parameters
signal item_picked_up(item_name: String, quantity: int)

# Signal with no type hints (works, but typed is recommended)
signal something_happened(data)

# Godot 3 (OLD — no longer works in Godot 4)
# button.connect("pressed", self, "_on_button_pressed")

# Godot 4 — callable-based connection
button.pressed.connect(_on_button_pressed)

func _on_button_pressed() -> void:
    print("Button was pressed!")

# Pass extra data via bind()
buy_button.pressed.connect(_on_item_button.bind("sword"))
sell_button.pressed.connect(_on_item_button.bind("shield"))

func _on_item_button(item_id: String) -> void:
    print("Selected item: ", item_id)

# Lambda — great for one-liners
button.pressed.connect(func(): print("Button pressed!"))

# Lambda with parameters
health_changed.connect(func(hp: int): health_label.text = str(hp))

# Multi-line lambda
enemy.died.connect(func():
    score += 100
    score_label.text = "Score: %d" % score
    print("Enemy defeated!")
)

# Disconnect a signal
button.pressed.disconnect(_on_button_pressed)

# Always check before disconnecting to avoid errors
if button.pressed.is_connected(_on_button_pressed):
    button.pressed.disconnect(_on_button_pressed)

func _ready() -> void:
    EventBus.player_died.connect(_on_player_died)

func _exit_tree() -> void:
    if EventBus.player_died.is_connected(_on_player_died):
        EventBus.player_died.disconnect(_on_player_died)

signal health_changed(new_health: int)
signal died

var health: int = 100

func take_damage(amount: int) -> void:
    health -= amount
    health_changed.emit(health)  # Emit with argument

    if health <= 0:
        died.emit()  # Emit with no arguments

func play_cutscene() -> void:
    # Wait for a timer
    await get_tree().create_timer(2.0).timeout

    # Wait for an animation to finish
    animation_player.play("intro")
    await animation_player.animation_finished

    # Wait for player input (custom signal)
    dialogue_label.text = "Press any key to continue..."
    await player_pressed_continue

    # Continue execution after the signal fires
    print("Cutscene complete!")

# Single parameter — returns the value directly
var final_health: int = await health_changed
print("Health is now: ", final_health)

# Multiple parameters — returns an array
var result = await item_picked_up
var item_name: String = result[0]
var quantity: int = result[1]

# Auto-disconnects after firing once
enemy.died.connect(_on_first_kill, CONNECT_ONE_SHOT)

func _on_first_kill() -> void:
    unlock_achievement("first_blood")

# Called at end of frame — safe for scene tree changes
button.pressed.connect(_on_restart, CONNECT_DEFERRED)

func _on_restart() -> void:
    get_tree().reload_current_scene()

# One-shot AND deferred
trigger.body_entered.connect(_on_trigger, CONNECT_ONE_SHOT | CONNECT_DEFERRED)

extends Node

# Define all global signals in one place
signal player_died
signal score_changed(new_score: int)
signal level_completed(level_id: int)
signal item_collected(item_name: String)
signal settings_changed

func die() -> void:
    # Any script can emit global signals
    EventBus.player_died.emit()

func _ready() -> void:
    # Any script can listen to global signals
    EventBus.score_changed.connect(_on_score_changed)
    EventBus.player_died.connect(_on_player_died)

func _on_score_changed(new_score: int) -> void:
    score_label.text = "Score: %d" % new_score

func _on_player_died() -> void:
    game_over_screen.show()

signal inventory_updated

func _ready() -> void:
    # Connect to all slot children
    for slot in get_children():
        if slot.has_signal("item_changed"):
            slot.item_changed.connect(_on_slot_changed)

func _on_slot_changed() -> void:
    inventory_updated.emit()

func run_level() -> void:
    spawn_enemies()
    await EventBus.all_enemies_defeated

    show_treasure_chest()
    await EventBus.chest_opened

    play_exit_animation()
    await get_tree().create_timer(1.5).timeout

    load_next_level()

func spawn_enemy(pos: Vector2) -> void:
    var enemy = enemy_scene.instantiate()
    enemy.position = pos

    # Connect signals before adding to scene tree
    enemy.died.connect(_on_enemy_died.bind(enemy))
    enemy.health_changed.connect(_on_enemy_health_changed)

    add_child(enemy)

func _on_enemy_died(enemy: Node) -> void:
    enemies_alive -= 1
    enemy.queue_free()

# WRONG — Godot 3 syntax, will not compile
button.connect("pressed", self, "_on_button_pressed")

# CORRECT — Godot 4 syntax
button.pressed.connect(_on_button_pressed)

# AVOID — deprecated, no compile-time checks
emit_signal("health_changed", health)

# PREFER — type-safe, catches typos at compile time
health_changed.emit(health)

# Safe emission pattern
for connection in my_signal.get_connections():
    if is_instance_valid(connection["callable"].get_object()):
        pass  # Connection is still valid
my_signal.emit()  # Godot handles invalid connections gracefully in 4.x

# WRONG — never disconnects, leaks across scene changes
func _ready() -> void:
    EventBus.score_changed.connect(_on_score_changed)

# CORRECT — clean disconnect
func _ready() -> void:
    EventBus.score_changed.connect(_on_score_changed)

func _exit_tree() -> void:
    EventBus.score_changed.disconnect(_on_score_changed)

# Guard against double-connection
func _ready() -> void:
    if not EventBus.score_changed.is_connected(_on_score_changed):
        EventBus.score_changed.connect(_on_score_changed)