[Signal] is a built-in [Variant] type that represents a signal of an [Object] instance. Like all [Variant] types, it can be stored in variables and passed to functions. Signals allow all connected [Callable]s (and by extension their respective objects) to listen and react to events, without directly referencing one another. This keeps the code flexible and easier to manage. You can check whether an [Object] has a given signal name using [method Object.has_signal].
Connecting signals is one of the most common operations in Godot and the API gives many options to do so, which are described further down. The code block below shows the recommended approach.
[codeblocks]
[gdscript]
func _ready():
var button = Button.new()
# `button_down` here is a Signal Variant type. We therefore call the Signal.connect() method, not Object.connect().
# See discussion below for a more in-depth overview of the API.
button.button_down.connect(_on_button_down)
# This assumes that a `Player` class exists, which defines a `hit` signal.
var player = Player.new()
# We use Signal.connect() again, and we also use the Callable.bind() method,
# which returns a new Callable with the parameter binds.
print("Hit with weapon %s for %d damage." % [weapon_type, damage])
[/gdscript]
[csharp]
public override void _Ready()
{
var button = new Button();
// C# supports passing signals as events, so we can use this idiomatic construct:
button.ButtonDown += OnButtonDown;
// This assumes that a `Player` class exists, which defines a `Hit` signal.
var player = new Player();
// We can use lambdas when we need to bind additional parameters.
player.Hit += () => OnPlayerHit("sword", 100);
}
private void OnButtonDown()
{
GD.Print("Button down!");
}
private void OnPlayerHit(string weaponType, int damage)
{
GD.Print($"Hit with weapon {weaponType} for {damage} damage.");
}
[/csharp]
[/codeblocks]
[b][code skip-lint]Object.connect()[/code] or [code skip-lint]Signal.connect()[/code]?[/b]
As seen above, the recommended method to connect signals is not [method Object.connect]. The code block below shows the four options for connecting signals, using either this legacy method or the recommended [method Signal.connect], and using either an implicit [Callable] or a manually defined one.
[codeblocks]
[gdscript]
func _ready():
var button = Button.new()
# Option 1: Object.connect() with an implicit Callable for the defined function.
button.connect("button_down", _on_button_down)
# Option 2: Object.connect() with a constructed Callable using a target object and method name.
// Option 3: GodotObject.Connect() with a constructed Callable using a target object and method name.
button.Connect(Button.SignalName.ButtonDown, new Callable(this, MethodName.OnButtonDown));
}
private void OnButtonDown()
{
GD.Print("Button down!");
}
[/csharp]
[/codeblocks]
While all options have the same outcome ([code]button[/code]'s [signal BaseButton.button_down] signal will be connected to [code]_on_button_down[/code]), [b]option 3[/b] offers the best validation: it will print a compile-time error if either the [code]button_down[/code] [Signal] or the [code]_on_button_down[/code] [Callable] are not defined. On the other hand, [b]option 2[/b] only relies on string names and will only be able to validate either names at runtime: it will generate an error at runtime if [code]"button_down"[/code] is not a signal, or if [code]"_on_button_down"[/code] is not a method in the object [code]self[/code]. The main reason for using options 1, 2, or 4 would be if you actually need to use strings (e.g. to connect signals programmatically based on strings read from a configuration file). Otherwise, option 3 is the recommended (and fastest) method.
[b]Binding and passing parameters:[/b]
The syntax to bind parameters is through [method Callable.bind], which returns a copy of the [Callable] with its parameters bound.
When calling [method emit] or [method Object.emit_signal], the signal parameters can be also passed. The examples below show the relationship between these signal parameters and bound parameters.
[codeblocks]
[gdscript]
func _ready():
# This assumes that a `Player` class exists, which defines a `hit` signal.
Connects this signal to the specified [param callable]. Optional [param flags] can be also added to configure the connection's behavior (see [enum Object.ConnectFlags] constants). You can provide additional arguments to the connected [param callable] by using [method Callable.bind].
A signal can only be connected once to the same [Callable]. If the signal is already connected, this method returns [constant ERR_INVALID_PARAMETER] and generates an error, unless the signal is connected with [constant Object.CONNECT_REFERENCE_COUNTED]. To prevent this, use [method is_connected] first to check for existing connections.
Disconnects this signal from the specified [Callable]. If the connection does not exist, generates an error. Use [method is_connected] to make sure that the connection exists.
Emits this signal. All [Callable]s connected to this signal will be triggered. This method supports a variable number of arguments, so parameters can be passed as a comma separated list.
