Instancing with signals

Signals provide a way to decouple game objects, allowing you to avoid forcing a fixed arrangement of nodes. One sign that a signal might be called for is when you find yourself using get_parent(). Referring directly to a node’s parent means that you can’t easily move that node to another location in the scene tree. This can be especially problematic when you are instancing objects at runtime and may want to place them in an arbitrary location in the running scene tree.

Below we’ll consider an example of such a situation: firing bullets.

Exemplo de Disparo

Consider a player character that can rotate and shoot towards the mouse. Every time the mouse button is clicked, we create an instance of the bullet at the player’s location. See Instâncias for details.

We’ll use an Area2D for the bullet, which moves in a straight line at a given velocity:

extends Area2D

var velocity = Vector2.ZERO

func _physics_process(delta):
    position += velocity * delta
public class Bullet : Area2D
{
    Vector2 Velocity = new Vector2();

    public override void _PhysicsProcess(float delta)
    {
        Position += Velocity * delta;
    }
}

Entretanto, se as balas forem anexadas ao jogador, então elas continuarão “anexadas” ao jogador enquanto ele gira:

../../_images/signals_shoot1.gif

Ao contrário disso, nós precisamos que as balas sejam independentes do movimento do jogador - uma vez disparadas, elas devem continuar se movendo em linha reta e o jogador não pode mais afetá-las. Invés de serem adicionadas a árvore da cena como filha do jogador, faz mais sentido adicionar as balas como filha da cena “principal” do jogo, a qual deve ser o nó pai do jogador ou em uma hierarquia mais alta.

You could do this by adding the bullet to the main scene directly:

var bullet_instance = Bullet.instance()
get_parent().add_child(bullet_instance)
Node bulletInstance = Bullet.Instance();
GetParent().AddChild(bulletInstance);

However, this will lead to a different problem. Now if you try to test your “Player” scene independently, it will crash on shooting, because there is no parent node to access. This makes it a lot harder to test your player code independently and also means that if you decide to change your main scene’s node structure, the player’s parent may no longer be the appropriate node to receive the bullets.

A solução para isso é usar um sinal para “emitir” as balas do jogador. Com isso não há mais necessidade do jogador “saber” o que acontece com as balas depois de disparadas - qualquer nó que estiver conectado ao sinal pode “receber” as balas e tomar a ação apropriada para gerar elas.

Aqui está o código para o jogador usar sinais para emitir a bala:

extends Sprite

signal shoot(bullet, direction, location)

var Bullet = preload("res://Bullet.tscn")

func _input(event):
    if event is InputEventMouseButton:
        if event.button_index == BUTTON_LEFT and event.pressed:
            emit_signal("shoot", Bullet, rotation, position)

func _process(delta):
    look_at(get_global_mouse_position())
public class Player : Sprite
{
    [Signal]
    delegate void Shoot(PackedScene bullet, Vector2 direction, Vector2 location);

    private PackedScene _bullet = GD.Load<PackedScene>("res://Bullet.tscn");

    public override void _Input(InputEvent event)
    {
        if (input is InputEventMouseButton mouseButton)
        {
            if (mouseButton.ButtonIndex == (int)ButtonList.Left && mouseButton.Pressed)
            {
                EmitSignal(nameof(Shoot), _bullet, Rotation, Position);
            }
        }
    }

    public override _Process(float delta)
    {
        LookAt(GetGlobalMousePosition());
    }
}

Na cena principal, conectamos então o sinal do jogador (aparecerá na guia “Nó”).

func _on_Player_shoot(Bullet, direction, location):
    var b = Bullet.instance()
    add_child(b)
    b.rotation = direction
    b.position = location
    b.velocity = b.velocity.rotated(direction)
public void _on_Player_Shoot(PackedScene bullet, Vector2 direction, Vector2 location)
{
    var bulletInstance = (Bullet)bullet.Instance();
    AddChild(bulletInstance);
    bulletInstance.Rotation = direction;
    bulletInstance.Position = location;
    bulletInstance.Velocity = bulletInstance.Velocity.Rotated(direction);
}

Agora as balas terão seu movimento conservado independente da rotação do jogador:

../../_images/signals_shoot2.gif