Instantiate scenes with factory like pattern and singleton lifetime behavior

Help Programming
,
1

Godot Version

4.2.1 stable

Question

I’ve discovered an behavior, which I can’t really explain.

I have a scene, which is created using ".instantiate()".
Contrary to my expectations, I have already learned in Github that scenes, which are created ".instantiate()", the pseudoconstructor "._init()" is not executed.

Therefore u aren’t able to pass parameters while instantiating scenes/nodes using this method.
Heres the link: Make it possible to pass arguments to `PackedScene.instance()` · Issue #1513 · godotengine/godot-proposals · GitHub

Since it’s not a solution for me to fill the properties by hand, after every ".instantiate()", of the respective scene, i’d like to have a clean API for instantiation.
I tried to create a method that i could just call myself (something like my own “pseudoconstructor”).
While this solution is not optimal, as it does not get called automatically it’s better than nothing.

However, I encountered strange behavior that I can’t explain.

Here is a simple example:

laser.gd

class_name Laser
extends Area2D

var speed: int = 0
var direction: Vector2 = Vector2(0,0)

# expected to be a instance method
func custom_psuedoconstructor(spawn_position, direction, speed):
	print(spawn_position) # (268.7128, 598.5436)
	print(direction) # (-0.973798, 0.227414)
	print(speed) # 3000
	
	position = spawn_position
	direction = direction
	speed = speed
	rotation_degrees = rad_to_deg(direction.angle())

# should process with values from member variables but it isn't
func _process(delta):
	print(speed) # 0
	print(direction) # (0, 0)
	
	position += direction * speed * delta

level.gd


extends Node2D

# Import the laser scene
var laser_scene: PackedScene = preload("res://scenes/projectiles/laser.tscn")

func _ready():
	# Changes the arrow shape of the cursor.
	Input.set_custom_mouse_cursor(arrow)

func _on_player_player_laser_shooted(pos, direction):
	var laser = laser_scene.instantiate() as Laser
	
	# This works as expected and changes the member variables that are used
	# by the instance
	#laser.position = pos
	#laser.rotation_degrees = rad_to_deg(direction.angle())
	#laser.direction = direction
	
	# This does not work as expected and leads to the behavior described
	laser.custom_psuedoconstructor(pos, direction, 3000)
	$Projectiles.add_child(laser)

I assumed that the variables and functions that are within a script are created at object or instance level and are not created as global Functions and variables.

I couldn’t find andy information in the GDScript documentation about whether there is a distinction between instance methods, member variables, and and singletons vs global variables, functions and multiples.

I’m still a beginner in Godot and GDScript and I think I’m doing something fundamentally wrong here.

Can someone help me and explain how to use Global Functions or Instance Methods and member variables in GDScript?

Thank you in advance - Danloc

2

You named the method parameters with the same name as the properties of the class and you are assigning the method parameters to the method parameters:

# expected to be a instance method
func custom_psuedoconstructor(spawn_position, direction, speed):
	print(spawn_position) # (268.7128, 598.5436)
	print(direction) # (-0.973798, 0.227414)
	print(speed) # 3000
	
	position = spawn_position 
	direction = direction # <---- HERE
	speed = speed # <---- HERE
	rotation_degrees = rad_to_deg(direction.angle())

If you want to assign it to the properties of the class then you need to use self:

# expected to be a instance method
func custom_psuedoconstructor(spawn_position, direction, speed):
	print(spawn_position) # (268.7128, 598.5436)
	print(direction) # (-0.973798, 0.227414)
	print(speed) # 3000
	
	position = spawn_position 
	self.direction = direction # <---- Assigning the method parameter 'direction' to the property of the class 'direction'
	self.speed = speed # <---- Assigning the method parameter 'speed' to the property of the class 'speed'
	rotation_degrees = rad_to_deg(direction.angle())

Not useful to your issue but _init() gets called when using instantiate(), it just can’t have any required parameter or it will fail as the engine call the method with no parameters.

3

Thanks that works! And thanks for the clarification.

could you provide a link in the docs where i can read more about it?

Is there a possibility in GDScript to have global functions instead of instance methods?

4

Here’s the GDScript documentation GDScript — Godot Engine (stable) documentation in English