Why does the add_child() method change position?

[Pityke]

This is my shoot function:
(I used a Node2D for a shooting point as suggested in a thread)

func shoot():
	var rocket_instance = rocket_scene.instantiate()
	rocket_instance.global_position = shooting_point.global_position
	print(global_position)
	print(shooting_point.global_position)
	print(rocket_instance.global_position)

	add_child(rocket_instance, true)

	print(global_position)
	print(shooting_point.global_position)
	print(rocket_instance.global_position)

This is the console result:

global pos
(564.1352, 342.0016)
shooting point pos
(611.1352, 342.0016)
rocket_instance pos
(611.1352, 342.0016)


global pos
(564.1352, 342.0016)
shooting point pos
(611.1352, 342.0016)
rocket_instance pos
(1175.27, 684.0033)

As you can see rocket_instance.global_position changed after I added the child.
Can you explain why the add_child() method will change the instance’s position?

If I use add_child() first then change the position with rocket_instance.global_position = shooting_point.global_position it works as I intended

I think the rocket_instance’s position is different because the add_child() will add the player’s location on top of the shooting_point.global_position .

[Pityke]

I found this thread in the next course:
https://community.gamedev.tv/t/why-does-the-order-of-adding-child-and-setting-position-matter/232905

According to this if I set the global position before adding the child it won’t matter, because:

its not part of the scene tree at that point, and doesnt have any transform information

But clearly it does something, if I would remove the

rocket_instance.global_position = shooting_point.global_position

line, the rocket would spawn from the center of the player node

[BH67]

@Pityke

I think the rocket_instance’s position is different because the add_child() will add the player’s location on top of the shooting_point.global_position .

That’s the idea. Because everything is based in Node2Ds, the rocket will inherit the transform of its parent and it becomes an offset. I don’t remember offhand if this still happens when the rocket has Top Level ticked in its Inspector, but the problem is still fixed easily enough either way.

@Pityke

If I use add_child() first then change the position with rocket_instance.global_position = shooting_point.global_position it works as I intended

This is exactly why I consider it a best practice to place your add_child() line immediately after your instantiate() line.

@Pityke

But clearly [set the global position before adding the child] does something

I agree, as you’ve clearly shown. Don’t get me wrong, Shape definitely has the right idea, I think things just got lost in translation a little. All the other properties of a node exist at that point and can be modified before add_child(), so it’s less that the transform doesn’t exist and more that it’s easily overwritten unintentionally if you do it that way.

You’re writing excellent topics; keep it up =)