Problem with state machine

leakybob

Hi,

I am following the GDquest tutorial for finite state machines, and I'm running into an issue with referencing the Player class.

The assertion fails and it seems that "player" is returning null.

PlayerState class

extends State


class_name PlayerState


# Typed reference to the player node.
var player:  Player


func _ready() -> void:

	yield(owner, "ready")

	player = owner as Player

	assert(player != null)

State class


extends Node
class_name State
var state_machine = null
func handle_input(_event: InputEvent) -> void:
	pass
func update(_delta: float) -> void:
	pass
func physics_update(_delta: float) -> void:
	pass
func enter () -> void:
	pass
func exit() -> void:
	pass

StateMachine class

class_name StateMachine
extends Node

signal transitioned(state_name)

export var initial_state := NodePath()

onready var state: State = get_node(initial_state)


func _ready() -> void:
	yield(owner, "ready")
	
	for child in get_children():
		child.state_machine =  self
	state.enter()

func _unhandled_input(event: InputEvent) -> void:
	state.handle_input(event)


func _process(delta: float) -> void:
	state.update(delta)


func _physics_process(delta: float) -> void:
	state.physics_update(delta)

func transition_to(target_state_name: String) -> void:
	
	if not has_node(target_state_name):
		return

	state.exit()
	state = get_node(target_state_name)
	state.enter()
	emit_signal("transitioned", state.name)

duane

You're assigning owner to player, but I can't see anyplace that you've defined owner. Did you define it somewhere else? Is it actually being assigned there?

leakybob

I haven't defined owner. I thought owner was automatically set as the root node (in this case, Player)? Sorry, I am a Godot novice

duane

@leakybob said: I thought owner was automatically set as the root node (in this case, Player)?

I don't know if there's a system variable like that, but you could assign owner in the ready() function of the Player node:

_ready():
	owner = self

or you could get it from any node by it's path:

	owner = get_node("/root/Player")

leakybob

When trying to assign owner in the ready() function of Player, it returns this error: "Condition "p_owner == this" is true." Does that mean it is already true?

I appreciate the help!

duane

I was mistaken. "owner" is a property of every node, so you can't declare it as a variable. "owner" is preset when the node is already attached in the scene tree. So, your State class (extending Node) should also have an owner, but only if you've already added it to the scene tree.

If Idle and Walk are States, then I think their owner should be StateMachine (their parent). StateMachine's owner should be your Player. So you might have to say

	player = owner.owner

to get the player node. Or, you could just assign it with

	player = get_node("/root/Player")

which is the way I'm used to doing it.

Edit: No, I'm wrong again. After a little testing, I found that owner is set to a node's parent's owner when you add a child node. So, Idle and Walk should have the same owner as StateMachine, which ought to be Player. I'm pretty sure that "/root/Player" will always work, though there's probably a better way to do it.

Edit 2: Actually, since your Player is a scene that will be added as a node, even "/root/Player" probably won't work. You'd have to use

	player = get_parent().get_parent()

which is pretty ugly.

DaveTheCoder

Which nodes are the scripts in your post attached to?

duane

Oddly enough, when I make a small project with the pasted code above, it can find Player without any problems. :/

leakybob

@duane said: Oddly enough, when I make a small project with the pasted code above, it can find Player without any problems. :/

This really helped.

It turned out that there was a duplicate Player class in a Kinematic2d script, so once I deleted that, no error returned. Now I just have to get the state machine working properly!

Thanks again!

leakybob

@DaveTheCoder said: Which nodes are the scripts in your post attached to?

Turns out that I had a duplicate script messing things up. Thanks!

leakybob

I am now getting this error: "Invalid call. Nonexistent function 'transition_to' in base 'Nil'."


extends PlayerState

func enter() -> void:
	player.anim_state_machine.travel("Run")
	

func exit() -> void:
	pass


func _physics_process(delta: float) -> void:

	if player.is_on_floor() and is_zero_approx(player._velocity.x) :
		state_machine.transition_to("Idle")
		return


	player._apply_gravity(delta)
	player._velocity = player.move_and_slide(player._velocity, player.UP_DIRECTION)
	player._horizontal_direction = (Input.get_action_strength("move_right") - Input.get_action_strength("move_left"))
	player._velocity.x = player._horizontal_direction * player.speed

transition_to is defined in State.gd, but I'm not sure if I have to reference it again somewhere else in the script to make it work?

duane

@leakybob said: I am now getting this error: "Invalid call. Nonexistent function 'transition_to' in base 'Nil'."

The problem there is that the state_machine property in your PlayerState is still null. "state_machine" is set by the ready() function of your StateMachine class, and it works normally in my test project, but you may be using it before the ready() function finishes.

You could try putting this at the top of your _physics_process()

if state_machine == null:
	return

That might keep it from starting before the _ready() functions have all run.

leakybob

You could try putting this at the top of your _physics_process()
if state_machine == null:
	return
That might keep it from starting before the _ready() functions have all run.

In the children of StateMachine? It doesn't return an error when I put that in, but it doesn't get a chance to transition or do anything else. I tried only putting "pass" and print in the _physics_process() and all it did was return "idle" and "run" every other frame. It seems to be cycling through all of the states without being told to transition. Is this normal in a state machine?

duane

@leakybob said: In the children of StateMachine?

In the code you pasted (most recently) above.

It doesn't return an error when I put that in, but it doesn't get a chance to transition or do anything else. I tried only putting "pass" and print in the _physics_process() and all it did was return "idle" and "run" every other frame. It seems to be cycling through all of the states without being told to transition. Is this normal in a state machine?

"Normal" isn't really an issue -- the question is how to get it to do what you want. If you put the check for null in the _physics_process(), but it's never getting past that, then we know that the "state_machine" parameter is never getting assigned. That should happen before any other state activity.

When the StateMachine node runs its _ready() method, it will look through all of its children (not grandchildren, or anything else) and assign their "state_machine" property. So all of your nodes that inherit from State need to be children of the StateMachine node. Is that the case?

leakybob

If you put the check for null in the _physics_process(), but it's never getting past that, then we know that the "state_machine" parameter is never getting assigned. That should happen before any other state activity.

It's never getting past that, so it must not be be getting assigned.

When the StateMachine node runs its _ready() method, it will look through all of its children (not grandchildren, or anything else) and assign their "state_machine" property. So all of your nodes that inherit from State need to be children of the StateMachine node. Is that the case?

Yes, this is the case.

duane

@leakybob said: Yes, this is the case.

I'm not sure that there's any clue in the code you posted as to why "state_machine" isn't being assigned. You might have to attach a zip of your project (or a subset), since there's no easy way to examine the tree otherwise. In godot, the scene_tree is at least as important as the code, but harder to post.

I think I know why your state is toggling though. Anything you put in the new State's enter() method is run every time the StateMachine runs the old State's transition_to() method. You put a transition to another state in the class you posted [that inherits PlayerState()], so every time it tries to transition to that state, it's going to trigger a transition to the Run state.

Since I don't know what actual state object that code was copied from, it's hard to be more precise. But, if it's the Run object, it doesn't need to transition. If it's another state, it shouldn't automatically transition to Run in enter().

leakybob

I am attaching the project. This is my first state machine so I'm quite lost in the woods...my spaghetti code just isn't scalable! Again, any help is hugely appreciated!

duane

Ok! It took me about a minute to spot it in the project, but it was a code issue. You put "ready()" at the top of StateMachine, but it has to be "_ready()" to run when it's supposed to. System-oriented methods usually start with an underscore.

You're still going to have some issues, but that will get you past that error.

leakybob

@duane said: Ok! It took me about a minute to spot it in the project, but it was a code issue. You put "ready()" at the top of StateMachine, but it has to be "_ready()" to run when it's supposed to. System-oriented methods usually start with an underscore.

You're still going to have some issues, but that will get you past that error.

I would have never noticed that underscore. Thank you!! Now to get the rest of this thing working