Hello, I have implemented a node-based state machine based on examples. If I use @export variables in the base state class, everything works fine, but I have to manually add all these variables in the object inspector. I would like to initialize the player and animation_player variables in one place so that I can use them in child states.
I tried initializing them with @onready in PlayerState, and player surprisingly started working, but now there is a problem with AninationPlayer. Now the program crashes when it needs to play the animation in enter() in IdleState.
I tried to understand what the problem might be, apparently in the order of ready() calls in parents and descendants.
Below is the code that works, but not quite the way I need it to:
@abstract
class_name State extends Node
signal transitioned(new_state_name: StringName)
func enter() -> void:
pass
func exit() -> void:
pass
func update(_delta: float) -> void:
pass
func physics_update(_delta: float) -> void:
pass
func handle_input(_event: InputEvent) -> void:
pass
class_name StateMachine extends Node
@export var current_state: State
var states: Dictionary = {}
func _ready() -> void:
for state in get_children():
if state is State:
states[state.name] = state
state.transitioned.connect(on_state_transitioned)
else:
push_warning('State is not State!')
current_state.enter()
func _process(delta: float) -> void:
if current_state.has_method('update'):
current_state.update(delta)
func _physics_process(delta: float) -> void:
if current_state.has_method('physics_update'):
current_state.physics_update(delta)
func _handle_input(event: InputEvent) -> void:
if current_state.has_method('handle_input'):
current_state.handle_input(event)
func on_state_transitioned(new_state_name: StringName) -> void:
var new_state = states.get(new_state_name)
if new_state != null and new_state != current_state:
current_state.exit()
current_state = new_state
current_state.enter()
else:
push_warning('New state is null or current state!')
class_name PlayerState extends State
const PLAYER_STATES_NAMES: Dictionary = {
'IDLE': 'IdleState',
'WALK': 'WalkState',
'USE': 'UseState',
}
@export var player: Player
@export var animation_player: AnimationPlayer
func handle_input(_event: InputEvent) -> void:
if _event.is_action_pressed('use'):
transitioned.emit(PLAYER_STATES_NAMES.get('USE'))
func update_state_animation(state_prefix: String, last_direction: Vector2) -> void:
match last_direction:
Vector2.UP:
animation_player.play(state_prefix + '_up')
Vector2.DOWN:
animation_player.play(state_prefix + '_down')
Vector2.LEFT:
animation_player.play(state_prefix + '_left')
Vector2.RIGHT:
animation_player.play(state_prefix + '_right')
class_name IdleState extends PlayerState
const IDLE_ANIMATION_PREFIX: String = 'idle'
func enter() -> void:
update_state_animation(IDLE_ANIMATION_PREFIX, player.last_direction)
func physics_update(_delta: float) -> void:
if player.direction != Vector2.ZERO:
transitioned.emit(PLAYER_STATES_NAMES.get('WALK'))
1 Like
Post the scene structure.
Also the code that crashes.
When I do this, my code immediately crashes with an animation error because AnimationPlayer is null.
class_name PlayerState extends State
const PLAYER_STATES_NAMES: Dictionary = {
'IDLE': 'IdleState',
'WALK': 'WalkState',
'USE': 'UseState',
}
@onready var animation_player: AnimationPlayer = $AnimationPlayer # <-----
@onready var player: Player = owner as Player # <-----
func handle_input(_event: InputEvent) -> void:
if _event.is_action_pressed('use'):
transitioned.emit(PLAYER_STATES_NAMES.get('USE'))
func update_state_animation(state_prefix: String, last_direction: Vector2) -> void:
match last_direction:
Vector2.UP:
animation_player.play(state_prefix + '_up')
Vector2.DOWN:
animation_player.play(state_prefix + '_down') # <----- ERROR
Vector2.LEFT:
animation_player.play(state_prefix + '_left')
Vector2.RIGHT:
animation_player.play(state_prefix + '_right')
class_name IdleState extends PlayerState
const IDLE_ANIMATION_PREFIX: String = 'idle'
func enter() -> void:
update_state_animation(IDLE_ANIMATION_PREFIX, player.last_direction)
func physics_update(_delta: float) -> void:
if player.direction != Vector2.ZERO:
transitioned.emit(PLAYER_STATES_NAMES.get('WALK'))
Yeah, so here’s what’s happening. You’ve got a race condition. Nodes are initialized bottom to top. Your StateMachine is being created before your AnimationPlayer. StateMachine enters _ready() which triggers enter() in your IdleState. Since AnimationPlayer doesn’t exist yet, IdleState cannot create its @onready variable and it crashes.
Potentially easy fix, move AnimationPlayer lower in the tree. Better fix, make your StateMachine wait for Player to be ready before starting.
func _ready() -> void:
get_parent().ready.connect(_on_player_ready)
func _on_player_ready() -> void:
for state in get_children():
if state is State:
states[state.name] = state
state.transitioned.connect(on_state_transitioned)
else:
push_warning('State is not State!')
current_state.enter()
Also, FWIW, your abstract keyword doesn’t need to be at the top of the State class, as you haven’t made any abstract functions. You also do not need update(), physics_update() or handle_input() functions. You can just turn the existing functions off and re-enable them when needed.
class_name State extends Node
signal transitioned(new_state_name: StringName)
func _ready() -> void:
set_process(false)
set_physics_process(false)
set_process_input(false)
set_process_unhandled_input(false)
func enter() -> void:
pass
func exit() -> void:
pass
Then on enter() and exit() just turn what you need on and off again. Then you can simplify your StateMachine:
class_name StateMachine extends Node
@export var current_state: State
var states: Dictionary = {}
func _ready() -> void:
for state in get_children():
if state is State:
states[state.name] = state
state.transitioned.connect(on_state_transitioned)
else:
push_warning('State is not State!')
current_state.enter()
func on_state_transitioned(new_state_name: StringName) -> void:
var new_state = states.get(new_state_name)
if new_state != null and new_state != current_state:
current_state.exit()
current_state = new_state
current_state.enter()
else:
push_warning('New state is null or current state!')
If you want to see another way to do a state machine, check out my State Machine plugin. It’s a pull instead of push method. It’s a little more complicated than yours to use, but each state is atomic and can be added and removed at runtime. It also handles race conditions.
1 Like
animation_player is null because the node path you used is incorrect. From the perspective of a state node the path is $"../../AnimationPlayer"
The order appears to be fine here. Btw _ready() is called top to bottom, depth first. Only _input() and _exit_tree() are called bottom to top.
1 Like
Noted. Thanks for the correction.
1 Like
