State Machine

State Machines in Godot 4.3+

Choose the right FSM pattern for your complexity level. All examples target Godot 4.3+ with no deprecated APIs.

Related skills: player-controller for movement state integration, ai-navigation for AI state patterns, resource-pattern for resource-based state configuration, animation-system for AnimationTree states driven by FSM, dialogue-system for dialogue flow as a state machine, ability-system for caster state gating (casting/stunned), limboai for the LimboAI addon's HSM (BTState) if you need a behavior tree alongside your FSM, beehave for a GDScript-only BT alternative.

When to reach for an addon: This skill covers the built-in FSM patterns (enum, node-based, resource-based). If your agent needs a full behavior tree, see limboai (C++ + HSM, Godot 4.6+) or beehave (pure GDScript, Godot 4.1+) instead.

1. Approach Comparison

2. Approach 1: Enum-Based (Simplest)

Use when you have a small number of states and no significant enter/exit logic.

GDScript

gdscriptCopy code
extends CharacterBody2D

enum State { IDLE, PATROL, CHASE, ATTACK }

@export var patrol_range: float = 200.0
@export var chase_range: float = 300.0
@export var attack_range: float = 50.0
@export var speed: float = 80.0

var current_state: State = State.IDLE
var patrol_target: Vector2 = Vector2.ZERO

@onready var player: Node2D = get_tree().get_first_node_in_group("player")


func _physics_process(delta: float) -> void:
	match current_state:
		State.IDLE:
			_state_idle()
		State.PATROL:
			_state_patrol()
		State.CHASE:
			_state_chase()
		State.ATTACK:
			_state_attack()

	move_and_slide()


func _state_idle() -> void:
	velocity = Vector2.ZERO
	if _player_in_range(chase_range):
		current_state = State.CHASE
	elif randf() < 0.005:
		patrol_target = global_position + Vector2(randf_range(-patrol_range, patrol_range), 0.0)
		current_state = State.PATROL


func _state_patrol() -> void:
	var direction := (patrol_target - global_position)
	if direction.length() < 4.0:
		current_state = State.IDLE
		return
	velocity = direction.normalized() * speed
	if _player_in_range(chase_range):
		current_state = State.CHASE


func _state_chase() -> void:
	if not is_instance_valid(player):
		current_state = State.IDLE
		return
	if _player_in_range(attack_range):
		current_state = State.ATTACK
		return
	if not _player_in_range(chase_range):
		current_state = State.PATROL
		return
	velocity = (player.global_position - global_position).normalized() * speed


func _state_attack() -> void:
	velocity = Vector2.ZERO
	if not _player_in_range(attack_range):
		current_state = State.CHASE


func _player_in_range(range: float) -> bool:
	if not is_instance_valid(player):
		return false
	return global_position.distance_to(player.global_position) <= range

C# Equivalent

csharpCopy code
using Godot;

public partial class SimpleEnemy : CharacterBody2D
{
    private enum State { Idle, Patrol, Chase, Attack }

    [Export] public float PatrolRange { get; set; } = 200f;
    [Export] public float ChaseRange  { get; set; } = 300f;
    [Export] public float AttackRange { get; set; } = 50f;
    [Export] public float Speed       { get; set; } = 80f;

    private State _currentState = State.Idle;
    private Vector2 _patrolTarget = Vector2.Zero;
    private Node2D _player;

    public override void _Ready()
    {
        _player = GetTree().GetFirstNodeInGroup("player") as Node2D;
    }

    public override void _PhysicsProcess(double delta)
    {
        switch (_currentState)
        {
            case State.Idle:   StateIdle();   break;
            case State.Patrol: StatePatrol(); break;
            case State.Chase:  StateChase();  break;
            case State.Attack: StateAttack(); break;
        }
        MoveAndSlide();
    }

    private void StateIdle()
    {
        Velocity = Vector2.Zero;
        if (PlayerInRange(ChaseRange))
        {
            _currentState = State.Chase;
        }
        else if (GD.Randf() < 0.005f)
        {
            _patrolTarget = GlobalPosition + new Vector2(GD.RandRange(-PatrolRange, PatrolRange), 0f);
            _currentState = State.Patrol;
        }
    }

    private void StatePatrol()
    {
        var direction = _patrolTarget - GlobalPosition;
        if (direction.Length() < 4f) { _currentState = State.Idle; return; }
        Velocity = direction.Normalized() * Speed;
        if (PlayerInRange(ChaseRange)) _currentState = State.Chase;
    }

    private void StateChase()
    {
        if (!IsInstanceValid(_player)) { _currentState = State.Idle; return; }
        if (PlayerInRange(AttackRange)) { _currentState = State.Attack; return; }
        if (!PlayerInRange(ChaseRange)) { _currentState = State.Patrol; return; }
        Velocity = (_player.GlobalPosition - GlobalPosition).Normalized() * Speed;
    }

    private void StateAttack()
    {
        Velocity = Vector2.Zero;
        if (!PlayerInRange(AttackRange)) _currentState = State.Chase;
    }

    private bool PlayerInRange(float range) =>
        IsInstanceValid(_player) && GlobalPosition.DistanceTo(_player.GlobalPosition) <= range;
}

When to upgrade away from enum-based:

• Enter/exit logic starts duplicating across state methods
• Animation sync requires explicit enter/exit hooks
• The match/switch block grows beyond ~100 lines

3. Approach 2: Node-Based (Recommended for Characters)

Each state is its own node. The StateMachine node delegates input and process calls to whichever state is active, and states trigger transitions by name.

Scene Tree

Copy code
Player (CharacterBody2D)
└── StateMachine (Node)
    ├── Idle  (State)
    ├── Run   (State)
    ├── Jump  (State)
    └── Attack (State)

State Base Class

GDScript (state.gd)

gdscriptCopy code
class_name State
extends Node

## Populated by StateMachine._ready()
var entity: CharacterBody2D
var state_machine: StateMachine


## Called when this state becomes active.
func enter() -> void:
	pass


## Called when this state is deactivated.
func exit() -> void:
	pass


## Mirrors _process. Return a state name string to transition, or "" to stay.
func update(delta: float) -> String:
	return ""


## Mirrors _physics_process. Return a state name string to transition, or "".
func physics_update(delta: float) -> String:
	return ""


## Mirrors _unhandled_input.
func handle_input(event: InputEvent) -> String:
	return ""

C# (State.cs)

csharpCopy code
using Godot;

public partial class State : Node
{
    /// Populated by StateMachine._Ready()
    public CharacterBody2D Entity { get; set; }
    public StateMachine StateMachine { get; set; }

    public virtual void Enter() { }
    public virtual void Exit() { }
    public virtual string Update(double delta) => string.Empty;
    public virtual string PhysicsUpdate(double delta) => string.Empty;
    public virtual string HandleInput(InputEvent @event) => string.Empty;
}

StateMachine Class

GDScript (state_machine.gd)

gdscriptCopy code
class_name StateMachine
extends Node

@export var initial_state: State

var current_state: State
var states: Dictionary = {}


func _ready() -> void:
	for child in get_children():
		if child is State:
			states[child.name] = child
			child.entity = owner as CharacterBody2D
			child.state_machine = self

	if initial_state:
		current_state = initial_state
		current_state.enter()


func _unhandled_input(event: InputEvent) -> void:
	var next := current_state.handle_input(event)
	if next:
		transition_to(next)


func _process(delta: float) -> void:
	var next := current_state.update(delta)
	if next:
		transition_to(next)


func _physics_process(delta: float) -> void:
	var next := current_state.physics_update(delta)
	if next:
		transition_to(next)


func transition_to(state_name: String) -> void:
	if not states.has(state_name):
		push_error("StateMachine: unknown state '%s'" % state_name)
		return
	current_state.exit()
	current_state = states[state_name]
	current_state.enter()

C# (StateMachine.cs)

csharpCopy code
using System.Collections.Generic;
using Godot;

public partial class StateMachine : Node
{
    [Export] public State InitialState { get; set; }

    public State CurrentState { get; private set; }
    private readonly Dictionary<string, State> _states = new();

    public override void _Ready()
    {
        foreach (var child in GetChildren())
        {
            if (child is State state)
            {
                _states[state.Name] = state;
                state.Entity = Owner as CharacterBody2D;
                state.StateMachine = this;
            }
        }

        if (InitialState != null)
        {
            CurrentState = InitialState;
            CurrentState.Enter();
        }
    }

    public override void _UnhandledInput(InputEvent @event)
    {
        var next = CurrentState.HandleInput(@event);
        if (!string.IsNullOrEmpty(next)) TransitionTo(next);
    }

    public override void _Process(double delta)
    {
        var next = CurrentState.Update(delta);
        if (!string.IsNullOrEmpty(next)) TransitionTo(next);
    }

    public override void _PhysicsProcess(double delta)
    {
        var next = CurrentState.PhysicsUpdate(delta);
        if (!string.IsNullOrEmpty(next)) TransitionTo(next);
    }

    public void TransitionTo(string stateName)
    {
        if (!_states.TryGetValue(stateName, out var next))
        {
            GD.PushError($"StateMachine: unknown state '{stateName}'");
            return;
        }
        CurrentState.Exit();
        CurrentState = next;
        CurrentState.Enter();
    }
}

Concrete Example: IdleState

GDScript (idle_state.gd)

gdscriptCopy code
class_name IdleState
extends State


func enter() -> void:
	entity.get_node("AnimationPlayer").play("idle")


func physics_update(delta: float) -> String:
	if not entity.is_on_floor():
		return "Jump"
	if Input.get_axis("move_left", "move_right") != 0.0:
		return "Run"
	return ""


func handle_input(event: InputEvent) -> String:
	if event.is_action_pressed("jump") and entity.is_on_floor():
		return "Jump"
	if event.is_action_pressed("attack"):
		return "Attack"
	return ""

4. Approach 3: Resource-Based (Data-Driven)

Use when designers need to configure states in the Godot Inspector without modifying code.

StateData Resource

gdscriptCopy code
class_name StateData
extends Resource

@export var state_name: String = ""
@export var animation_name: String = ""
@export var move_speed: float = 0.0
@export var can_transition_to: Array[String] = []

Export an Array[StateData] on your AI controller. Designers populate each entry in the Inspector — no code changes needed to tune behavior or add states. The runtime reads can_transition_to to validate transitions and picks animation_name / move_speed for each active state.

csharpCopy code
using Godot;

[GlobalClass]
public partial class StateData : Resource
{
    [Export] public string StateName { get; set; } = string.Empty;
    [Export] public string AnimationName { get; set; } = string.Empty;
    [Export] public float MoveSpeed { get; set; } = 0f;
    [Export] public Godot.Collections.Array<string> CanTransitionTo { get; set; } = new();
}

Attach an Array[StateData] export on your AI controller class ([Export] public Godot.Collections.Array<StateData> States). At runtime, look up the active StateData by StateName and read AnimationName / MoveSpeed to drive behavior; use CanTransitionTo to guard TransitionTo calls.

5. Hierarchical and Parallel State Machines

When a flat FSM grows beyond ~8 states or spans multiple concerns (movement + combat + animation), split into hierarchical machines (states own sub-state machines, e.g. OnGround containing Idle/Walk/Run) or parallel machines (independent FSMs for movement, combat, animation running side-by-side). Both keep state counts additive instead of multiplicative.

See references/hierarchical-and-parallel.md for full scene trees, HierarchicalState base class, parallel-machine character example, and a "which to choose" comparison table — GDScript and C# for each.

6. Decision Flowchart

Copy code
Start
  │
  ▼
Fewer than 5 states?
  ├─ Yes ──────────────────────────────────► Enum-Based
  └─ No
       │
       ▼
     Multiple independent concerns
     (movement + combat + animation)?
       ├─ Yes ──────────────────────────────► Parallel State Machines
       └─ No
            │
            ▼
          States naturally nest
          (sub-states within states)?
            ├─ Yes ────────────────────────► Hierarchical State Machine
            └─ No
                 │
                 ▼
               Designers need to configure
               states in the Inspector?
                 ├─ Yes ──────────────────► Resource-Based
                 └─ No ──────────────────► Node-Based

7. Implementation Checklist

• Chose the approach that matches actual complexity (enum / node / resource)
• Every state has explicit enter() and exit() methods (or equivalent)
• All transitions are named explicitly — no implicit fallthrough between states
• Animations are started in enter() and cleaned up in exit() where needed
• No circular transition loops that could cause infinite recursion in a single frame
• Flat FSM is replaced with hierarchical or parallel when states exceed ~8 or span multiple concerns
• Parallel state machines don't modify the same state (e.g., both setting velocity) — one concern per machine