create abstract state machine class

scenes/classes/state.gd

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+## Virtual base class for all states.
+## Extend this class and override its methods to implement a state.
+@abstract class_name State extends Node
+
+## Emitted when the state finishes and wants to transition to another state.
+@warning_ignore("unused_signal")
+signal finished(next_state_path: String, data: Dictionary)
+
+## Called by the state machine when receiving unhandled input events.
+@abstract func handle_input(_event: InputEvent) -> void
+
+## Called by the state machine on the engine's main loop tick.
+@abstract func state_update(_delta: float) -> void
+
+## Called by the state machine on the engine's physics update tick.
+@abstract func state_physics_update(_delta: float) -> void
+
+## Called by the state machine upon changing the active state. The `data` parameter
+## is a dictionary with arbitrary data the state can use to initialize itself.
+@abstract func enter(previous_state_path: String, data: Dictionary = {}) -> void
+
+## Called by the state machine before changing the active state. Use this function
+## to clean up the state.
+@abstract func exit() -> void

scenes/classes/state.gd.uid

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+uid://bpjerj832q2gf

scenes/classes/state.tscn

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+[gd_scene load_steps=2 format=3 uid="uid://kyeowhvglgj2"]
+
+[ext_resource type="Script" uid="uid://bpjerj832q2gf" path="res://scenes/classes/state.gd" id="1_m4c3p"]
+
+[node name="State" type="Node"]
+script = ExtResource("1_m4c3p")

scenes/classes/state_machine.gd

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+## Virtual base class for all nodes that deal directly with states.
+## Extend this class and override its methods to implement a state machine.
+@abstract class_name StateMachine extends Node
+
+## The initial state of the state machine. If not set, the first child node is used.
+@export var initial_state: State
+
+## The state machine's current loaded state
+@onready var state: State = _get_initial_state()
+
+
+func _ready() -> void:
+	for state_node: State in find_children("*", "State"):
+		state_node.finished.connect(_transition_to_next_state)
+		
+		await owner.ready
+		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)
+
+
+## Called when initial state is not specified.
+## Returns the first child node by default.
+## Override if necessary, but don't call it directly.
+func _get_initial_state() -> State:
+	return initial_state if initial_state != null else get_child(0)
+
+
+## Transitions the active state out after receiving a finished signal.
+func _transition_to_next_state(target_state_path: String, data: Dictionary = {}) -> void:
+	if not has_node(target_state_path):
+		printerr(owner.name + ": Trying to transition to state " + target_state_path + " but it does not exist.")
+		return
+	
+	var previous_state_path: StringName = state.name
+	state.exit()
+	state = get_node(target_state_path)
+	state.enter(previous_state_path, data)

scenes/classes/state_machine.gd.uid

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+uid://dqjaxgmyxq3rx

scenes/classes/state_machine.tscn

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+[gd_scene load_steps=2 format=3 uid="uid://cy4h0xcy2kscg"]
+
+[ext_resource type="Script" uid="uid://dqjaxgmyxq3rx" path="res://scenes/classes/state_machine.gd" id="1_rnohx"]
+
+[node name="StateMachine" type="Node"]
+script = ExtResource("1_rnohx")