callable_state_machine.gd

class_name CallableStateMachine

var state_dictionary = {}
var current_state: String


func add_states(
	normal_state_callable: Callable,
	enter_state_callable: Callable,
	leave_state_callable: Callable
):
	state_dictionary[normal_state_callable.get_method()] = {
		"normal": normal_state_callable,
		"enter": enter_state_callable,
		"leave": leave_state_callable
	}


func set_initial_state(state_callable: Callable):
	var state_name = state_callable.get_method()
	if state_dictionary.has(state_name):
		_set_state(state_name)
	else:
		push_warning("No state with name " + state_name)


func update():
	if current_state != null:
		(state_dictionary[current_state].normal as Callable).call()


func change_state(state_callable: Callable):
	var state_name = state_callable.get_method()
	if state_dictionary.has(state_name):
		_set_state.call_deferred(state_name)
	else:
		push_warning("No state with name " + state_name)


func _set_state(state_name: String):
	if current_state:
		var leave_callable = state_dictionary[current_state].leave as Callable
		if !leave_callable.is_null():
			leave_callable.call()
	
	current_state = state_name
	var enter_callable = state_dictionary[current_state].enter as Callable
	if !enter_callable.is_null():
		enter_callable.call()

@mbnewsom All your code can be in a single script. I've updated it below, also fixed the typo in the State._init declaration.

I'll probably update this code further once I test it with the new variadic arguments, so we will be allowed to declare this as a single external Node, extend its script to declare the states and give arguments to .update() and .state_change(), allowing transient variables, like delta and controller inputs, being passed into, instead of holding global references, to access these values inside the state functions.

# callable_state_machine.gd
class_name CallableStateMachine


class_name State

    var normal: Callable
    var enter: Callable
    var leave: Callable

    func _init(normal: Callable, enter: Callable, leave: Callable):
        self.normal = normal
        self.enter = enter
        self.leave = leave


var state_dictionary: Dictionary[String, State] = {}
var current_state: String

func add_state(
    normal_state_callable: Callable,
    enter_state_callable: Callable,
    leave_state_callable: Callable
):
    state_dictionary[normal_state_callable.get_method()] = \
        State.new(normal_state_callable, enter_state_callable, leave_state_callable)

func set_initial_state(state_callable: Callable):
    var state_name = state_callable.get_method()
    if state_dictionary.has(state_name):
        _set_state(state_name)
    else:
        push_warning("No state with name " + state_name)


func update():
    if current_state != "":  #null here was a bug. Strings default to "", not null
        state_dictionary[current_state].normal.call()


func change_state(state_callable: Callable):
    var state_name = state_callable.get_method()
    if state_dictionary.has(state_name):
        _set_state.call_deferred(state_name)
    else:
        push_warning("No state with name " + state_name)


func _set_state(state_name: String):
    if current_state:
        var leave_callable = state_dictionary[current_state].leave
        if leave_callable.is_valid():
	    leave_callable.call()
	
    current_state = state_name
    var enter_callable = state_dictionary[current_state].enter
    if enter_callable.is_valid():
        enter_callable.call()