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()

Throwing my hat in the ring with a simpler version using State objects defined inside the StateMachine class. I include an example of how .. concise it can be used. Granted, you dont have to use only lambdas like i am here, but I think it is pretty neat that you can.

https://gist.github.com/alkemann/481252a7f8d3d84e606a049a3d6c6b9d