[Python] How to set reactive value from callback in non-reactive module?

I'm building a app using Shiny for Python and Tinkerforge hardware modules. I separated all hardware-related code into a class and create an instance of it when starting the app. Tinkerforge offers callback registration for button state change events. So I can execute a function whenever the button changes state.

I want to trigger a reactive chain in the app when the button is depressed, I also want to know how long the button was pressed. However it seems, that while I can make use of global variables within the callback, the same is not true of reactive values. There is "no current reactive context". So how can I make use of this callback to trigger events in the Shiny app?

Here is a MWE akin to what I have, I'm unsure of how to build one without Tinkerforge:

# TinkerforgeModule.py
from tinkerforge.ip_connection import IPConnection
from tinkerforge.bricklet_rgb_led_button import BrickletRGBLEDButton

class TinkerforgeModule (object):
def __init__(self):
ipcon = IPConnection() # Create IP connection to tinkerforge
self.rgb_button = BrickletRGBLEDButton('24fu', ipcon)
ipcon.connect('127.0.0.1', 4223)

def register_hw_button_cb(self, func):
self.rgb_button.register_callback(
BrickletRGBLEDButton.CALLBACK_BUTTON_STATE_CHANGED,
func
)


# app.py
from shiny import App, ui, render, reactive
from datetime import datetime
import TinkerforgeModule

app_ui = ui.page_auto(
ui.output_text("result_output")
)

def server(input, output, session):

hw_button_last_press = None
triggerMe = reactive.value(None)

def hw_button_callback(state):
global hw_button_last_press
if state == 1: # BrickletRGBLEDButton.BUTTON_STATE_RELEASED = 1
press_time = datetime.now() - hw_button_last_press
print(f"Button released after {press_time} seconds.")

# The line below does not work
# triggerMe.set(f"Button released after {press_time} seconds.")

else: # BrickletRGBLEDButton.BUTTON_STATE_PRESSED = 0
hw_button_last_press = datetime.now()
print("Button pressed.")

# The line below does not work
# triggerMe.set("Button pressed.")

tfm = TinkerforgeModule.TinkerforgeModule()
tfm.register_hw_button_cb(hw_button_callback)

@output
@render.text
def result_output():
return(f"Message: {triggerMe()}")

app = App(app_ui, server)


Registering the callback works, the printing happens when the button is pressed and released. However when it comes to setting the reactive value the app crashes. Placing a breakpoint and executing step by step I get a RuntimeError: No current reactive context.

Initially I just polled the state of the button every 50ms using reactive.invalidate_later. But the callback would be a much cleaner way of doing it.

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