tutorials/wireless/hub-to-device/pc-communications: improve demo

- Remove extraneous Bleak code. - Add synchronization between hub program and pc program. - Handle case when device is not found. - Handle early hub disconnection.

Author: dlech

tutorials/wireless/hub-to-device/pc-communication/demo.py

@@ -1,8 +1,9 @@
 # SPDX-License-Identifier: MIT
 # Copyright (c) 2020 Henrik Blidh
-# Copyright (c) 2022 The Pybricks Authors
+# Copyright (c) 2022-2023 The Pybricks Authors
 
 import asyncio
+from contextlib import suppress
 from bleak import BleakScanner, BleakClient
 
 UART_SERVICE_UUID = "6E400001-B5A3-F393-E0A9-E50E24DCCA9E"
@@ -14,54 +15,68 @@
 HUB_NAME = "Pybricks Hub"
 
 
-def hub_filter(device, ad):
- return device.name and device.name.lower() == HUB_NAME.lower()
+async def main():
+ main_task = asyncio.current_task()
 
+ def handle_disconnect(_):
+ print("Hub was disconnected.")
 
-def handle_disconnect(_):
- print("Hub was disconnected.")
+ # If the hub disconnects before this program is done,
+ # cancel this program so it doesn't get stuck waiting
+ # forever.
+ if not main_task.done():
+ main_task.cancel()
 
+ ready_event = asyncio.Event()
 
-def handle_rx(_, data: bytearray):
- print("Received:", data)
+ def handle_rx(_, data: bytearray):
+ if data == b"rdy":
+ ready_event.set()
+ else:
+ print("Received:", data)
 
+ # Do a Bluetooth scan to find the hub.
+ device = await BleakScanner.find_device_by_name(HUB_NAME)
 
-async def main():
- # Find the device and initialize client.
- device = await BleakScanner.find_device_by_filter(hub_filter)
- client = BleakClient(device, disconnected_callback=handle_disconnect)
+ if device is None:
+ print(f"could not find hub with name: {HUB_NAME}")
+ return
 
- # Shorthand for sending some data to the hub.
- async def send(client, data):
- await client.write_gatt_char(rx_char, data)
+ # Connect to the hub.
+ async with BleakClient(device, handle_disconnect) as client:
 
- try:
- # Connect and get services.
- await client.connect()
+ # Subscribe to notifications from the hub.
  await client.start_notify(UART_TX_CHAR_UUID, handle_rx)
- nus = client.services.get_service(UART_SERVICE_UUID)
- rx_char = nus.get_characteristic(UART_RX_CHAR_UUID)
+
+ # Shorthand for sending some data to the hub.
+ async def send(data):
+ # Wait for hub to say that it is ready to receive data.
+ await ready_event.wait()
+ # Prepare for the next ready event.
+ ready_event.clear()
+ # Send the data to the hub.
+ await client.write_gatt_char(UART_RX_CHAR_UUID, data)
 
  # Tell user to start program on the hub.
  print("Start the program on the hub now with the button.")
 
  # Send a few messages to the hub.
  for i in range(5):
- await send(client, b"fwd")
+ await send(b"fwd")
  await asyncio.sleep(1)
- await send(client, b"rev")
+ await send(b"rev")
  await asyncio.sleep(1)
+ print(".", end="", flush=True)
 
  # Send a message to indicate stop.
- await send(client, b"bye")
+ await send(b"bye")
+
+ print("done.")
 
- except Exception as e:
- # Handle exceptions.
- print(e)
- finally:
- # Disconnect when we are done.
- await client.disconnect()
+ # Hub disconnects here when async with block exits.
 
 
 # Run the main async program.
-asyncio.run(main())
+if __name__ == "__main__":
+ with suppress(asyncio.CancelledError):
+ asyncio.run(main())

tutorials/wireless/hub-to-device/pc-communication/main.py

@@ -18,6 +18,9 @@
 
 while True:
 
+ # Let the remote program know we are ready for a command.
+ stdout.buffer.write(b"rdy")
+
  # Optional: Check available input.
  while not keyboard.poll(0):
  # Optional: Do something here.
@@ -35,6 +38,3 @@
  break
  else:
  motor.stop()
- 
- # Send a response.
- stdout.buffer.write(b"OK")