Allow callbacks for connection/disconnection

[beachcitiessoftware]

Describe the solution you'd like
It would be great if the BlessServer could expose some callbacks, for when a connection / disconnection occurs.

[thodpap]

I found out that there are some functions that you can set for just callbacks. Suppose you have

def handle_start_notifying(data):
 print(data)
def handle_stop_notifying(data):
 print(data)
def message_handler(message):
 print(data) # message format 

server = BlessServer(...)
...
await server.start()

server.app.StartNotify = handle_start_notifying # It seems that the input is None every time
server.app.StopNotify = handle_stop_notifying # It seems that the input is None every time

server.app.bus.add_message_handler(message_handler)

Check this for the message_handler object

[kevincar]

Changes have been implemented and merged into the latest on the repo.

These changes more specifically implement what @thodpap pointed out, using the subscription callbacks. Though, the example above is Linux-specific, it is now implemented across all backends using the on_subscribe and on_unsubscribe callbacks.

Outside of Linux, the APIs provided by Windows and MacOS provide access to the GATT layer of ble rather than the GAP or LINK layers, which limits are ability to determine when/whether a central device is connecting. Implementing this would require us to develop non-supported hacky lower-level functions to listen into the OS's HCI commands and data to determine the true link-layer connection, which the OS's could change how this is implemented across system times and OS updates.

So for now, the most appropriate method to handle this is to use the provided callbacks, which indicate that a central device is requesting use of data on the peripheral.