documentation for creating observable streams

[graingert]

I've got a functions:

def get_producer():
    tool = Tool()

    async def start(listener):
        def on_event(event):
            listener(event)
        tool.on_scan_result = on_event
        tool.start_scan()
        await tool.transport.drain()

    async def stop():
        tool.stop_scanning()
        await tool.transport.drain()

    return { "start": start, "stop": stop } # my old api.
    return AsyncObservable( # what goes here? )

And I'd like to be able to turn it into an observable stream, that automatically starts/stops scanning when the obserables are added/removed but I'm not sure how.

[graingert]

/cc @dbrattli