This is a proof of concept for an idea to allow users to handle events using await. An event handling workflow can then be written as a single async function, which can be much easier than listening to multiple events and maintaining state across the handlers.

Example implementing drag:

@canvas.add_event_task
async def foo(for_event):
    while True:

        # Wait for pointer down
        event = await for_event("pointer_down")

        # Does this select the current position of the active block?
        width, height = canvas.get_logical_size()
        x = int(w * event["x"] / width)
        y = int(h * event["y"] / height)
        if [x, y] != currentpos:
            print("nope", x, y)
            continue

        # Move until pointer up
        while True:
            event = await for_event("pointer_move", "pointer_up")
            if event["event_type"] == "pointer_up":
                break

            width, height = canvas.get_logical_size()
            x = int(w * event["x"] / width)
            y = int(h * event["y"] / height)
            print(x, y)
            currentpos[:] = x, y

The use-case that triggered me to think about this more is the lasso tool in FPL: fastplotlib/fastplotlib#837

I'm curious to what @Korijn thinks of this idea from a design/API perspective, and what users like @kushalkolar, @hmaarrfk, @claydugo, @kingbedjed think of writing handlers this way. I think it should be an alternative to add_event_handler, not replace it.

Some notes regarding async:

• Yes, this works with any backend.
• Yes, you can also await sleep(), but you should from rendercanvas.asyncs import sleep to make it portable.
• Although, if you know the underlying loop is asyncio, you can do any asyncio stuff :)

Some caveats:

• If an exception occurs in the task, it stops the task completely, unless you add a try except.

Some ideas:

• The argument into the function can be the for_event() instead of the emitted object (see example above).
• Use canvas.add_event_task("pointer_down", func) so the task is initiated for every mouse down, avoiding one while loop.
• Maybe could write this as a generator instead of a an async function to make it feel more 'confined' to events.