Canvas not scaled when the notebook is rendered with voila about ipycanvas HOT 10 CLOSED

To reproduce the issue: https://github.com/fastscape-lem/gilbert-board

from ipycanvas.

Thanks for filling this issue! I'll give it a try. Are you running voila with --enable_nbextensions=True?

from ipycanvas.

Are you running voila with --enable_nbextensions=True

Just tried it but no apparent effect.

from ipycanvas.

It seems like the first commands are lost in the void... Never received by the Canvas. I will investigate a bit.

from ipycanvas.

I think I've got a workaround for your scaled image. Every time you redraw the image, you should do something like:

canvas.save()
canvas.scale(scale)
canvas.put_image_data(data, 0, 0)
canvas.restore()

EDIT: You would also need to remove the scale call in the board constructor

And it should work.

But it does not fix the first render... I've got the feeling it can be a bug in the Voila widget manager, some comm messages that are not handled correctly when the widget is displayed.

from ipycanvas.

The problem is actually that ipycanvas is almost stateless. And between the execution of the Notebook and the front-end initialization, some messages are sent and there is no Canvas view for receiving those messages...

I cannot think of a good way to tackle this issue right now. I can only think of ugly workarounds...

from ipycanvas.

I cannot think of a good way to tackle this issue right now. I can only think of ugly workarounds...

Thanks for investigating anyway!

Regarding the 1st render, I guess I could add a code cell in the notebook (below the one that outputs the canvas) that triggers redrawing the canvas (that will be executed but hidden in the voila rendered dashboard).

from ipycanvas.

Regarding the 1st render, I guess I could add a code cell in the notebook (below the one that outputs the canvas) that triggers redrawing the canvas (that will be executed but hidden in the voila rendered dashboard).

Mmm it doesn't work. I guess a message has to come from the front-end then?!

from ipycanvas.

No, it cannot work... Voila waits for the entire Notebook to be executed before creating any widget view. It will eventually be fixed when Voila will support progressive widget rendering (It's on the roadmap).
What you could eventually do is execute this redraw in a separate thread in the last cell.

Something like: (disclaimer: this is not the final solution, this is very ugly)

from time import sleep
from threading import Thread

class Redraw(Thread):
    def __init__(self, canvas):
        self.canvas = canvas
        super(Redraw, self).__init__()

    def run(self):
        sleep(1) # wait one or two seconds and hope that the frontend has time to initialize
        # redraw here
        # ...

Redraw(canvas).start()

from ipycanvas.

I'm thinking of adding an event on the Canvas that will be triggered when the front-end is ready, so you know when you need to redraw the Canvas. It would allow you to do that more properly.

from ipycanvas.