Hi Roberto

In your example you don’t have a canvas object that is able to show drawing callbacks.

Please see the example http://doc.robofont.com/api/mojo/mojo-canvas/

The slider just tells the canvas object to redraw itself. While getting a draw callback some drawBot methods are called to draw something into the canvas

hope this helps

good luck!

oh, here is an example, it is required to install the drawBot extension, get it here: https://github.com/typemytype/drawBotRoboFontExtension

import drawBot
from drawBot.ui.drawView import DrawView

from vanilla import *

class Test(object):
    
    def __init__(self):
        
        self.w = Window((400, 400), minSize=(200, 200))
        
        self.w.s = Slider((10, 10, -10, 22), minValue=10, maxValue=100, value=50, callback=self.sliderCallback)
        self.w.c = DrawView((10, 40, -10, -10))
        
        self.w.open()

    def sliderCallback(self, sender):
        value = sender.get()
        # start a new drawing
        drawBot.newDrawing()
        # add a page
        drawBot.newPage(100, 100)
        # draw something
        drawBot.rect(10, 10, value, value)
        # get the pdf data
        pdf = drawBot.pdfImage()
        # set the pdf data in the canvas
        self.w.c.setPDFDocument(pdf)

Test()

good luck!