Hello
how can I get a message or do anything else when nothing is drawn on a canvas?
I do not find the canvas options the right choice
You could test for any pixels that do not match the background colour of your canvas. You would have to iterate over every pixel to do this, which could take some time for a large canvas.
Alternatively, introduce a button/buttons to commence drawing/complete drawing by the user getting them to confirm that they have drawn something they wish to save etc.