How do I change the screen so that I don't lose the content of the previous screen. Ex .: On a canvas I have a picture on the canvas and when changing the screen and returning to the canvas the picture that was on it does not disappear. Thank you for your help.
You would need to save everything to a tinydb, then restore it when returning to the screen. This could be complex if you are drawing on the canvas, with the need to record every drawing action so that it is redrawn. Or you could save the canvas as an image and set this to your canvas on return.
The easiest way is to not use other screens, but to use "virtual screens" (vertical or horizontal arrangements where you switch their visibility as you need them), then nothing is lost.
How are you switching screens? If you open a new screen and then close it, the content should remain. If you are always using "open another screen" and never closing screens, then each call always creates a new screen (the others are in memory in the back stack). If you could show how you're switching screens we'd have a better idea of what you could do to address your issue.