I mean how to get the blank screen, when switch to a new menu, I need to clean the current screen.
What do you mean by switch to a new menu? Please explain in details …
If you want to clear the whole screen maybe this is the one you want:
but as @avinash said, explain in details.
If my new menu you mean by a sub menu that pops when an item from first menu is selected then you need to have a different screen for the new menu. You have to switch the screen when first menu item is selected.
when using lv_obj_clean(lv_scr_act()) to clean the whole screen, the littlevgl inserts a blank screen that causing a flash. How to solve it?
How do you mean “causing a flash”? Can send a video and a super simple code snippet to reproduce?