Users have to make the number on top (39) by using this numbers in the grid. In this case, they would tap 7 and 5 and swipe right to multiply. This converts the 7 in 35 and the 5 in a dead or used cell. Then they select all and swipe up to add them and make 39.
I want users to be able to undo moves, and they have a button alongside other buttons like go to the next or previous level to do so. But this is UX and I want the undo action to be a gesture too and I don't know which one is best: tapping a used cell once to undo or double tapping anywhere in the screen that is not interactive, like the background outside the board or, again, used cells.
- Tapping once in the background is too broad.
- Tapping once in a dead cell requires the user to "find" a dead cell. They are clearly visible but their position changes from level to level.
- Double tapping in the background brings to the users a constant interaction across all levels they can quickly rely on. But double tapping is evil, right?