Tap or double tap do undo?

Question

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?

Answer 1

The best way to find out if gestures work is to test! Also provide a tutorial and have a help section always available to look them up. Be realistic on how familiar players will be with the game. To give some feedback on your ideas for undo gestures:

You are right that (double) tapping the background is a bit arbitrary.

Tapping a "dead" cell to undo raises a lot of questions: Which one to tap when there are multiple dead cells? Can you also tap dead cells from a few moves earlier? And will that undo the current move only or also that earlier move? It is maybe the better idea to undo but I don't think it is particular clear.

I would add a button for undo. But it concerns me that the design relies so much on icons! What I assume is the reset icon can also be an undo icon. Don't you think players will think that too? (again: Test!)

An idea to overcome the ambiguous icons is to use text instead. And add a help section to lookup how the game works.

Answer 2

There is an existing gesture for undo / redo actions which is the shaking the device, described at apple human interface guidelines.

If the option to shake the device is not a valid alternative, there's the possibility to implement the swipe left (if it has no other action assigned) or the 3-finger swipe as used in Android devices to delete emails from the mailbox.

Three-finger swipe: Initiates undo when swiping left, initiates redo when swiping right.