Many things will be impossible unless you delay the initial action. I mean, how do you really undo sending an email? You can't. What you do instead, and I'm guessing this is what gmail does, is to delay the sending for 15 seconds to give the user time to regret.
Seems like delaying the action will almost always be possible. Guess i could give the users the misconception that they can undo whenever they want, which is just..
Sometimes the actual deletion must occur before a mistake is realized. I have often seen undo implemented as a 'soft' delete which means the read-side needs to be 'undo aware'. Sometimes undo is implemented by backing up all related data to a separate location which is also non-trivial. Managing the 'restore' half needs logic to account for conflicts and changed state.
It doesn't change the fact that Undo is an amazingly useful feature but its not always as simple as 'delay the action'.
Seems like delaying the action will almost always be possible. Guess i could give the users the misconception that they can undo whenever they want, which is just..