How do i undo an action?
Example flagging something or unflagging something
For me it works with a left swipe on the item, for flagging. If the item is not flagged I get the option to flag it. If it is flagged already, I can unflag it.
Further stuff (read/unread, marked etc.) is possible to do and undo in the info panel of the specific item.
Well, sure. But once you unflag it, its gone from the smart group. Finding it is really really painful.
I am used to having an “Undo” button somewhere normally
If you Undo a flag setting, wouldn’t it become unflagging and thus be removed from the smart group?
Exactly. So flagging it again isn’t easy, hence I am surprised to not find a way to “Undo” it quickly.
You are speaking about when you have made a mistake in unflagging, (and by extension any other similar mistake). It seems to me that users can not really expect a developer to anticipate every possible error that a user could make and cater for them, the App would soon become unmanageable and unusable! This, surely, does not happen that often, if it does then I would hazard a guess that there is something wrong with your workflow, but presumably when you have made such an error you realise quickly that you have done so and also remember what the subject of the unflagged item is; a quick search for that item will soon recover it and you can then flag it again.
Hope this helps.
It is not about errors or mistakes. Just have a button that undos the last action. This has been a staple in UI since… forever.
all the time actually. cmd+z is a really useful shortcut 
that is how i actually found out about it. i couldn’t find it again because i remembered maybe 4 words from the title and it wouldn’t show up in the top 20 in the search results. was a real pain to flag it again.
If it is not “about errors…” then I can not think of why you would unflag an item without wanting to in the first place, that is why I say that perhaps there is something wrong with your workflow if this is happening “all the time”. I do not use flags much and certainly not for smart groups, I much prefer setting up a smart group by keyword which is much harder to delete than a flag.
Sorry, I was talking in general: let me undo my last action; no matter the action.
In terms of unflagging/flagging, yes, that would mostly be an error.
To be honest, there are those times that I’m quickly dealing with a number of actions. Then I fall into a hole. Undo is really is very handy in those cases. I can’t tell you the number of times I messed things up due to a series of quick and mindless operations that resulted in an error; I’d hold my breath and cmd-z. When that command works, I feel relieved and continue on my way.
IIRC, the Apple library includes the framework for undo which can be added to an app. Or am I thinking of Swing in Java?