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Implementing UNDO in my application UI.

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So far the need occurred to me only in an application whose only undoable part was the assisted filling of a big table. Not being particularly sophisticated, I created a finite depth queue, which stored the last N states of that table. Each value change of that control - enqueue at opposite end a new state. Each call of the user menu 'Undo' - dequeue one value and Property/write Value it back to the table. The table itself was not so big that keeping a few historical copies of it in memory was tallying. As simple as that, but certainly not too representative of an application with a more complex state to be rolled back.

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I also have not really had to implement undo for a sophisticated application. I have implemented "Restore Ro Default" and limited undo for settings,  configurations and such but have not been called upon for the kinds of undo required for things like drawing. I would imagine the same method could be used though.

I used the SQlite Savepoints for configurations and a default table which could be copied to the main table for "Restore to Default". The latter is an example library in the SQLite Toolkit for LabVIEW. Other similar operations can also be realised like restoring the state of the UI after exiting (form position, controls etc) but that's not really what you are asking here; just an indication of the usefulness of these approaches.


Addendum: There is a description of a method for Undo/Redo using SQLite triggers. Automatic Undo/Redo Using SQLite

They over complicated it by doing it in an "Object Oriented" fashion but I might use something similar for the redo while keeping the savepoint method for undo.

Edited by ShaunR
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I implemented undo several times in my applications, mainly because I usually do editors.

I did it many ways, usually I check if the data/state changes (after the event structure) and push the data on a FIFO. For most of the time, simply putting all raw data was sufficient. Once (in C win32, not in Labview) I had too much data, so I had to use "command pattern", which means only commands with relevant parameters were pushed on the undo stack and the initial raw data was saved with the undo "object".
It had limited depth (depending on available memory), so the initial datas had to be updated if memory would run out. So the oldest command was performed on the initial data.
Of course in some cases the "parameters" of the commands were bigger data (like paste from clipboard), but it worked pretty well. One caveat: it was quite slow to undo, since all the commands in the command stack had to be sequentially applied on the initial data. If all commands are reverseable without computing errors (which pretty much rules out math operations and delete and such), the stack could re reversely traversed so no slowdown.

Back to labview: I can't really exmplain after all these years what's happening, so I'll just post the code of "the most sophisticated" undo I did. It pushes the complete state to the undo stack, because the data was small (sequence editor). The word "manager" in the name is a bad sign, but here it goes.
Note1: it's implememted in a way that selection changes are not added to the stack, only the last one, or I don't really remember. So you can just ignore the "overwrite" case.
Note2: it's an old code from my newbie times. It fails the single responsibility principle badly,  I think both "undo stack" and "stack index" should be action enginified, the outputs besides "output data" is only for optimizing recalculation of data so simply ignore them.

EDIT: I've found a later implementation, so changed the vi. The data change check is outside the vi.




Edited by Lipko
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