[Dev] Undo and commit()

[John Anderson]

Alec Flett wrote:

> John Anderson wrote:
>
>> I was hoping, someday, to have the commit point be the unit of undo, 
>> e.g. if you did a delete, you'd commit after the operation. Of course 
>> this requires the ability to roll back changes to previous commit 
>> points, which doesn't yet exist.
>
>
> Personally, I think that trying to tie Undo transactions to repository 
> transactions is going to land us in a heap of trouble. I'm all for 
> thinking of Undo-able actions as a series of repository changes, but 
> there is a disconnect between what the user probably considers 
> "undoable" events and what the repository considers uncommittable.
>
> Here are a few scenarios that I think would cause us trouble if we 
> were to tie undo points directly to calls to commit()/uncommit(). I'm 
> using the term "undoable change" to be a user-comprehensible change, 
> such as deleting an event or changing data in the detail view - one 
> that the user would consider "undoable" by selecting Edit->Undo. I'm 
> using the term "uncommit" to refer to playing back a series of changes 
> in reverse order - this may or may not mean simply rolling back, as 
> you'll see below.
>
> Scenario 1: an undoable change may actually span multiple commits
> A user makes an undoable change that causes commit() to be called 
> three times just because of the codepath it follows happens to have an 
> extra commit() or two.. (maybe it goes through sharing and that causes 
> some repository view manipulation) Now any sort of "Undo" command 
> would have to be run three times to uncommit that single user action

In past systems that used a mechanism like I'm proposing, you can avoid 
this problem quite easily. Organize your code into operations that do 
work, but don't commit. A command calls one or more routines that the 
work then the command does a commit at the end (Actually, to be more 
precise, each time a command was begun it commited the last commands 
changes)

>
> Scenario 2: A user manipulates the UI after making an undoable change
> A user makes an undoable change, and then clicks on another 
> collection. Thanks to CPIA, changes in the UI are also changes in the 
> repository. This means that "Undo" operation might actually just cause 
> the user to switch back to their preview collection. It would take a 
> 2nd Undo operation to actually "undo" the original change.

I'm not sure I understand this, but if you mean that undo moves the user 
interface back to exactly what it was when you did the last commit, yes 
that's the way it works and this is what you expect -- undo should get 
you back to exactly where you were, view included.

>
> Scenario 3: Some changes might occur in different repository views
> A user makes an undoable change, and that change may cause changes in 
> multiple views - i.e. sharing. What exactly do we "undo" when we uncommit?

If I understand this, another variation of it is the following: Suppose 
while you're doing some operation, new mail arrives, when you undo you 
don't want to undo the getting of the new mail. This is a real problem. 
One possible solution is to save information about changes to the 
repository from other threads and when you undo (roll back) replay 
(merge) those changes into the repository.

>
> Personally, I think we need to be very explicit about undoable 
> changes. I think we need to bracket such changes with some sort of 
> begin/end undoable transaction mechanism. We can still exploit all the 
> benefits of the repository though - for instance:

Currently, there is no obvious rule about when to do a commit, so it 
ends up being a little arbitrary. This would eliminate the any confusion 
about when to do commit.

> 1) an undoable transaction might just be a pair of repository version 
> numbers - and we just play back all the changes backwards from the 
> newest version to the oldest. (i..e even if we're at version 103, the 
> previous undoable change might be from revision 100 to 101, so we'd 
> just play the changes backwards between 101 and 100)

Yes, I agree something like this makes sense

> 2) notifications would fire when we play back the changes, so the UI 
> would (hopefully) stay up to date

To keep the UI up to date, all we need to do is unrender the UI, roll 
back the repository, then rerender it (which also calls synchronizeWiget)

> 3) perhaps we could even look ahead at the changes to be played back, 
> to see if an undo operation is even valid. (i.e. if it might cause 
> conflicts or something)
> 4) Redo support would be easy - even multiple redo support like in 
> Word/etc.

>
> An explicit Undo mechanism gives us another advantage: we could assign 
> user-visible names to the undo actions.. and then we could display 
> that user-visible name in the edit menu, so it might say "Undo Cut" 
> instead of just "Undo"

In another system I used based on a begin/end mechanism like commit, 
commit took an argument which was the name of the command, e.g. "Cut", 
which was used to keep the menu up to date.

>
> Here's an example of this system in action:
>
> UndoManager.BeginTransaction(_(u"Cut"))
> self.CutSomething()
> UndoManger.EndTransaction()

If your BeginTransaction happened to call commit, I think we could 
eliminate explicit commits and our proposals would be identical.

>
> Maybe there's even a more pythonic way to handle this:
>
> UndoManager.DoTransaction(_(u"Cut"), self.CutSomething)
>
> Alec
>
>>>
>>>
>>> Alec
>>>
>>>>Sincerely,
>>>>Jeffrey
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>>>>
>>>
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>
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