Re: Synchronising across multiple NSUndoManager stacks

FROM : Graham Cox
DATE : Mon Mar 24 04:52:04 2008

Thanks for the considered reply, Jerry - much appreciated.

I hadn't thought of just using a global undo stack, and having tried 
it, it's actually a pretty good solution. Obviously there are times 
that the result of the undo targets a document other than the current 
one and that can seem a bit confusing, but apart from that there are 
no other obvious downsides. I might modify the menu strings to 
indicate the target if it's not the current doc to help clarify it, 
but that's a UI issue that can wait.

thanks!


------
S.O.S.



On 22 Mar 2008, at 4:15 pm, Jerry Krinock wrote:
>
> On 2008 Mar, 21, at 17:50, Graham Cox wrote:
>

>> So the question is, is this a requirement that others have come 
>> across and if so, how did you solve it?

>
> Not exactly that requirement, but something like it: I've had to 
> deal with objects that can be moved among open documents.  My 
> solution was to use one global undo stack, and also to clear the 
> stack whenever any document was closed.
>
> It looks like all of your documents and objects live in the same 
> "globe".  Therefore the global undo stack is the most natural 
> solution.  With the Cocoa default scheme, each document having its 
> own undo stack, I even found it difficult to troubleshoot my undo 
> stacks because, as I user, when moving between documents, it was 
> hard for me to say what was ^expected^ behavior when I clicked 
> 'Undo' while this document was active or 'Redo' while that document 
> was active.  Whoa!  But once I put everything into one undo stack, 
> even I could understand it.
>
> Consider how often that Undo gets used.  Then consider how much of 
> your time it is worth to handle all the pathological cases without 
> introducing bugs, and how many other spec requirements for really 
> useful features you need to meet before your deadline.  If Undo/Redo 
> works as expected 90% of the time that the user wants it and is 
> grayed out for the other 10%, that may be good enough.
>
> Probably a computer science guru would know for sure, but since none 
> of them have replied for over 4 hours I'll give you my opinion based 
> on engineering common-sense:  Trying to solve the general 'Undo' 
> problem is difficult in theory and intractable in practice.  Imagine 
> a multi-player networked computer game where, against the advice of 
> intellectuals, you directed your armies to invade another nation 
> five years ago, but now realize that the intellectuals were 
> correct.  You see why games don't have 'Undo'.
>
> Now, I see that you've considered a global Undo stack as your second 
> alternative, but I think you could simplify it even further, and 
> move on.
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