On Dec 6, 2009, at 5:53 AM, Graham Cox wrote:
>
> On 06/12/2009, at 8:12 PM, Uli Kusterer wrote:
>
>> I thought that was what undo groups were for? Open a group at the start of
>> the drag manually, close it at the end, and everything in between gets
>> lumped into one big action at the end. It may get "replayed" internally, but
>> will all be triggered by one Cmd-Z.
>
>
> Indeed, undo groups are great. Unfortunately, NSUndoManager has a bug where
> if you open a group (on mouse down, say), do nothing (no drag), and close it
> again (on mouse up), an empty Undo task appears in the Undo menu. It's
> harmless, in that it does nothing, but it's also a nuisance, since the user
> doesn't expect this and reports it as a bug with your app. It's working
> around this bug that is horrible and surprisingly complicated (for two
> reasons - one, you can't peek at the top of the undo stack to see what's
> there and two, even if you could there's no way to tell whether the task
> there is empty, because the 'tasks' are all private classes. Therefore you
> have to come up with another way either to detect this case, or to prevent it
> from happening. Either way, it's a complicated and nasty hack). Incidentally
> I have reported this bug but it came back as a dupe. It's been there since I
> started with Cocoa, on 10.2.
I ran into the same problem and my bug was also flagged as a duplicate. Here is
my submission which includes my workaround:
> 14-Aug-2008 09:31 AM Paul Bruneau:
> * SUMMARY
> When trying to utilize undo grouping using what seems to be the most
> straightforward way, the undo group can be "empty" with no actions in it, yet
> the system will still put this empty group into the undo stack.
>
> * STEPS TO REPRODUCE
> In my case, I am dealing with user actions in an NSView subclass. I use
> -mouseDown, -mouseDragged, and -mouseUp.
>
> The way that seemed at first to be best is to initiate an undo group in
> -mouseDown, create undo actions during -mouseDragged, and finally to
> terminate the undo grouping in -mouseUp. The user clicks objects in the view
> and drags them around. This results in many operations over the course of the
> dragging, all of which I want to be in one undo group.
>
> But if I implement my grouping code in mouseDown and mouseUp, I have the
> problem where if the user just clicks on one of my objects without moving it,
> the undo group is "empty", containing no actions.
>
>
> * RESULTS
> The results of this are that the empty undo group is put on the stack, and
> the user can Undo it, but nothing happens (because there was no action to
> undo).
>
> What I would like to see, is if an undo group has no actions in it, it should
> be discarded. Surely this is what Cocoa does when it is doing its normal
> "event loop" based grouping. It doesn't put an empty undo group into the
> stack with every pass of the event loop--why does it put my empty undo group
> into the stack?
>
> Or perhaps consider this a documentation enhancement request. Maybe there is
> an easy solution for me that I don't know about.
>
>
> * NOTES
> In my particular case, I was able to perform a workaround by implementing the
> -beginUndoGrouping message in my -mouseDragged method.
>
> By doing it this way, I am assured that the user is actually dragging around
> one of the objects in my view rather that just clicking on one, or clicking a
> blank area of the view.
>
> But it is kind of kludgy because -mouseDragged is repeatedly called while the
> user is dragging. So I had to test to see if an undo group already exists
> like so:
>
> - (void)mouseDragged:(NSEvent *)event
> {
> if ( dragging == YES )
> {
> if ( [[myUndoManager] groupingLevel] == 0 )
> {
> //start custom undo grouping
> [[myUndoManager] setGroupsByEvent:NO];
> [[myUndoManager] beginUndoGrouping];
>
> }
> ...
>
> But I don't find this optimal. It would get a lot more complex if I did any
> nested groups I think.
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