On 10 Nov 2009, at 22:29, Quincey Morris wrote:
> On Nov 10, 2009, at 14:08, jonat...@mugginsoft.com wrote:
>
>> I accept all of this except that when observing NSArrayController
>> -arrangedObjects it seems that NSKeyValueChangeSetting is all you ever get,
>> regardless of the nature of the changes to the underlying content.
>
> If you're really getting a stream of NSKeyValueChangeSetting notifications
> for the same array property during undo, it would be worth submitting a bug
> report. The behavior indicates that whatever is responsible for KVO
> compliance (which might be Core Data itself, or the NSArrayController) isn't
> being careful about how it's applying the changes. It's still an
> implementation detail, but your expectation of *one* of those non-specific
> notifications (or several of the more fine-grained notifications) seem
> reasonable.
>
>> But from an NSUndoManager perspective it would seem like a rational
>> expectation that is not met in practice.
>
> It's very unlikely that this is the undo manager's fault. It sounds more like
> a slight flaw in Core Data, or an "impedance mismatch" between Core Data and
> NSArrayController.
>
After a lot of experimentation I can define the issue better and a reasonable
solution.
scenario:
1. CoreData NSManagedObject subclass.
2. KVO notifications raised for MO subclass properties.
3. N x MO subclass entries added to NSArrayController with -addObjects:. This
invokes a single change to NSArrayController -arrangedObjects.
4. When undo is invoked NSArrayController -arrangedObjects is modified N times.
For me this N times modification of arrangedObjects was a problem.
The underlying issue is that NSArrayController -rearrangeObjects gets called
multiple times.
The best solution seems to be to subclass NSArrayController and override
-rearrangeObjects as a no-op if we are undoing.
/*
rearrange objects
*/
- (void)rearrangeObjects
{
// rearranging during undo causes performance issues
if (![[[self managedObjectContext] undoManager] isUndoing]) {
[super rearrangeObjects];
}
}
The mixture of bindings, KVO, CoreData and undo is a bit of a lethal brew.
And CoreData undo is definitely no free lunch.
It's easy to throw it all various technologies into the mix but it's another
matter to get them to collaborate effectively.
>
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