On 14 Nov 2009, at 02:11, David Catmull wrote:
> In my Core Data app, I have objects with a date property, and I want to
> maintain a list of all years that I have objects in. I'm having trouble
> settling on an approach.
>
> Currently, I'm listening for
> NSManagedObjectContextObjectsDidChangeNotification. This is working fine for
> additions and deletions, but it's harder for changes, mainly because I don't
> know the old value. If I did, I could check if the old year still has any
> other objects in it. As it is I have to refresh the whole list. I also get
> more notifications than I need, since I only care about date changes on this
> one entity.
You could cache the old year in a transient property and detect the change that
way.
>
> Another option is to use KVO so I can be notified only of date changes. I'm
> assuming that registering as an observer for potentially thousands of Core
> Data objects doesn't carry too much overhead. The down side is that if lots
> of objects change at once, I get lots of individual callbacks, and I worry
> that that would be a bottleneck.
>
I find it hard to get a combination of KVO, bindings, CoreData and undo that
works cleanly and efficiently.
I faced a perhaps similar situation and implemented a system of coalescing
notifications.
My NSManagedObject subclass raises manual KVO notifications for use in bindings
put each property modified also calls subclass method
-postPropertyChangeNotification: (see below).
This sends a single notification for each modified keypath at the end of the
runloop.
This means you get one notification regardless of how many changes there are.
So regardless of how many objects get their Year property modified you get one
notification and can form your year list.
Another subclass method +keyPathsThatDoNotEnqueueCoalescingNotifications
provides a way of overriding the coalescing for individual keypaths.
One point would be to consider if you want to support undo.
CoreData will undo your changes to your data model and post KVO notes but it
won't access your model accessor's during undo.
Its possible to build a series of observations and notifications that works
when doing, but fails when undoing.
So if you want undo support I would try to implement it now.
To get undo support to function so that the GUI was always in sync with the
model i found it necessary to listen for
NSManagedObjectContextObjectsDidChangeNotification as well.
/*
post property change notification
provides an alternative to observing all model properties
*/
- (void)postPropertyChangeNotification:(NSString *)keyPath
{
// build our notification
NSDictionary *userInfo = [NSDictionary
dictionaryWithObjectsAndKeys:keyPath, @"keyPath", nil];
NSNotification *notification = [NSNotification
notificationWithName:@"MGSUnitEntryUpdated" object:self userInfo:userInfo];
// the default is that we shall coalesce these notifications
NSUInteger coalesceMask = NSNotificationCoalescingOnName;
// determine any keyPaths that require distinct notifications
// eg: date as the receiver of the notification treats it differently
to all other keypaths
NSSet *nonCoalescingKeypaths = [[self class]
keyPathsThatDoNotEnqueueCoalescingNotifications];
if ([nonCoalescingKeypaths containsObject:keyPath]) {
coalesceMask = NSNotificationNoCoalescing;
}
// enqueue the notification
[[NSNotificationQueue defaultQueue] enqueueNotification:notification
postingStyle:NSPostASAP // run at end of runloop
coalesceMask:coalesceMask
forModes:nil];
}
/*
keypaths that do not enqueue coalescing notifications
*/
+ (NSSet *)keyPathsThatDoNotEnqueueCoalescingNotifications
{
NSSet *keyPaths = [super
keyPathsThatDoNotEnqueueCoalescingNotifications];
// we require that separate uncoalesced notifications are sent for
these property changes
keyPaths = [keyPaths setByAddingObjectsFromSet: [NSSet setWithObjects:
@"date", nil]];
return keyPaths;
}
> Recommendations? Other options?
>
> Thanks,
>
> --
> David Catmull
> uncom...@uncommonplace.com
> http://www.uncommonplace.com/
>
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