objective c - Core-Data willSave: method

Question

I have an attribute modificationDate in my Entity A. I want to set its value whenever NSManagedObject is saved. However, if i try to do that in NSManagedObject willSave: method, i get an error:

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Failed to process pending changes before save.  The context is still dirty after 100 attempts.  Typically this recursive dirtying is caused by a bad validation method, -willSave, or notification handler.' ***

So, i'm wondering, what's the best way to set the value of modificationDate?

Answer 1

In fact the apple docs (which are only half read in the accepted answer) don't recommend this method. They explicitly say you should use NSManagedObjectContextWillSaveNotification. An example might be:

@interface TrackedEntity : NSManagedObject
@property (nonatomic, retain) NSDate* lastModified;
@end

@implementation TrackedEntity
@dynamic lastModified;

+ (void) load {
    @autoreleasepool {
       [[NSNotificationCenter defaultCenter] addObserver: (id)[self class]
                                                selector: @selector(objectContextWillSave:)
                                                    name: NSManagedObjectContextWillSaveNotification
                                                  object: nil];
    }
}

+ (void) objectContextWillSave: (NSNotification*) notification {
   NSManagedObjectContext* context = [notification object];
   NSSet* allModified = [context.insertedObjects setByAddingObjectsFromSet: context.updatedObjects];
   NSPredicate* predicate = [NSPredicate predicateWithFormat: @"self isKindOfClass: %@", [self class]];
   NSSet* modifiable = [allModified filteredSetUsingPredicate: predicate];
   [modifiable makeObjectsPerformSelector: @selector(setLastModified:) withObject: [NSDate date]];
}
@end

I use this (with a few other methods: primary key for example) as an abstract base class for most core data projects.