Hi!
I came across a strange problem today:
I have a simple Core Data Application whose managed object model has
one entity with one attribute. I set up an AppController class that
inserts one instance of that entity's backing class (NSManagedObject)
into the MOC:
- (void)awakeFromNib {
myObject = [NSEntityDescription
insertNewObjectForEntityForName:@"MyEntity" inManagedObjectContext:
[[NSApp delegate] managedObjectContext]];
}
Then, there is an IBAction that is called every time I press a button
in the GUI:
- (IBAction)doIt:(id)sender {
NSLog(@"before: object's retain count: %i", [myObject retainCount]);
// Call the setter of myBoolAttribute. This will change object's
retain count
[myObject setValue:[NSNumber numberWithBool:YES]
forKey:@"myBoolAttribute"];
NSLog(@"after : objects's retain count: %i\n\n", [myObject
retainCount]);
}
The strange thing is that setting the myBoolAttribute increases
object's retain count every time. I tried doing a
[managedObjectContext setRetainsRegisteredObjects:NO]; in the Xcode
template's AppDelegate class, right after the managed object context
is created (line 93, the line right after alloc+init), but it just
makes the first value printed by the above method start lower by one
(as expected).
I did nothing more than the above. No special settings for the entity
or the class, nothing. Then, I created an NSManagedObject subclass for
testing different, self-implemented setters for myAttribute:
@interface MyEntityClass : NSManagedObject { }
@property(retain, nonatomic) NSNumber *myBoolAttribute;
@end
@implementation MyEntityClass
@dynamic myBoolAttribute;
// 1. Using Core Data's generated setter causes self's retain count to
be changed
// 2. Using this setter causes self's retain count to be changed
- (void)setMyBoolAttribute:(NSNumber *)value {
[self willChangeValueForKey:@"myBoolAttribute"];
[self setPrimitiveValue:value forKey:@"myBoolAttribute"];
[self didChangeValueForKey:@"myBoolAttribute"];
}
// 3. Using this setter does not cause self's retain count to be
changed - but it's wrong
- (void)setMyBoolAttribute:(NSNumber *)value {
[self setPrimitiveValue:value forKey:@"myBoolAttribute"];
}
// 4. This setter is stupid and should cause a crash later on - but it
doesn't
- (void)setMyBoolAttribute:(NSNumber *)value {
[self willChangeValueForKey:@"myBoolAttribute"];
[self setPrimitiveValue:value forKey:@"myBoolAttribute"];
[self didChangeValueForKey:@"myBoolAttribute"];
[self release];
}
@end
After a few NSLog()'s in setter #3 it is clear, that -
willChangeValueForKey: increases the retain count. Why does it do that?
After that, I tried adding additional attributes to my entity and to
make everything even worse, *all* attributes' retain counts are
increased by -willChangeValueForKey:
This is not the first time I'm writing a Core Data application and I
just can't see where the problem lies. I was really thinking long and
hard about this before I wrote this mail, but it just doesn't make
sense to me...
Any comments are greatly appreciated!
Marco
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