Question

The documentation is pretty confusing on this one:

The implementation of accessor methods
you write for subclasses of
NSManagedObject is typically different
from those you write for other
classes.

If you do not provide custom instance
variables, you retrieve property
values from and save values into the
internal store using primitive
accessor methods. You must ensure that
you invoke the relevant access and
change notification methods
(willAccessValueForKey:,
didAccessValueForKey:,
willChangeValueForKey:,
didChangeValueForKey:,
willChangeValueForKey:withSetMutation:usingObjects:,
and
didChangeValueForKey:withSetMutation:usingObjects:).
NSManagedObject disables automatic
key-value observing (KVO, see
Key-Value Observing Programming Guide)
change notifications, and the
primitive accessor methods do not
invoke the access and change
notification methods.

In accessor methods for properties
that are not defined in the entity
model, you can either enable automatic
change notifications or invoke the
appropriate change notification
methods.

Are there any examples that show how these look like?

Answer 1

If you go into the Data Model editor, select and entity attribute and then choose “Copy Method Implementation to the Clipboard”. It will generate accessors for you. Here is the accessors for the default “timeStamp” property in a Core Data template project:

- (NSDate *)timeStamp 
{
    NSDate * tmpValue;

    [self willAccessValueForKey:@"timeStamp"];
    tmpValue = [self primitiveValueForKey:@"timeStamp"];
    [self didAccessValueForKey:@"timeStamp"];

    return tmpValue;
}

- (void)setTimeStamp:(NSDate *)value 
{
    [self willChangeValueForKey:@"timeStamp"];
    [self setPrimitiveValue:value forKey:@"timeStamp"];
    [self didChangeValueForKey:@"timeStamp"];
}

The basic idea here is that you have to bracket any primitive value changes with willChange... and didChange... calls so that the context knows something is in the process of being changed.

You usually don’t have to fiddle with your own accessors unless you have side effects for setting the attribute.