Dynamic Parameters - Core Data - page 178

In this method, we receive the name of the value that the caller is attempting
to retrieve. We use this name to retrieve the NSManagedObject via the -findParameter:
method and return the NSManagedObject object's value property. If there is no
parameter with the passed-in name, we check the defaults NSDictionary to see
whether there is a default for it. If there is no default set, we let the -valueForKey:
method return nil to the caller.
-findParameter:
CDPreferences/DocumentPreferences.m
- (NSManagedObject*)findParameter:(NSString*)name;
{
NSManagedObjectContext *moc;
NSManagedObject *param;
NSError *error = nil;
moc = [[self associatedDocument] managedObjectContext];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:[NSEntityDescription entityForName:@ "Parameter"
inManagedObjectContext:moc]];
[request setPredicate:[NSPredicate predicateWithFormat:@ "name == %@" , name]];
param = [[moc executeFetchRequest:request error:&error] lastObject];
if (error) {
DLog(@ "Error fetching parameter: %@\n%@" , [error localizedDescription],
[error userInfo]);
return nil;
}
[request release], request = nil;
return param;
}
In the -findParameter: method, we construct an NSFetchRequest against the
parameters table using a compare on the name property to filter it down to a
single result. Assuming there is no error on the fetch, we return the NSManage-
dObject . In this method, we are using the -lastObject method on the resulting
array as a convenience. -lastObject automatically checks for an empty array
and returns nil if the array is empty. This reduces the code complexity and
gives us the result we want in a single call. If there is an error accessing the
Core Data stack, we report the error and return nil . Note that we do not create
a parameter if there is not one in this method. We intentionally separate this
out so that we are not creating potentially empty parameters. This allows us
to request a parameter and check whether it is nil without the concern of
parameters being generated unnecessarily.