Objective-C Fundamentals

Building the PocketTasks application
247
variable of the type NSManagedObject and an init method called initWithPerson:
that looks like the following listing.
Listing 12.13
Initializing TasksViewController with a person
(TasksViewController.m)
- (id)initWithPerson:(NSManagedObject *)aPerson {
if ((self = [super initWithStyle:UITableViewStylePlain])) {
NSManagedObjectContext *moc = [aPerson managedObjectContext];
person = [aPerson retain];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:[NSEntityDescription entityForName:@"Task"
inManagedObjectContext:moc]];
[request setSortDescriptors:
[NSArray arrayWithObject:
[NSSortDescriptor sortDescriptorWithKey:@"name"
ascending:YES]]];
[request setPredicate:
[NSPredicate predicateWithFormat:@"person == %@", person]];
resultsController = [[NSFetchedResultsController alloc]
initWithFetchRequest:request
managedObjectContext:moc
sectionNameKeyPath:nil
cacheName:nil];
resultsController.delegate = self;
[request release];
NSError *error = nil;
}
if (![resultsController performFetch:&error]) {
NSLog(@"Error while performing fetch: %@", error);
}
}
return self;
Most of this method should look familiar, but you’ll notice one difference: the use of a
predicate. What does the predicate do? It’s a filter statement, much like the WHERE
clause in a SQL query. In this case you want to fetch only tasks that belong to a given
person, so you create a predicate that uses the person relationship, defined in the
Task entity, as a filter criteria to return only those tasks whose person relationship
matches the specified person.
The following listing shows the rest of the interesting methods.
Listing 12.14
Necessary methods in TasksViewController.m
- (void)viewDidLoad {
[super viewDidLoad];
UIBarButtonItem *addButton =
[[UIBarButtonItem alloc]
Add an Add button to
right navigation bar