quellish

- (void) fetchThingsWithCompletion:(void (^)(NSArray *, NSError *))completion{
    [context performBlock:^{
         result = [context executeFetch:...
          if (completion != nil){
              completion(result, error);
          }
    }];
}

quellish

group = dispatch_group_create();
dispatch_group_enter(group);
[context performBlock:^{
    result = [context executeFetch:...
    dispatch_group_leave(group);
}];
groupResult = dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

quellish

context = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
[context setParentContext:someOtherContext];
result = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:context sectionNameKeyPath:nil cacheName:nil];

quellish

[[[self fetchedResultsController] managedObjectContext] performBlock:^{
    NSError *fetchError = nil;
    
    if (![self fetchedResultsController] performFetch:&fetchError]){
        /// handle the error. Don't just log it.
    } else {
        // Update the view from the main queue.
        [[NSOperationQueue mainQueue] addOperationWithBlock:^{
            [tableView reloadData];
        }];

quellish

- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
    [[NSOperationQueue mainQueue] addOperationWithBlock:^{
        [[self tableView] endUpdates];
    }];
}

quellish

NSManagedObject *someObject = [[self fetchedResultsController] objectAtIndexPath:indexPath];
value = [someObject lastName];

quellish

NSManagedObject *someObject = [[self fetchedResultsController] objectAtIndexPath:indexPath];
[[someObject managedObjectContext] performBlock:^{
    value = [someObject lastName];
}];

quellish

- (void) tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath {
    NSManagedObject	*row	= [[self fetchedResultsController] objectAtIndexPath:indexPath];

    [[row managedObjectContext] performBlock:^{
        NSString	*text	= nil;

        // Fire a fault.
        text = [row lastName];
        // Update the user interface with the value.
        [[NSOperationQueue mainQueue] addOperationWithBlock:^{

quellish

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    NSManagedObjectContext  *rootContext                = nil;

    // Override point for customization after application launch.

    // 1. Create an NSManagedObjectContext with private queue concurrency
    rootContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
    // 2. Instantiate a concrete NSPersistentStore by adding it to the coordinator asynchronously.
    [rootContext performBlock:^{

quellish

/**
Informal protocol that traverses the responder chain until a concrete class provides
an instance of NSManagedObjectContext. If no responder provides on, UIApplication is extended to forward
the request to it's delegate. We extend the UIApplicationDelegate to add this new behavior.

Concrete responders can provide optional implementations of managedObjectContext. For example, a responder such as
a view may call [self managedObjectContext], which would start walking up the responder chain. When it gets to the view controller that owns the view, the view controller may have a concrete implementation of managedObjectContext. This can be a property, or a method that creates a new context, etc. The instance vended by the view controller method is what the view that called [self managedObjectContext] gets as a response from that method.

Both the responder chain and informal protocols are incredible powerful concepts that are often under utilized by developers outside of Apple.