CoreData performFetch in viewDidLoad not working

问题

When my main view controller is loaded and it calls viewDidLoad I am performing a fetch request and retiring an array or my core data objects:

+ (NSArray *)getData {

    // Fetch Data
    NSError *error = nil;
    if (![[[AppDelegate instance] fetchedResultsController] performFetch:&error]) {
        // Update to handle the error appropriately.
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }

    NSArray *items = [[AppDelegate instance].fetchedResultsController fetchedObjects];
    return items;

}//end

For some reason, this returns an empty array when called from viewDidLoad.

If I call the same method from viewDidAppear: it works correctly and returns the NSArray of my CoreData objects.

Is there a reason why this won't work in viewDidLoad?

EDIT:

Fetched Results Controller method:

/**
 * The controller the gets our results from core data
 *
 * @version $Revision: 0.1
 */
- (NSFetchedResultsController *)fetchedResultsController {

    if (fetchedResultsController != nil) {
        return fetchedResultsController;
    }

    // Create and configure a fetch request
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"SiteConfig" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];
    // Create the sort descriptors array
    NSSortDescriptor *sectionTitle = [[NSSortDescriptor alloc] initWithKey:@"createdDate" ascending:NO];
    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sectionTitle, nil];
    [fetchRequest setSortDescriptors:sortDescriptors];

    // Create and initialize the fetch results controller
    NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:@"Root"];
    self.fetchedResultsController = aFetchedResultsController;
    fetchedResultsController.delegate = self;

    // Memory management

    return fetchedResultsController;

}//end

回答1:

My guess is that your view controller is part of your MainWindow.xib, and so its viewDidLoad is being called before your app delegate gets the core data context ready.

You probably want to run this in viewWillAppear anyway, so that you'll get new data if you leave the screen and return. The other option would be to ensure the app delegate responds to fetchedResultsController by getting the core data stack ready if it isn't already.

来源：https://stackoverflow.com/questions/11334753/coredata-performfetch-in-viewdidload-not-working