核心数据：表格视图获取对象的FRC数量为零

Question

I am implementing core data in two table view controllers. The first display bank name, city and state, the second displays bank's detail info like zip, closing date etc. Accordingly I have two entities in my model. First is called 'BankInfo', second one is called 'BankDetails'. The two entities are in a one-to-one relationship.

Problem arises when I segue from the first controller to the second. It appears that number of fetched objects in the second table view controller is always zero. It means FRC fetches nothing. What's going wrong?

The first table is working fine. And I am sure the second entity is filled with data properly. And I tried removing the predicate, in which case FRC should return all objects in entity. But it still returns nothing..

#import "SecondViewController.h"
#import "AppDelegate.h"

@interface SecondViewController () <NSFetchedResultsControllerDelegate>
@property (nonatomic,strong) NSFetchedResultsController *fetchedResultsController;
@end

@implementation SecondViewController
@synthesize bankInfo = _bankInfo;
@synthesize fetchedResultsController = _fetchedResultsController;

- (void)viewDidLoad
{
    [super viewDidLoad];

    self.fetchedResultsController.delegate = self;

    NSError *error;

    if (![self.fetchedResultsController performFetch:&error]) {
        // Update to handle the error appropriately.
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        exit(-1);  // Fail
    }

    [self.tableView reloadData];

    NSLog(@"Frist fetched object is %@",[[self.fetchedResultsController fetchedObjects] objectAtIndex:0]);
}


- (void)setBankInfo:(BankInfo *)bankInfo
{
    _bankInfo = bankInfo;
}

- (NSFetchedResultsController *)fetchedResultsController
{
    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"BankDetails"];

    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"info.state = %@",self.bankInfo.state];
    [request setPredicate:predicate];

    [request setSortDescriptors: [NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:@"zip"
                                                                                    ascending:YES
                                                                                     ]]];
    AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
    NSManagedObjectContext *context = appDelegate.managedObjectContext;

    _fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request managedObjectContext:context sectionNameKeyPath:nil cacheName:nil];

    return _fetchedResultsController;
}
// FRC delegate methods...
@end

Answer 1

Your getter function fetchedResultsController creates a new FRC each time it is called.

So when you call

[[self.fetchedResultsController fetchedObjects] objectAtIndex:0]

then fetchedResultsController is called, creates a new FRC, stores it in _fetchedResultsController , and returns the new FRC. Since performFetch has never been called on this new FRC, fetchedObjects is empty. Also the delegate is not set for the new FRC.

The same thing happens probably in the table view delegate methods.

You can verify easily if this is the problem by setting a breakpoint in fetchedResultsController or adding some NSLog() to that method.

What you perhaps wanted to do is the lazy getter function . It also makes sense to set the delegate and perform the fetch in that method.

- (NSFetchedResultsController *)fetchedResultsController
{
    if (_fetchedResultsController == nil) {
        /* ... create request, predicate, sort descriptor ... */
        _fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request managedObjectContext:context sectionNameKeyPath:nil cacheName:nil];
        _fetchedResultsController.delegate = self;
        [_fetchedResultsController performFetch:&error]; // error handling omitted
    }
    return _fetchedResultsController;
}

If the search criteria change, you just set _fetchedResultsController = nil to force the creation of a new FRC.