My Last Word on NSFetchedResultsController

Somehow, I never posted my final fix for the various NSFetchedResultsController problems. I submitted a bug report to Apple to get the actual row counts out of NSFetchedResultsController including any deferred inserts and deletes, but went ahead with developing a workaround that keeps track of the inserts and deletes locally. I thought I had posted this information, but I guess I never did (Sorry!). I am optimistic that these issues will be fixed in 3.2, but haven't had time to really run the new version of NSFetchedResultsController through its paces, and even if I had, the NDA would keep me from being able to tell you what I had found. Nevertheless, the fact that it has been updated makes me optimistic that this workaround is temporary and that Apple has finally shipped a production-ready version of NSFetchedResultsController. In the meantime…

I'm going to give the changes in context of the Chapter 7 application from More iPhone 3 Development, but these changes are generic and could just be copied to any project using an NSFetchedResultsController and the delegate methods from Chapter 2. I don't usually encourage copy-and-paste coding, but for a temporary workaround it makes sense.

In HeroListViewController.h, we have to add an instance variable to keep track of the deferred inserts and deletes:

#import <UIKit/UIKit.h>

#define kSelectedTabDefaultsKey @"Selected Tab"
enum {
    kByName = 0,
    kBySecretIdentity,
};
@class ManagedObjectEditor;
@interface HeroListViewController : UIViewController  <UITableViewDelegate, UITableViewDataSource, UITabBarDelegate, UIAlertViewDelegate, NSFetchedResultsControllerDelegate>{
    
    UITableView *tableView;
    UITabBar    *tabBar;
    ManagedObjectEditor *detailController;
    
@private
    NSFetchedResultsController *_fetchedResultsController;
    NSUInteger sectionInsertCount;
}
@property (nonatomic, retain) IBOutlet UITableView *tableView;
@property (nonatomic, retain) IBOutlet UITabBar *tabBar;
@property (nonatomic, retain) IBOutlet ManagedObjectEditor *detailController;
@property (nonatomic, readonly) NSFetchedResultsController *fetchedResultsController;
- (void)addHero;
- (IBAction)toggleEdit;
@end

Then, we need to make some changes to two of the NSFetchedResultsController delegate methods. Easiest thing is to probably just replace the existing version with these new versions. The only difference from the previous version is that we keep track of insertions and deletions for each transaction and then use the row count for sections is determined by adding the deferred insertion / deletion count to the number reported by the NSFetchedResultsController:

- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller {
    sectionInsertCount = 0;
    [self.tableView beginUpdates];
}
- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
    [self.tableView endUpdates];
}
- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath {
    switch(type) {
        case NSFetchedResultsChangeInsert:
            [self.tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
        case NSFetchedResultsChangeDelete:
            [self.tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
        case NSFetchedResultsChangeUpdate: {
            NSString *sectionKeyPath = [controller sectionNameKeyPath];
            if (sectionKeyPath == nil)
                break;
            NSManagedObject *changedObject = [controller objectAtIndexPath:indexPath];
            NSArray *keyParts = [sectionKeyPath componentsSeparatedByString:@"."];
            id currentKeyValue = [changedObject valueForKeyPath:sectionKeyPath];
            for (int i = 0; i < [keyParts count] - 1; i++) {
                NSString *onePart = [keyParts objectAtIndex:i];
                changedObject = [changedObject valueForKey:onePart];
            }
            sectionKeyPath = [keyParts lastObject];
            NSDictionary *committedValues = [changedObject committedValuesForKeys:nil];
            
            if ([[committedValues valueForKeyPath:sectionKeyPath] isEqual:currentKeyValue])
                break;
            
            NSUInteger tableSectionCount = [self.tableView numberOfSections];
            NSUInteger frcSectionCount = [[controller sections] count];
            if (tableSectionCount + sectionInsertCount != frcSectionCount) {
                // Need to insert a section
                NSArray *sections = controller.sections;
                NSInteger newSectionLocation = -1;
                for (id oneSection in sections) {
                    NSString *sectionName = [oneSection name];
                    if ([currentKeyValue isEqual:sectionName]) {
                        newSectionLocation = [sections indexOfObject:oneSection];
                        break;
                    }
                }
                if (newSectionLocation == -1)
                    return; // uh oh
                
                if (!((newSectionLocation == 0) && (tableSectionCount == 1) && ([self.tableView numberOfRowsInSection:0] == 0))) {
                    [self.tableView insertSections:[NSIndexSet indexSetWithIndex:newSectionLocation] withRowAnimation:UITableViewRowAnimationFade];
                    sectionInsertCount++;
                }
                
                NSUInteger indices[2] = {newSectionLocation, 0};
                newIndexPath = [[[NSIndexPath alloc] initWithIndexes:indices length:2] autorelease];
            }
        }
        case NSFetchedResultsChangeMove:
            if (newIndexPath != nil) {
                
                NSUInteger tableSectionCount = [self.tableView numberOfSections];
                NSUInteger frcSectionCount = [[controller sections] count];
                if (frcSectionCount != tableSectionCount + sectionInsertCount)  {
                    [self.tableView insertSections:[NSIndexSet indexSetWithIndex:[newIndexPath section]] withRowAnimation:UITableViewRowAnimationNone];
                    sectionInsertCount++;
                }
                
                
                [self.tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
                [self.tableView insertRowsAtIndexPaths: [NSArray arrayWithObject:newIndexPath]
                                      withRowAnimation: UITableViewRowAnimationRight];
                
            }
            else {
                [self.tableView reloadSections:[NSIndexSet indexSetWithIndex:[indexPath section]] withRowAnimation:UITableViewRowAnimationFade];
            }
            break;
        default:
            break;
    }
}
- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type {
    switch(type) {
            
        case NSFetchedResultsChangeInsert:
            if (!((sectionIndex == 0) && ([self.tableView numberOfSections] == 1))) {
                [self.tableView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
                sectionInsertCount++;
            }
            
            break;
        case NSFetchedResultsChangeDelete:
            if (!((sectionIndex == 0) && ([self.tableView numberOfSections] == 1) )) {
                [self.tableView deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
                sectionInsertCount--;
            }
            break;
        case NSFetchedResultsChangeMove:
            break;
        case NSFetchedResultsChangeUpdate: 
            break;
        default:
            break;
    }
}

You can find the fixed version of the Chapter 7 application right here.

Posted in: Book project,NSFetchedResultsController