General-purpose implementation of NSFetchedResultsControllerDelegate

KDJFetchedResultsControllerContentUpdater.h

#import <UIKit/UIKit.h>
#import <CoreData/CoreData.h>

// Protocol adopted by an object that can configure a table view cell with data from an object
@protocol KDJTableViewCellConfigurationDelegate <NSObject>
- (void)configureCell:(UITableViewCell *)cell withObject:(NSObject *)object;
@end


// General-purpose implementation of NSFetchedResultsControllerDelegate
@interface KDJFetchedResultsControllerContentUpdater : NSObject <NSFetchedResultsControllerDelegate>

// Table view to be updated
@property (nonatomic, weak) UITableView *tableView;

// Delegate that will configure a cell with updated data when a managed object changes
@property (nonatomic, weak) id<KDJTableViewCellConfigurationDelegate> cellConfigurationDelegate;

@end

KDJFetchedResultsControllerContentUpdater.m

#import "KDJFetchedResultsControllerContentUpdater.h"

@implementation KDJFetchedResultsControllerContentUpdater

- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller {
    [self.tableView beginUpdates];
}

- (void)controller:(NSFetchedResultsController *)controller
  didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo
           atIndex:(NSUInteger)sectionIndex
     forChangeType:(NSFetchedResultsChangeType)type {

    switch(type) {
        case NSFetchedResultsChangeInsert:
            [self.tableView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex]
                          withRowAnimation:UITableViewRowAnimationFade];
            break;
            
        case NSFetchedResultsChangeDelete:
            [self.tableView deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex]
                          withRowAnimation:UITableViewRowAnimationFade];
            break;
    }
}

- (void)controller:(NSFetchedResultsController *)controller
   didChangeObject:(id)anObject
       atIndexPath:(NSIndexPath *)indexPath
     forChangeType:(NSFetchedResultsChangeType)type
      newIndexPath:(NSIndexPath *)newIndexPath {

    UITableView *tableView = self.tableView;
    
    switch(type) {
        case NSFetchedResultsChangeInsert:
            [tableView insertRowsAtIndexPaths:@[newIndexPath]
                             withRowAnimation:UITableViewRowAnimationFade];
            break;
            
        case NSFetchedResultsChangeDelete:
            [tableView deleteRowsAtIndexPaths:@[indexPath]
                             withRowAnimation:UITableViewRowAnimationFade];
            break;
            
        case NSFetchedResultsChangeUpdate: {
            UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath];
            if (cell != nil) {
                [self.cellConfigurationDelegate configureCell:cell
                                                   withObject:anObject];
            }}
            break;

        case NSFetchedResultsChangeMove:
            [tableView deleteRowsAtIndexPaths:@[indexPath]
                             withRowAnimation:UITableViewRowAnimationFade];
            [tableView insertRowsAtIndexPaths:@[newIndexPath]
                             withRowAnimation:UITableViewRowAnimationFade];
            break;
    }
}

- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
    [self.tableView endUpdates];
}

@end

It was annoying to copy the NSFetchedResultsControllerDelegate boilerplate into every table view controller with a fetched results controller, so I created this little reusable class.

To use it, declare your table view to implement the KDJTableViewCellConfigurationDelegate protocol and implement a method like this:

- (void)configureCell:(UITableViewCell *)cell withObject:(NSManagedObject *)managedObject {
    // TODO: configure the cell with data from the managed object
}

(You should probably call this from your -tableView:cellForRowAtIndexPath: method to avoid duplication of cell-configuration code.)

Then, initialize the fetched results controller's delegate like this:

NSFetchedResultsController *frc = [[NSFetchedResultsController alloc] initWithFetchRequest: /* ... */];
self.fetchedResultsController = frc;

KDJFetchedResultsControllerContentUpdater *contentUpdater = [[KDJFetchedResultsControllerContentUpdater alloc] init];
contentUpdater.tableView = self.tableView;
contentUpdater.cellConfigurationDelegate = self;
self.contentUpdater = contentUpdater;
frc.delegate = contentUpdater;

Make sure you retain the content updater yourself, as the fetched results controller won't retain its delegate.

BTW, this snippet of boilerplate from the NSFetchedResultsControllerDelegate documentation is buggy:

case NSFetchedResultsChangeUpdate:
    [self configureCell:[tableView cellForRowAtIndexPath:indexPath] atIndexPath:indexPath];
    break;

See these links for explanation and fixes:

• http://oleb.net/blog/2013/02/nsfetchedresultscontroller-documentation-bug/
• http://stackoverflow.com/questions/11432556/nsrangeexception-exception-in-nsfetchedresultschangeupdate-event-of-nsfetchedres
• http://wangling.me/2011/09/bugs-of-nsfetchedresultscontrollerdelegate-template-code.html