NSFetchedResultsController multiple entities for U

问题:

I have two entities one called Post and one called User. Post<<---->User is the relationship in core data. I am using a NSFetchedResultsController to fetch all Post records in my core data stack and then displaying them in a UITableView. Each cell has an image and that image corresponds to a User.profilePicture.

Upon initializing I do not download the profile picture from the server, I only download when it scrolls past that cell (lazy load). Once I download it I save the downloaded image to the corresponding User.profilePicture in the core data stack.

Is there a way for controllerDidChangeContent to be called when I update the User entity?? My current understanding is that my NSFetchedResultsController can only follow the Post entity since that is what I initially set it to do and cannot traverse and monitor updates across a relationship, is that true?

回答1:

Sadly I know only of an UGLY solution for this issue.

In your User .m file implements the setProfilePicture: like this:

//NOT TESTED IN A MULTITHREADED ENV
- (void) setProfilePicture:(NSData *)data
{
    [self willChangeValueForKey:@"profilePicture"];
    [self setPrimitiveValue:data forKey:@"profilePicture"];
    [self.posts enumerateObjectsUsingBlock:^(Post* p, BOOL *stop) {
        [p willChangeValueForKey:@"user"];
        [p didChangeValueForKey:@"user"];
    }];
    [self didChangeValueForKey:@"profilePicture"];
}

This will notify the FRC that the Post element has changes.

You might find additional information here

Edit:

To fetch the data on access you can add this to your User.m:

//UNTESTED
+ (void) mergeToMain:(NSNotification*)notification
{
    AppDelegate* appDel = (AppDelegate*)[[UIApplication sharedApplication] delegate];
    [appDel.managedObjectContext performSelectorOnMainThread:@selector(mergeChangesFromContextDidSaveNotification:) 
                                                  withObject:notification 
                                               waitUntilDone:YES];
}

- (NSData*)_profilePicture
{
    return [self primitiveValueForKey:@"profilePicture"];
}

- (NSData*) profilePicture
{
    [self willAccessValueForKey:@"profilePicture"];
    NSData* picData = [self primitiveValueForKey:@"profilePicture"];
    if (!name) {
        __block NSManagedObjectID* objectID = self.objectID;
        //This solves the multiple downloads per item by using a single queue
        //for all profile pictures download.
        //There are more concurrent ways to accomplish that
        dispatch_async(downloadSerialQueue, ^{ //define some serial queue for assuring you down download multiple times the same object
            NSError* error = nil;
            AppDelegate* appDel = (AppDelegate*)[[UIApplication sharedApplication] delegate];
            NSManagedObjectContext* context = [[NSManagedObjectContext alloc] init];
            [context setPersistentStoreCoordinator:appDel.persistentStoreCoordinator];
            [context setUndoManager:nil];
            User* user = (User*)[context existingObjectWithID:objectID error:&error];
            if (user && [user _profilePicture] == nil) {
                NSData *data = //[method to retrieve data from server];
                if (data) {
                    if (user) {
                        user.profilePicture = data;
                    } else {
                        NSLog(@"ERROR:: error fetching user: %@",error);
                        return;
                    }
                    [[NSNotificationCenter defaultCenter] addObserver:[self class] selector:@selector(mergeToMain:) name:NSManagedObjectContextDidSaveNotification object:context];
                    [context save:&error];
                    [[NSNotificationCenter defaultCenter] removeObserver:[self class] name:NSManagedObjectContextDidSaveNotification object:context];
                }                    
            }
        });
    }
    [self didAccessValueForKey:@"profilePicture"];
    return picData;
}

回答2:

I think this issue can be solved without NSFetchedResultsController involved.

1. use SDWebImage, SDWebImage can load images from remote server asynchronously, just do this:

   [myImageView setImageWithURL:[NSURL URLWithString:@"http://www.domain.com/path/to/image.jpg"]
                  placeholderImage:[UIImage imageNamed:@"placeholder.png"]];
   

2. use KVO, add a observer to User entity and update corresponding image view accordingly. But the code for KVO is rather complex, ReactiveCocoa can simplify them:

   [RACAble(user.profilePicture) subscribeNext:^(UIImage *image) {
       [myImageView setImage:image];
   }];