NSFetchedResultsController multiple entities for UITableView - ios

Question

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?

Answer 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;
}

Answer 2

I think this issue can be solved without NSFetchedResultsController involved.
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"]];
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];
}];