从核心数据加载UICollectionView

Question

I'm developing an iOS app and at a certain point I store images draw by the user in Core Data. Now I would like to load all the recorded images in a UICollectionView so the user could select one and share it on social networks. Everything in my app works except this last part. I followed all sorts of tutorials on the net but all examples I could find on UICollectionView were using images from Flickr or similar websites.

Here is my code at this point:

#import "LibraryViewController.h" (The name of the class we're in)
#import "SocialViewController.h"
#import "CollectionViewCell.h"


static NSString *cellIdentifier = @"MemeCell";

@implementation LibraryViewController {
    NSMutableArray *_objectChanges;
    NSMutableArray *_sectionChanges;
}


#pragma mark - View Controller LifeCycle

- (void)awakeFromNib
{
    [super awakeFromNib];
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    _objectChanges = [NSMutableArray array];
    _sectionChanges = [NSMutableArray array];

    self.title = @"Meme collection";
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    if ([[segue identifier] isEqualToString:@"share"]) {
        NSIndexPath *indexPath = [[self.collectionView indexPathsForSelectedItems] lastObject];
        NSManagedObject *object = [[self fetchedResultsController] objectAtIndexPath:indexPath];
        SocialViewController *destViewController = segue.destinationViewController;
        [destViewController setDetailItem:object];
    }
}

#pragma mark - UICollectionView

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{

    id <NSFetchedResultsSectionInfo> sectionInfo = [self.fetchedResultsController sections][section];
    return [sectionInfo numberOfObjects];
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    CollectionViewCell *cell = (CollectionViewCell *)[collectionView dequeueReusableCellWithReuseIdentifier:cellIdentifier
                                                                                           forIndexPath:indexPath];

    NSManagedObject *object = [self.fetchedResultsController objectAtIndexPath:indexPath];
    [cell setImage:[UIImage imageWithData:[object valueForKey:@"image"]]];

    return cell;
}


#pragma mark - Fetched results controller

- (NSFetchedResultsController *)fetchedResultsController
{
    if (_fetchedResultsController != nil) {
        return _fetchedResultsController;
    }

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    // Edit the entity name as appropriate.
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Meme" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];

    [fetchRequest setFetchBatchSize:20];
    NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:@"Master"];
    aFetchedResultsController.delegate = self;
    self.fetchedResultsController = aFetchedResultsController;

    NSError *error = nil;
    if (![self.fetchedResultsController performFetch:&error]) {
    // Replace this implementation with code to handle the error appropriately.
    // abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
    NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
    abort();
}

    return _fetchedResultsController;
}

- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo
       atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type
{
    NSMutableDictionary *change = [NSMutableDictionary new];

    switch(type) {
        case NSFetchedResultsChangeInsert:
            change[@(type)] = @(sectionIndex);
            break;
        case NSFetchedResultsChangeDelete:
            change[@(type)] = @(sectionIndex);
            break;
    }

    [_sectionChanges addObject:change];
}

- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject
   atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type
  newIndexPath:(NSIndexPath *)newIndexPath
{
    NSMutableDictionary *change = [NSMutableDictionary new];
    switch(type)
    {
        case NSFetchedResultsChangeInsert:
            change[@(type)] = newIndexPath;
            break;
        case NSFetchedResultsChangeDelete:
            change[@(type)] = indexPath;
            break;
        case NSFetchedResultsChangeUpdate:
            change[@(type)] = indexPath;
            break;
        case NSFetchedResultsChangeMove:
            change[@(type)] = @[indexPath, newIndexPath];
            break;
    }
    [_objectChanges addObject:change];
}

- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller
{
    if ([_sectionChanges count] > 0)
    {
        [self.collectionView performBatchUpdates:^{

            for (NSDictionary *change in _sectionChanges)
            {
                [change enumerateKeysAndObjectsUsingBlock:^(NSNumber *key, id obj, BOOL *stop) {

                    NSFetchedResultsChangeType type = [key unsignedIntegerValue];
                    switch (type)
                    {
                        case NSFetchedResultsChangeInsert:
                            [self.collectionView insertSections:[NSIndexSet indexSetWithIndex:[obj unsignedIntegerValue]]];
                            break;
                        case NSFetchedResultsChangeDelete:
                            [self.collectionView deleteSections:[NSIndexSet indexSetWithIndex:[obj unsignedIntegerValue]]];
                            break;
                        case NSFetchedResultsChangeUpdate:
                            [self.collectionView reloadSections:[NSIndexSet indexSetWithIndex:[obj unsignedIntegerValue]]];
                            break;
                    }
                }];
            }
        } completion:nil];
    }

    if ([_objectChanges count] > 0 && [_sectionChanges count] == 0)
    {
        [self.collectionView performBatchUpdates:^{

            for (NSDictionary *change in _objectChanges)
            {
                [change enumerateKeysAndObjectsUsingBlock:^(NSNumber *key, id obj, BOOL *stop) {

                    NSFetchedResultsChangeType type = [key unsignedIntegerValue];
                    switch (type)
                    {
                        case NSFetchedResultsChangeInsert:
                            [self.collectionView insertItemsAtIndexPaths:@[obj]];
                            break;
                        case NSFetchedResultsChangeDelete:
                            [self.collectionView deleteItemsAtIndexPaths:@[obj]];
                            break;
                        case NSFetchedResultsChangeUpdate:
                            [self.collectionView reloadItemsAtIndexPaths:@[obj]];
                            break;
                        case NSFetchedResultsChangeMove:
                            [self.collectionView moveItemAtIndexPath:obj[0] toIndexPath:obj[1]];
                            break;
                    }
                }];
            }
        } completion:nil];
    }

    [_sectionChanges removeAllObjects];
    [_objectChanges removeAllObjects];
}

@end

When I run and try to access this ViewController, the app crashes with this error message: 2013-07-25 23:08:11.832 MemeGen[87259:c07] * Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '+entityForName: nil is not a legal NSManagedObjectContext parameter searching for entity name 'Meme'' * First throw call stack:

and it points to this line of code:

    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Meme" inManagedObjectContext:self.managedObjectContext];

Obviously the self.managedObjectContext is nil. How should I assign it? And when the user launches the application for the first time and the core data is empty, how should I manage it ? Forbidding its access as long as there are no images?

Otherwise, I know how to store the draw images on the file system. If someone knew a way to load it in the UICollectionView it could also be a solution I could accept.

Answer 1

I would not recommend storing images in core data unless you are using the external storage functionality. Large binary objects are rarely something you want to store inside something like core data. You can, of course, store filesystem paths or URLs on core data objects that point to images on the filesystem.

To use Core Data with a UICollectionView, take a look at NSFetchedResultsController. Here's an example of using the two together to get you started.

Answer 2

You will want to get a reference to your ManagedObjectContext. Oftentimes this gets created in your application delegate. In which case you will want something like this -

NSManagedObjectContext *aManagedObjectContext = ((AppDelegate *)[UIApplication sharedApplication].delegate).managedObjectContext;

If you want you can do this in your viewDidLoad and assign it to self.managedObjectContext.