iphone - 从核心数据加载 UICollectionView

Question

我正在开发一个 iOS 应用程序，在某个时候我将用户绘制的图像存储在 Core Data 中。现在我想将所有记录的图像加载到 UICollectionView 中，以便用户可以选择一个并在社交网络上分享。除了最后一部分，我的应用程序中的所有内容都有效。我遵循了网上的各种教程，但我在 UICollectionView 上找到的所有示例都使用了来自 Flickr 或类似网站的图像。

这是我此时的代码:

#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

当我运行并尝试访问此 ViewController 时，应用程序崩溃并显示以下错误消息:2013-07-25 23:08:11.832 MemeGen[87259:c07] * 由于未捕获的异常“NSInvalidArgumentException”而终止应用程序，原因:“+entityForName:nil 不是搜索实体名称“Meme”的合法 NSManagedObjectContext 参数'* 首先抛出调用栈:

它指向这行代码:

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

显然 self.managedObjectContext 是 nil。我应该如何分配它？而当用户第一次启动应用时，核心数据是空的，我该如何管理呢？只要没有图片就禁止访问？

否则，我知道如何将绘图图像存储在文件系统中。如果有人知道在 UICollectionView 中加载它的方法，它也可能是我可以接受的解决方案。

Answer 1

除非您使用外部存储功能，否则我不建议将图像存储在核心数据中。大型二进制对象很少是您想要存储在核心数据之类的东西中的东西。当然，您可以将文件系统路径或 URL 存储在指向文件系统上图像的核心数据对象上。

要将 Core Data 与 UICollectionView 一起使用，请查看 NSFetchedResultsController。这是一个 example结合使用这两者来帮助您入门。