從核心數據加載UICollectionView

Question

我正在開發一個iOS應用程序，並且在某個時刻我將用戶繪制的圖像存儲在Core Data中。 現在我想在UICollectionView中加載所有錄制的圖像，以便用戶可以選擇一個並在社交網絡上共享。 我的應用程序中的所有內容都工作，除了最后一部 我在網上關注了各種各樣的教程，但我在UICollectionView上找到的所有例子都使用了Flickr或類似網站的圖片。

這是我的代碼：

#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

當我運行並嘗試訪問此ViewController時，應用程序崩潰並顯示以下錯誤消息：2013-07-25 23：08：11.832 MemeGen [87259：c07] *由於未捕獲的異常'NSInvalidArgumentException'而終止應用程序，原因：'+ entityForName ：nil不是合法的NSManagedObjectContext參數，用於搜索實體名稱'Meme''* First throw call stack：

它指向這行代碼：

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

顯然self.managedObjectContext是nil。 我該如何分配？ 當用戶第一次啟動應用程序並且核心數據為空時，我該如何管理它？ 只要沒有圖像，禁止訪問？

否則，我知道如何將繪制圖像存儲在文件系統上。 如果有人知道在UICollectionView中加載它的方法，它也可以是我能接受的解決方案。

Answer 1

除非您使用外部存儲功能，否則我不建議將圖像存儲在核心數據中。 大型二進制對象很少是您想要存儲在核心數據之類的東西。 當然，您可以在指向文件系統上的圖像的核心數據對象上存儲文件系統路徑或URL。

要將Core Data與UICollectionView一起使用，請查看NSFetchedResultsController。 這是一個將兩者結合使用以幫助您入門的示例 。

Answer 2

您將需要獲取對ManagedObjectContext的引用。 通常，這會在您的應用程序委托中創建。 在這種情況下你會想要這樣的東西 -

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

如果需要，可以在viewDidLoad中執行此操作並將其分配給self.managedObjectContext。