从核心数据加载UICollectionView

  • Z时代
  • 2024-01-10
  • 分类：问答

我正在开发一个iOS应用程序，并在某个时间点存储用户在Core Data中绘制的图像。 现在我想将所有录制的图像加载到UICollectionView中，以便用户可以选择一个并在社交网络上共享。 除了这最后一部分，我的应用程序中的所有东西都可以工作。 我跟着网上的各种教程，但我可以在UICollectionView上找到的所有例子都是使用来自Flickr或类似网站的图片。从核心数据加载UICollectionView

这是我在这一点上代码：

#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，并尝试访问该视图控制器，应用程序崩溃与此错误消息： 2013年7月25日23：08：11.832 MemeGen [ 87259：C07] *终止应用程序由于未捕获的异常“NSInvalidArgumentException”，原因是：“+ entityForName：无不是合法的NSManagedObjectContext参数搜索实体名称‘梅梅’” *第一掷调用堆栈：

和它指向这行代码：

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

显然self.managedObjectContext为零。 我应该如何分配它？当用户首次启动应用程序并且核心数据为空时，我应该如何管理它？禁止访问，只要没有图像？

否则，我知道如何将绘制图像存储在文件系统上。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。

回答：

您将希望获得对您的ManagedObjectContext的引用。通常这会在应用程序委托中创建。在这种情况下，你会想这样的事情 -

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

如果你愿意，你可以做到这一点在你的viewDidLoad并将其分配给self.managedObjectContext。

回答：

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。大的二进制对象很少是你想要存储在核心数据之类的东西。当然，您可以将文件系统路径或URL存储在指向文件系统映像的核心数据对象上。

要使用带有UICollectionView的Core Data，请查看NSFetchedResultsController。 这里有一个example使用这两者来帮助你入门。

以上是 从核心数据加载UICollectionView 的全部内容， 来源链接： utcz.com/qa/260001.html

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