我正在开发一个iOS应用程序，并且在某个时刻我将用户绘制的图像存储在Core Data中。 现在我想在UICollectionView中加载所有录制的图像，以便用户可以选择一个并在社交网络上共享。 我的应用程序中的所有内容都工作，除了最后一部 我在网上关注了各种各样的教程，但我在UICollectionView上找到的所有例子都使用了Flickr或类似网站的图片。
此处是我的代码：
#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] *由于未捕获异常'NSInvalidArgumentException'而终止应用程序，原因：'+ entityForName：nil不是合法的NSManagedObjectContext参数，用于搜索实体名称'Meme' “ * 首先抛出调用堆栈：
它指向这行代码：
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Meme" inManagedObjectContext:self.managedObjectContext];
显然self.managedObjectContext是nil。 我该如何分配？当用户第一次启动应用程序并且核心数据为空时，我该如何管理它？只要没有图像，就禁止访问？
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，它也可以是我能接受的解决方案。
答案 0 :(得分：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。当然，您可以在指向文件系统上的图像的核心数据对象上存储文件系统路径或URL。
要将Core Data与UICollectionView一起使用，请查看NSFetchedResultsController。 这是example使用这两者一起开始的。{/ p>
答案 1 :(得分：0)
您需要获取对ManagedObjectContext的引用。通常，这会在您的应用程序委托中创建。在这种情况下，你会想要这样的东西 -
NSManagedObjectContext *aManagedObjectContext = ((AppDelegate *)[UIApplication sharedApplication].delegate).managedObjectContext;
如果需要，可以在viewDidLoad中执行此操作并将其分配给self.managedObjectContext。