Question

好吧，我真的被困在这里：/在Apple的Collection View Programming Guide中插入一个项目，它说有两个步骤：

更新数据源对象中的数据。
调用集合视图的相应方法以插入或删除节或项目。

我被困在第二个。我希望新项目出现在集合视图的开头。我不知道要为insertItemsAtIndexPaths：参数添加什么。我知道它需要一个索引对象数组，但我不知道如何告诉它选择集合的开头作为它的索引值，我也很困惑如何告诉它其他任何东西。您可以在我的早期代码（cellForItemAtIndexPath）中看到我在填充集合时修改单元格内按钮元素的属性。如果我想在创建时修改新项目，我该怎么做？我真的很感激任何帮助，我觉得我在这里错过了很多，而且我真的很喜欢砖墙。

@implementation RepsCollectionViewController

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        // Custom initialization
    }
    return self;
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    NSError *error = nil;
    if (![self.fetchedResultsController performFetch:&error]) {
        NSLog(@"Error : %@", error);
        abort();
    }
}

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

#pragma mark - Collection view data source

-(NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section {
    id <NSFetchedResultsSectionInfo> secInfo = [self.fetchedResultsController.sections objectAtIndex:section];

    return [secInfo numberOfObjects];
}

-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {

    RepCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"Cell" forIndexPath:indexPath];

    Rep *rep = [self.fetchedResultsController objectAtIndexPath:indexPath];

    cell.repButton.pressed = [rep.completed boolValue];
    cell.repButton.indexPath = indexPath;

    return cell;
}

#pragma mark - Fetched Results Controller

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

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc]init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Rep" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];

    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc]initWithKey:@"dateTimeCreated" ascending:NO];
    [fetchRequest setSortDescriptors:[NSArray arrayWithObjects:sortDescriptor, nil]];

    _fetchedResultsController = [[NSFetchedResultsController alloc]initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:nil];

    _fetchedResultsController.delegate = self;

    return _fetchedResultsController;
}

#pragma mark - IBActions

- (IBAction)addRep:(id)sender {

    [NSEntityDescription insertNewObjectForEntityForName:@"Rep" inManagedObjectContext:self.managedObjectContext];

    [self.collectionView performBatchUpdates:^{

        [self.collectionView insertItemsAtIndexPaths:WHAT TO PUT HERE?];

    } completion:nil];
}

Answer 1

您似乎没有实施任何NSFetchedResultsControllerDelegate方法。对于您想要的功能，作为获取结果控制器的委托是关键。

NSFetchedResultsController的实例将通知它委托对与其获取请求匹配的对象集合的更改。它通过在其委托上调用各种NSFetchedResultsControllerDelegate方法来实现。例如，如果将新对象插入分配给与提取请求匹配的提取结果控制器的NSManagedObjectContext中，则会调用以下NSFetchedResultsControllerDelegate方法：

•controllerWillChangeContent:

•controller:didChangeObject:atIndexPath:forChangeType:newIndexPath:

•controllerDidChangeContent:

controller:didChangeObject:atIndexPath:forChangeType:newIndexPath:是您在集合视图中调用insertItemsAtIndexPaths:方法的地方，告诉它新项目在其数据源中可用。这看起来像是：

- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath {
    UICollectionView *collection = self.collectionView;
    switch(type) {
                case NSFetchedResultsChangeInsert:
                [collection insertItemsAtIndexPaths:@[newIndexPath] ];
                break;
    }
}

显然，一个真实的例子应该正确处理其他更改类型，这只是一个说明性的例子。您还应该实现其他NSFetchedResultsControllerDelegate方法。使用批量更新进行集合视图更改不是必需的，可以轻松实现。

Answer 2

我也不能正确理解你的问题。但这对你有帮助，

从Core Data获取所有数据后

[self.collectionView reloadData];

在此之前将所有数据都放到self.fetchedResultsController

添加项目时更新UICollectionView（核心数据），卡在insertItemsAtIndexPaths上

2 个答案: