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

Question

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

1. 更新数据源对象中的数据。
2. 调用 Collection View 的适当方法以插入或删除部分或项目。

我卡在第二个了。我希望新项目出现在 Collection View 的开头。我不知道为 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始作为它的索引值，而且我也很困惑如何告诉它其他任何东西。您可以在我之前的代码 (cellForItemAtIndexPath) 中看到，我在填充集合时修改了单元格中按钮元素的属性。如果我想在创建时修改新项目，我该怎么做？我真的很感谢这方面的任何帮助，我觉得我在这里错过了很多，而且真的碰壁了。

@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

您似乎没有实现任何 NSFetchedResultsControllerDelegate 方法。对于您想要的功能，作为获取结果 Controller 的委托(delegate)是关键。

NSFetchedResultsController 的一个实例将通知它的代表对匹配它的获取请求的对象集合的更改。它通过在它的委托(delegate)上调用各种 NSFetchedResultsControllerDelegate 方法来做到这一点。例如，如果将一个新对象插入分配给匹配获取请求的获取结果 Controller 的 NSManagedObjectContext，将调用以下 NSFetchedResultsControllerDelegate 方法:

• controllerWillChangeContent:

• controller:didChangeObject:atIndexPath:forChangeType:newIndexPath:

• controllerDidChangeContent:

controller:didChangeObject:atIndexPath:forChangeType:newIndexPath: 是您调用 Collection View 上的 insertItemsAtIndexPaths: 方法的地方，告诉它新项目在它的数据源。这看起来像:

- (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变化类型，这只是一个说明性的例子。您还应该实现其他 NSFetchedResultsControllerDelegate 方法。不需要使用批量更新来更改 Collection View ，并且可以轻松实现。