iphone - “NSFetchedResultsController 不支持更改跟踪和使用 NSDictionaryResultType 获取请求”

Question

我有一个在 OS3+ 下运行良好的应用程序。但在OS4下不起作用。我收到以下错误消息:

'NSFetchedResultsController does not support both change tracking and fetch request's with NSDictionaryResultType'

这对这里的任何人来说都敲响了警钟吗？

- (NSFetchedResultsController *)fetchedResultsController {

    if (fetchedResultsController != nil) {
        return fetchedResultsController;
    }

    /*
     Set up the fetched results controller.
     */
    // Create the fetch request for the entity.
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    // Edit the entity name as appropriate.
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"myEntity" inManagedObjectContext:managedObjectContext];

    [fetchRequest setEntity:myEntity];

    [fetchRequest setResultType:NSDictionaryResultType];

    [fetchRequest setPropertiesToFetch :[NSArray arrayWithObjects:@"FIELD1",@"FIELD2",@"FIELD3",@"FIELD4",@"FIELD5",nil]];      

    // Setting unique values        
    [fetchRequest setReturnsDistinctResults:YES];       

    // Edit the sort key as appropriate.
    NSSortDescriptor *initialDescriptor = [[NSSortDescriptor alloc] initWithKey:@"FIELD1" ascending:YES];

    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:initialDescriptor, nil];

    [fetchRequest setSortDescriptors:sortDescriptors];      

    // Edit the section name key path and cache name if appropriate.
    // nil for section name key path means "no sections".       
    NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:@"FIELD1" cacheName:@"myCache"];

    aFetchedResultsController.delegate = self;
    self.fetchedResultsController = aFetchedResultsController;
    [aFetchedResultsController release];
    [fetchRequest release];
    [initialDescriptor release];
    [sortDescriptors release];      

    return fetchedResultsController;
}    

提前致谢。

Answer 1

该错误是指您尝试获取 NSDictionary 结果，但同时也期望获取的结果 Controller 监视更改。由于更改只会通过 NSManagedObjects 传播，因此获取的结果 Controller 无法再执行其工作。

使用 NSManagedObjectResultType 不起作用，因为 setPropertiesToFetch:不再适用。相反，在知道结果集后，您必须自己执行一些合并，这将使使用获取的结果 Controller 变得相当困难。

最好的答案是不要为获取的结果 Controller 设置缓存名称或委托(delegate)，在这种情况下它将not perform any change tracking .