iphone - 获取时核心数据中的重复记录

Question

各位，

我的 iPhone 项目中有一些涉及 Core Data 的谜题。在两个单独的 View Controller 实例中的 viewDidLoad 方法期间，我调用以下方法:

- (NSFetchedResultsController *)getStudentResults:(NSString *)classRoom forWeekStarting:(NSDate *)startDate andEnding:(NSDate *)endDate {
    AttendanceAppDelegate *appDelegate = (AttendanceAppDelegate *)[[UIApplication sharedApplication] delegate];

    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    NSEntityDescription *entityDescription = [NSEntityDescription entityForName:@"Student" inManagedObjectContext:[appDelegate managedObjectContext]];

    int secondsInAWeek = 60 * 60 * 24 * 7;
    NSDate *today = [[NSDate alloc] init];
    NSDate *nextWeek = [[NSDate alloc] initWithTimeIntervalSinceNow:secondsInAWeek];
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(student.classRoom like %@) AND (dateTime >= %@) AND (dateTime <= %@)", classRoom, startDate, endDate];
    [request setPredicate:predicate];
    [request setEntity:entityDescription];

    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"dateTime" ascending:YES];
    NSArray *descriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
    [request setSortDescriptors:descriptors];
    [descriptors release];
    [sortDescriptor release];
    [nextWeek release];
    [today release];


    NSFetchedResultsController *fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request managedObjectContext:[appDelegate managedObjectContext] sectionNameKeyPath:nil cacheName:@""];
    NSError *error; 
    if (![fetchedResultsController performFetch:&error])
        NSLog(@"Error performing fetch on fetchedResultsController: %@", [error localizedDescription]);

    if (fetchedResultsController == nil || [[fetchedResultsController fetchedObjects] count] == 0) { 
        Student *student = [NSEntityDescription insertNewObjectForEntityForName:@"Student" inManagedObjectContext:[appDelegate managedObjectContext]];
        NSData *classRoomStudentData = [[NSData alloc] initWithContentsOfFile:[[NSBundle mainBundle] pathForResource:@"STUDENT_ATTENDANCE" ofType:@"XML"]];
        [student setClassRoom:@"default"];
        [student buildWithStudentData:classRoomStudentData startingWithXPathNode:@"//attendance"];

        NSError *error; 
        if (![[appDelegate managedObjectContext] save:&error]) { 
            NSLog(@"Error saving the managedObjectContext: %@", [error localizedDescription]);
        }

        if (![fetchedResultsController performFetch:&error])
            NSLog(@"Error performing fetch on fetchedResultsController: %@", [error localizedDescription]);
    }
    return fetchedResultsController;
}

每次运行应用程序时，Student 对象的数量都会随着原始实体的数量而增加(每个原始实体都会重复一次)。因此，如果我从 4 开始，下次我会得到 8，然后是 12，然后是 16，等等。我似乎无法弄清楚为什么，因为解析 XML 的代码仅在获取没有返回对象时才会被调用。

如有任何帮助，我们将不胜感激，谢谢。

-- 迈克尔

Answer 1

您不仅在获取未返回对象时插入新对象:查看您的代码

if (fetchedResultsController == nil || [[fetchedResultsController fetchedObjects] count] == 0)

当您无法正确初始化 NSfetchedResultsController 时，您也可以插入它们。因此，如果每次添加新对象时都无法正确初始化它。

建议初始化 NSfetchedResultsController，如 Apple 提供的所有示例代码所示。基本上，您首先验证 fetchedResultsController 是否为零。如果没有，您只需返回该变量即可。否则，您将对其进行初始化。

请参阅我对此问题的回答作为示例:

How to properly configure NSFetchedResultsController