iphone：NSFetchedResultsController 的 NSPredicate 问题

Posted 2023-02-27

【中文标题】iphone：NSFetchedResultsController 的 NSPredicate 问题

【英文标题】：iphone: NSPredicate problem with NSFetchedResultsController

【发布时间】：2010-09-30 20:30:43

【问题描述】：

我目前在我的系统中定义了一个“主题”实体：

@interface Topic :  NSManagedObject  


@property (nonatomic, retain) NSString * path;
@property (nonatomic, retain) NSString * name;
@property (nonatomic, retain) NSNumber * topicID;
@property (nonatomic, retain) NSNumber * parent;
@end

我想使用 NSFetchedResultsController 获取具有特定数字（例如 4001）的主题。我将我的定义为：

- (NSFetchedResultsController *)fetchedResultsController 

    if (_fetchedResultsController != nil) 
        return _fetchedResultsController;
    

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];

    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Topic" inManagedObjectContext:_context];
    [fetchRequest setEntity:entity];

    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"topicID == 4100"];
    [fetchRequest setPredicate:predicate];


    NSSortDescriptor *sort = [[NSSortDescriptor alloc] initWithKey:@"path" ascending:NO];
    [fetchRequest setSortDescriptors:[NSArray arrayWithObject:sort]];

    [fetchRequest setFetchBatchSize:20];

    NSFetchedResultsController *theFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:_context sectionNameKeyPath:nil cacheName:@"Root"];
    self.fetchedResultsController = theFetchedResultsController;
    _fetchedResultsController.delegate = self;

    [fetchRequest release];
    [theFetchedResultsController release];

    return _fetchedResultsController;    

..没有谓词，FetchedResultsController 工作正常.. 但是，当我添加它时，出现错误并且应用程序崩溃.. 有人可以告诉我我是否错误地定义了我的 NSPredicate 吗？

【参考方案1】：

尝试将 cacheName 设置为 nil：

NSFetchedResultsController *theFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:_context sectionNameKeyPath:nil cacheName:nil];

它在类似的情况下帮助了我。

【参考方案2】：

这样的东西有用吗？

[NSPredicate predicateWithFormat:@"topicID == %@", [NSNumber numberWithInt:4100]];