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如何正确配置nsfetchedresultsController

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Corey Floyd · 技术社区 · 16 年前

我在代码中放置了一个nsfetchedresultscontroller,这样我就可以对表视图数据进行很好的自动分区。

所以我要做一个测试来确保一切正常。我的持久性商店里只有一个图书实体。我将首先执行旧方法获取,然后尝试使用nsfetchedresultsController。这两个代码块之间的差别只有两行。

代码没有输入结果控制器:

 NSFetchRequest *request = [[NSFetchRequest alloc] init];
 NSEntityDescription *entity = [NSEntityDescription entityForName:kBookEntityName inManagedObjectContext:self.managedObjectContext];
 [request setEntity:entity];

 NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"title" ascending:NO];
 NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
 [request setSortDescriptors:sortDescriptors];
 [sortDescriptor release];
 [sortDescriptors release];

  //The following 2 lines will be replaced by the NSFetchedResultsController
 NSMutableArray *mutableFetchResults = [[[self.managedObjectContext executeFetchRequest:request error:nil] mutableCopy] autorelease];
 Book *result = (Book*)[mutableFetchResults objectAtIndex:0];

 NSString* title = [result valueForKey:@"title"];
 NSString* priority = [result valueForKeyPath:@"priority.name"];


 [request release];

现在,我用行替换nsfetchedresultscontroller:

NSFetchedResultsController* fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request managedObjectContext:self.managedObjectContext sectionNameKeyPath:@"title" cacheName:@"BookList"];                                                                                                                                                                                                                                                                                                           

Book *result = (Book*)[fetchedResultsController objectAtIndexPath:[NSIndexPath indexPathForRow:0 inSection:0]];

看起来很干练。第一个代码块正确地获取单个图书实体。但是,带有nsfetchedresultsController的代码没有。相反,它返回零。

我的问题是:在这个例子中,我是否正确地配置了nsfetchedresultscontroller?

(注意,nsfetchedResultsController的FetchedObjects属性也为零)

2 回复 | 直到 16 年前

David Maymudes 16 年前

我想你还是需要告诉 NSFetchedResultsController 要实际执行提取,请执行以下操作:

NSError *error;
BOOL success = [controller performFetch:&error];

(摘自 未保存的结果控制器 参考文献)

还有一件事似乎很奇怪:你真的想用“标题”作为 sectionNameKeyPath ?这基本上不会为每本书创建一个单独的部分吗?

Massimo Cafaro 16 年前

您只需要初始化一次nsfetchedresultsController,如下所示。

NSFetchedResultsController *fetchedResultsController;
NSManagedObjectContext *managedObjectContext;

在控制器的头中声明,并且ManagedObjectContext已正确初始化。

- (void)viewDidLoad {

    [super viewDidLoad];

    NSError *error;
    if (![[self fetchedResultsController] performFetch:&error]) {
        // Handle error
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);

    }

    NSLog(@"%d objects fetched", [[fetchedResultsController fetchedObjects] count]);
}

- (NSFetchedResultsController *)fetchedResultsController {

    if (fetchedResultsController != nil) {
        return fetchedResultsController;

    }

    /*
     Set up the fetched results controller.
     */



    // Create the fetch request for the entity.
    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    // Edit the entity name as appropriate.
    NSEntityDescription *entity = [NSEntityDescription entityForName:kBookEntityName inManagedObjectContext:self.managedObjectContext];
    [request setEntity:entity];

/* Optional settings

    [request setResultType:NSManagedObjectResultType];
    [request setIncludesPropertyValues:YES];
    [request setIncludesPendingChanges:NO];
    [request setReturnsObjectsAsFaults:NO];
*/  


    // Edit the sort key as appropriate.
     NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"title" ascending:NO];
 NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
 [request setSortDescriptors:sortDescriptors];
 [sortDescriptor release];
 [sortDescriptors release];;

    // 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:request managedObjectContext:managedObjectContext sectionNameKeyPath:@"title" cacheName:@"BookList"];
    aFetchedResultsController.delegate = self;
    self.fetchedResultsController = aFetchedResultsController;

    [aFetchedResultsController release];
    [request release];


    return fetchedResultsController;
}