NSFetchedResultsController 的实例需要非零 fetchRequest 和 ManagedObjectContext

Question

我正在尝试在 iphone 应用程序中使用核心数据，但是当我启动该应用程序时，出现此错误：

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'An instance of NSFetchedResultsController requires a non-nil fetchRequest and managedObjectContext'

代码是这样的：

#import "CocktailController.h"

@implementation CocktailController

@synthesize tableView;
@synthesize cocktailDetailController;
@synthesize fetchedResultsController=fetchedResultsController_, managedObjectContext=managedObjectContext_;

#pragma mark Instance Methods

- (void)configureCell:(UITableViewCell *)cell atIndexPath:(NSIndexPath *)indexPath {

    NSManagedObject *managedObject = [self.fetchedResultsController objectAtIndexPath:indexPath];
    cell.textLabel.text = [[managedObject valueForKey:@"name"] description];
}

- (void)viewDidLoad {
    [super viewDidLoad];
    self.title = @"Cocktails";
}

- (void)didReceiveMemoryWarning {
    // Releases the view if it doesn't have a superview.
    [super didReceiveMemoryWarning];

    // Release any cached data, images, etc that aren't in use.
}

- (void)dealloc {
    [fetchedResultsController_ release];
    [managedObjectContext_ release];
    [tableView release];
    [cocktails release];
    [super dealloc];
}

#pragma mark UITableViewDataSource Methods

- (UITableViewCell *)tableView:(UITableView *)tv cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    UITableViewCell *cell = [tv dequeueReusableCellWithIdentifier:@"cell"];
    if (cell == nil) {
        cell = [[[UITableViewCell alloc] initWithFrame:CGRectZero reuseIdentifier:@"cell"] autorelease];
    }
    Cocktail *cocktail = [cocktails objectAtIndex:indexPath.row];

    cell.textLabel.text = cocktail.name;
    return cell;
}

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView {
    return [[self.fetchedResultsController sections] count];
}


- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
    id <NSFetchedResultsSectionInfo> sectionInfo = [[self.fetchedResultsController sections] objectAtIndex:section];
    return [sectionInfo numberOfObjects];
}

- (void)tableView:(UITableView *)tv didSelectRowAtIndexPath:(NSIndexPath *)indexPath {
    [tv deselectRowAtIndexPath:indexPath animated:YES];
}

#pragma mark -
#pragma mark Fetched results controller

- (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:@"Cocktail" inManagedObjectContext:managedObjectContext];
    [fetchRequest setEntity:entity];

    // Set the batch size to a suitable number.
    [fetchRequest setFetchBatchSize:20];

    // Edit the sort key as appropriate.
    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:NO];
    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, 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:self.managedObjectContext sectionNameKeyPath:nil cacheName:@"Root"];
    aFetchedResultsController.delegate = self;
    self.fetchedResultsController = aFetchedResultsController;

    [aFetchedResultsController release];
    [fetchRequest release];
    [sortDescriptor release];
    [sortDescriptors release];

    NSError *error = nil;
    if (![fetchedResultsController_ performFetch:&error]) {
        /*
         Replace this implementation with code to handle the error appropriately.

         abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development. If it is not possible to recover from the error, display an alert panel that instructs the user to quit the application by pressing the Home button.
         */
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }

    return fetchedResultsController_;
}    


#pragma mark -
#pragma mark Fetched results controller delegate


- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller {
    [self.tableView beginUpdates];
}


- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo
           atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type {

    switch(type) {
        case NSFetchedResultsChangeInsert:
            [self.tableView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
            break;

        case NSFetchedResultsChangeDelete:
            [self.tableView deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
            break;
    }
}


- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject
       atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type
      newIndexPath:(NSIndexPath *)newIndexPath {

    UITableView *tableView = self.tableView;

    switch(type) {

        case NSFetchedResultsChangeInsert:
            [tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;

        case NSFetchedResultsChangeDelete:
            [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;

        case NSFetchedResultsChangeUpdate:
            [self configureCell:[tableView cellForRowAtIndexPath:indexPath] atIndexPath:indexPath];
            break;

        case NSFetchedResultsChangeMove:
            [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            [tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath]withRowAnimation:UITableViewRowAnimationFade];
            break;
    }
}


- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
    [self.tableView endUpdates];
}

它位于选项卡栏应用程序内部（并且具有导航控制器）

谢谢

@end

Answer 1

您确定正确设置了托管对象上下文吗？
从我所看到的情况来看，提取请求已正确创建，但您没有在该类中设置上下文。在显示视图之前，是否在另一个类中设置上下文？

通常我将上下文保留在应用程序委托中，然后在 viewDidLoad 或 fetchedResultsController 中设置类的上下文，如下所示：

AppDelegate *delegate = [[NSApplication sharedApplication] delegate];
self.managedObjectContext = delegate.managedObjectContext;

希望这有帮助！祝你好运！

Answer 2

无需在要使用 ManagedObjectContext 的任何控制器中包含 AppDelegate 依赖项的一个好方法是在应用程序完成启动时将对象传递给控制器​​。

在 AppDelegate.m 中，方法 didFinishLaunchingWithOptions 执行以下操作：

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    UITabBarController *tabBarController = (UITabBarController *)self.window.rootViewController;
    UINavigationController *navigationController = (UINavigationController *)[[tabBarController viewControllers] objectAtIndex:0];
    YourController  *controller = (YourController *)[[navigationController viewControllers] objectAtIndex:0];
    controller.managedObjectContext = self.managedObjectContext;

    return YES;
}

确保您的控制器上具有 ManagedObjectContext 属性。

Answer 3

这是一个迟到的回复，但我昨天遇到了完全相同的问题，所以根据我的代码：

NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];

我在以下行之后放置了上面的代码行：

_fetchedResultsController = [[NSFetchedResultsController alloc]initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:@"author" cacheName:nil];

所以构建出现了错误。据我了解，在将获取请求分配给获取结果控制器之前，必须将排序描述符设置为获取请求。

将它放在正确的位置解决了我的问题。希望这对面临这种情况的其他人有所帮助。