Iphone NSFetchedResultsController实例需要非nil fetchRequest和managedObjectContext

我试图在iphone应用程序中使用核心数据，但当我启动应用程序时，出现以下错误：

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

代码如下：

#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];
}

---

是否确实正确设置了托管对象上下文？ 从这里我可以看到，Fetch请求是正确创建的，但是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了上下文

通常我在应用程序委托中保留上下文，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

您确定正确设置了托管对象上下文吗？ 从这里我可以看到，Fetch请求是正确创建的，但是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了上下文

通常我在应用程序委托中保留上下文，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

在不必在任何控制器中包含AppDelegate依赖项的情况下使用managedObjectContext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;
}

---

确保控制器上有属性managedObjectContext。

在不必在任何控制器中包含AppDelegate依赖项的情况下，使用managedObjectContext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;
}

---

请确保控制器上有属性managedObjectContext。

回复很晚，但我昨天遇到了完全相同的问题，因此根据我的代码：

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

我将代码的上一行放在下一行之后：

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

因此，构建出现了错误。所以据我所知，排序描述符必须设置为fetch request，然后才能将fetch请求分配给fetch Results控制器

---

把它放在正确的地方解决了我的问题。希望这能帮助其他面临这种情况的人。

这是一个迟到的回复，但我昨天遇到了完全相同的问题，因此根据我的代码：

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

我将代码的上一行放在下一行之后：

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

因此，构建出现了错误。所以据我所知，排序描述符必须设置为fetch request，然后才能将fetch请求分配给fetch Results控制器

把它放在正确的地方解决了我的问题。希望这能帮助其他面临这种情况的人