Question

我知道其他人已经发布了同样的问题，但我无法得到解决问题的任何答案。我收到以下错误：

An instance of NSFetchedResultsController requires a non-nil fetchRequest and managedObjectContext

我已经加入了我的appdelegate.m：

WorkoutExercisesTVC *workoutExercisesTVC;
workoutExercisesTVC.managedObjectContext = self.managedObjectContext;
workoutExercisesTVC.fetchedResultsController = self.fetchedResultsController;

我的表视图中有控制器类：

#import "WorkoutExercisesTVC.h"

@implementation WorkoutExercisesTVC

@synthesize fetchedResultsController = __fetchedResultsController;
@synthesize managedObjectContext = __managedObjectContext;

- (void)setupFetchedResultsController
{   
    // 1 - Decide what Entity you want
    NSString *entityName = @"WorkoutExercise"; // Put your entity name here
    NSLog(@"Setting up a Fetched Results Controller for the Entity named %@", entityName);

    // 2 - Request that Entity
    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:entityName];

    // 3 - Filter it if you want
    //request.predicate = [NSPredicate predicateWithFormat:@"enity.name = Blah"];

    // 4 - Sort it if you want
    request.sortDescriptors = [NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:@"order"
                                                                                 ascending:YES
                                                                                  selector:@selector(localizedCaseInsensitiveCompare:)]];

    // 5 - Fetch it
    self.fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request
                                                                    managedObjectContext:self.managedObjectContext
                                                                      sectionNameKeyPath:nil
                                                                               cacheName:nil];
    [self performFetch];
}

我的错误发生在步骤5中的setupFetchedResultsController方法中（获取它）。

如果有人可以帮助我获取self.managedObjectContext，那么它将不再为null。让我知道您可能需要的其他信息！提前谢谢。

Answer 1

为什么不遵循苹果样本中熟悉的懒惰实例化模式？

-(NSFetchedResultsController*)fetchedResultsController {
   if (_fetchedResultsController) {
    return _fetchedResultsController;
   }

   // set your fetch request, entity, predicate, sort descriptors etc. 

   NSFetchedResultsController *aController = [[NSFetchedResultsController alloc] 
                initWithFetchRequest:request 
                managedObjectContext:self.managedObjectContext
                sectionNameKeyPath:nil
                cacheName:nil];

   NSError *error;
   aController.delegate = self; 

   [aController performFetch:&error]
   _fetchedResultsController = aController;

   return aController;
}

当您需要刷新表格视图时（如果实施NSFetchedResultsControllerDelegate方法则不需要）：

[self.fetchedResultsController performFetch:nil];
[self.tableView reloadData];

最后，设置断点并检查托管对象上下文是否为零 - 也许您没有从代理中正确检索它。

NSFetchedResultsController的一个实例需要一个非零的fetchRequest和managedObjectContext ios6

1 个答案: