objective-c - +entityForName : nil is not a legal NSManagedObjectContext parameter searching for entity name

Question

我正在尝试做的事情:让 NSFetchedResultsController 第一次工作以将 UITableView 连接到 Core Data。我是新手，确实没经验，关注过Ray Wenderlich's Tutorial逐字逐句。

错误:

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '+entityForName: nil is not a legal NSManagedObjectContext parameter searching for entity name 'Medicine''`.

我尝试了什么:我发现的其他 StackOverflow 问题都没有(包括 this和 this ) 有一个解释，除了 the second one这是有道理的。但是，他提到的方法已经写在我的应用程序委托(delegate)中，我将其导入到我正在使用的 TableViewController 类中。

非常感谢:对我正在做的事情/出了什么问题以及如何解决问题的解释，而不仅仅是答案/更正。

PMMedicinesTableViewController.m

#import "PMMedicinesTableViewController.h"
#import "PMAppDelegate.h"
#import "Medicine.h"

@interface PMMedicinesTableViewController ()

@property (nonatomic, strong) NSFetchedResultsController *fetchedResultsController;
@property (nonatomic, retain) NSManagedObjectContext *managedObjectContext;

@end


@implementation PMMedicinesTableViewController

@synthesize fetchedResultsController = _fetchedResultsController;

- (id)initWithStyle:(UITableViewStyle)style
{
    self = [super initWithStyle:style];
    if (self) {
        // Custom initialization
    }
    return self;
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    
    // Uncomment the following line to preserve selection between presentations.
    // self.clearsSelectionOnViewWillAppear = NO;
    
    // Uncomment the following line to display an Edit button in the navigation bar for this view controller.
    // self.navigationItem.rightBarButtonItem = self.editButtonItem;
    
    NSError *error;
    if (![[self fetchedResultsController] performFetch:&error]) {
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        exit(-1);  // Fail
    }

}

- (NSFetchedResultsController *)fetchedResultsController {
    
    if (_fetchedResultsController != nil) {
        return _fetchedResultsController;
    }
    
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription
                                   entityForName:@"Medicine" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];
    
    NSSortDescriptor *sort = [[NSSortDescriptor alloc]
                              initWithKey:@"name" ascending:NO];
    [fetchRequest setSortDescriptors:[NSArray arrayWithObject:sort]];
    
    [fetchRequest setFetchBatchSize:20];
    
    NSFetchedResultsController *theFetchedResultsController =
    [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
                                        managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil
                                                   cacheName:@"Root"];
    self.fetchedResultsController = theFetchedResultsController;
    _fetchedResultsController.delegate = self;
    
    return _fetchedResultsController;
    
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

#pragma mark - Table view data source

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    // Return the number of sections.
    return 1;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    id  sectionInfo =
    [[_fetchedResultsController sections] objectAtIndex:section];
    return [sectionInfo numberOfObjects];
}

- (void)configureCell:(UITableViewCell *)cell atIndexPath:(NSIndexPath *)indexPath {
    Medicine *med = [_fetchedResultsController objectAtIndexPath:indexPath];
    cell.textLabel.text = med.name;
    cell.detailTextLabel.text = [NSString stringWithFormat:@"%@",
                                 med.name];
}


- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"medicineCellIdentifier" forIndexPath:indexPath];
    
    // Set up the cell...
    [self configureCell:cell atIndexPath:indexPath];
    
    return cell;
}

提前致谢!

Answer 1

错误告诉您 self.managedObjectContext 为 nil，这意味着您尚未为该属性分配值。很可能您应该在创建或加载此 View Controller 的任何代码中完成此操作(例如，如果您使用的是 Storyboard，则在 prepareForSegue:sender: 中)。