Question

我正在尝试做什么：首次使用NSFetchedResultsController将UITableView连接到Core Data。我是一个新手，真的没经验，我跟着Ray Wenderlich's Tutorial一字一句地跟着。

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app委托中，我在TableViewController类中使用它来导入。

我非常感谢：解释我正在做什么/出了什么问题以及如何解决问题，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为零，这意味着您尚未为该属性分配值。很可能你应该在创建或加载此视图控制器的任何代码中完成此操作（例如，如果您使用的是故事板，则应在prepareForSegue:sender:中）。

+ entityForName：nil不是搜索实体名称的合法NSManagedObjectContext参数

PMMedicinesTableViewController.m

1 个答案: