第二次调用时 NSManagedObjectContext nil

Question

每次我想多次访问 Core Data 时都会出错。我有一个表格视图，我试图在其中接收数据并显示它。

我的 SecondViewController.h 中有一个变量：

#import <UIKit/UIKit.h>
#import "CooperTestModel.h"
#import "Event.h"
#import <CoreData/CoreData.h>

@interface SecondViewController : UITableViewController {
    NSManagedObjectContext *managedObjectContext;
    NSMutableArray *eventArray;
}

@property (nonatomic, strong) NSManagedObjectContext *managedObjectContext;
@property (nonatomic, strong) NSMutableArray *eventArray;

- (void) fetchRecords;

@end

和 SecondViewController.m

#import "SecondViewController.h"

@implementation SecondViewController
@synthesize managedObjectContext, eventArray;

在我的 AppDelegate 中，我得到一个设置为我的 ViewController 的对象：

- (void)applicationDidFinishLaunching:(UIApplication *)application {

    SecondViewController *tableController = [[SecondViewController alloc] init];
    tableController.managedObjectContext = [self managedObjectContext];

    self.navigationController = [[UINavigationController alloc] initWithRootViewController:tableController];

    [window addSubview: [self.navigationController view]];
    [window makeKeyAndVisible];

}


- (NSManagedObjectContext *) managedObjectContext {

    if (managedObjectContext != nil) {
        NSLog(@"managedObjectContext already in use. Returning instance.");
        return managedObjectContext;
    }

    NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
    if (coordinator != nil) {
        managedObjectContext = [[NSManagedObjectContext alloc] init];
        [managedObjectContext setPersistentStoreCoordinator: coordinator];
    }

    NSLog(@"managedObjectContext nil. Returning new instance.");
    return managedObjectContext;
}

然后我在tableView中获取数据：

- (void)fetchRecords {

    NSLog(@"Fetching records.");
    // Define our table/entity to use
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Event" inManagedObjectContext:managedObjectContext];
    // Setup the fetch request
    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    [request setEntity:entity];
    // Define how we will sort the records
    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"timeStamp" ascending:NO];
    NSArray *sortDescriptors = [NSArray arrayWithObject:sortDescriptor];
    [request setSortDescriptors:sortDescriptors];
    // Fetch the records and handle an error
    NSError *error;
    NSMutableArray *mutableFetchResults = [[managedObjectContext executeFetchRequest:request error:&error] mutableCopy];
    if (!mutableFetchResults) {
        NSLog(@"Error fetching data.");
    }
    // Save our fetched data to an array
    [self setEventArray: mutableFetchResults];

    NSLog(@"Records found: %i", [eventArray count]);
}

这在第一次尝试时效果很好，但后来我收到了这个错误：

2013-09-07 19:55:03.715 CooperTest[2697:11603] managedObjectContext nil. Returning new instance.
2013-09-07 19:55:03.729 CooperTest[2697:11603] Fetching records.
2013-09-07 19:55:03.732 CooperTest[2697:11603] Records found: 3
2013-09-07 19:55:03.734 CooperTest[2697:11603] Rows in table: 3
2013-09-07 19:55:04.874 CooperTest[2697:11603] Fetching records.
2013-09-07 19:55:04.878 CooperTest[2697:11603] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '+entityForName: nil is not a legal NSManagedObjectContext parameter searching for entity name 'Event''
libc++abi.dylib: terminate called throwing an exception

对此有什么想法吗？我没有设置新项目并勾选“使用核心数据”选项，因为它不存在。所以我在 AppDelegate 中添加了 CoreData.framework 和一些代码。模型和类也正确（我能够在fetchRecords 的末尾存储 3 条记录）。

Answer 1

最后我只是通过使用修复它

AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
managedObjectContext = appDelegate.managedObjectContext;

在 ViewControlelr 中而不是从 AppDelegate 传递它。