Question

我不知道为什么这个代码错误。请帮忙。我读了一些文章，我认为问题与上下文有关。我该怎么办？

这个程序是关于在viewcontroller中标记的coredata中显示数据。

AppDelegate.h    
#import <UIKit/UIKit.h>
#import <CoreData/CoreData.h>

@interface AppDelegate : UIResponder <UIApplicationDelegate>

@property (strong, nonatomic) UIWindow *window;

@property (readonly, strong, nonatomic) NSManagedObjectContext *managedObjectContext;
@property (readonly, strong, nonatomic) NSManagedObjectModel *managedObjectModel;
@property (readonly, strong, nonatomic) NSPersistentStoreCoordinator *persistentStoreCoordinator;

- (void)saveContext;
- (NSURL *)applicationDocumentsDirectory;

@end

AppDelegate.m
#import "AppDelegate.h"
#import "Test.h"
#import "ViewController.h"

@interface AppDelegate ()

@end

@implementation AppDelegate


- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
// Override point for customization after application launch.

NSManagedObjectContext *context = [self managedObjectContext];
Test *t = [NSEntityDescription insertNewObjectForEntityForName:@"Test"
                                        inManagedObjectContext:context];
t.name = @"please";



return YES;}

ViewController.h
#import <UIKit/UIKit.h>

@interface ViewController : UIViewController

@property (weak, nonatomic) IBOutlet UILabel *label;
@property (nonatomic,strong) NSArray *temp;
@property (nonatomic,strong) NSManagedObjectContext* managedObjectContext;
@end

ViewController.m
#import "ViewController.h"
#import "Test.h"

@interface ViewController ()

@end

@implementation ViewController
@synthesize managedObjectContext;
@synthesize temp;
- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];

    NSEntityDescription *entity = [NSEntityDescription
                                    entityForName:@"Test"  
                       inManagedObjectContext:managedObjectContext];
    [fetchRequest setEntity:entity];
    NSError *error;
self.temp = [managedObjectContext executeFetchRequest:fetchRequest error:&error];
for(Test *info in temp){

    _label.text = info.name;
}

}

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

@end

Test.h    
#import <Foundation/Foundation.h>
#import <CoreData/CoreData.h>


@interface Test : NSManagedObject

@property (nonatomic, retain) NSString * name;

@end

Test.m
#import "Test.h"


@implementation Test

@dynamic name;  
@end

我不知道为什么这个代码错误。请帮忙。我读了一些文章，我认为问题与上下文有关。我该怎么办？

Answer 1

在视图控制器中，替换以下行：

@synthesize managedObjectContext;

有了这个：

- (NSManagedObjectContext *) managedObjectContext {
    return ((AppDelegate *)[[UIApplication sharedApplication] delegate]).managedObjectContext;
}

此属性不会在视图控制器中存储另一个不同的对象上下文，而是返回您在应用程序委托中设置的对象上下文。

还有其他方法可以做到这一点，例如按照Singleton模式创建Core Data助手类（如@ NewYork167建议的那样），但这至少应该解决您当前的问题。

Answer 2

对于将来的任何参考，您还可以尝试子类化NSManagedObjectContext，如下所示：

@interface MyManagedObjectContext : NSManagedObjectContext

+ (MyManagedObjectContext *)mainThreadContext;

@end

@implementation MyManagedObjectContext

+ (MyManagedObjectContext *)mainThreadContext;
{
   static MyManagedObjectContext *moc;
   static dispatch_once_t onceToken;
   dispatch_once(&onceToken, ^{
       moc = [[self alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
       // Setup persistent store coordinator here
   });
   return moc;
}

@end

参考：Best practices for passing NSManagedObjectContext around to UITabBarController child view controllers?

如何将数据库从AppDelegate传递到ViewController

2 个答案: