managedObjectContext为nil [英] managedObjectContext is nil

问题描述

我正在尝试将Core Data添加到现有项目。我有：

I'm trying to add Core Data to an existing project. I've:

1）添加了核心数据框架
2）将访问器和属性添加到AppDelegate
3）创建数据模型文件

1)added the Core Data framework 2)added the accessors and properties to the AppDelegate 3)created the data model file

现在当我尝试调用
NSManagedObjectContext * context = [self managedObjectContext];
从视图控制器的上下文是nil和managedObjectContext从不触发。

Now when I try to call NSManagedObjectContext *context = [self managedObjectContext]; from a view controller the context is nil and the managedObjectContext never fires.

这里是AppDelegate：

Here is the AppDelegate:

#import "XXXAppDelegate.h"
#import <CoreData/CoreData.h>

@implementation XXXAppDelegate 


@synthesize window=_window;

@synthesize navigationController=_navigationController;

@synthesize managedObjectContext = _managedObjectContext;
@synthesize managedObjectModel = _managedObjectModel;
@synthesize persistentStoreCoordinator = _persistentStoreCoordinator;

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // Override point for customization after application launch.
    // Add the navigation controller's view to the window and display.
    self.window.rootViewController = self.navigationController;
    [self.window makeKeyAndVisible];

    return YES;
}

- (void)applicationWillResignActive:(UIApplication *)application
{
    /*
     Sent when the application is about to move from active to inactive state. This can occur for certain types of temporary interruptions (such as an incoming phone call or SMS message) or when the user quits the application and it begins the transition to the background state.
     Use this method to pause ongoing tasks, disable timers, and throttle down OpenGL ES frame rates. Games should use this method to pause the game.
     */
}

- (void)applicationDidEnterBackground:(UIApplication *)application
{
    /*
     Use this method to release shared resources, save user data, invalidate timers, and store enough application state information to restore your application to its current state in case it is terminated later. 
     If your application supports background execution, this method is called instead of applicationWillTerminate: when the user quits.
     */
}

- (void)applicationWillEnterForeground:(UIApplication *)application
{
    /*
     Called as part of the transition from the background to the inactive state; here you can undo many of the changes made on entering the background.
     */
}

- (void)applicationDidBecomeActive:(UIApplication *)application
{
    /*
     Restart any tasks that were paused (or not yet started) while the application was inactive. If the application was previously in the background, optionally refresh the user interface.
     */
}

- (void)applicationWillTerminate:(UIApplication *)application
{
    /*
     Called when the application is about to terminate.
     Save data if appropriate.
     See also applicationDidEnterBackground:.
     */
}

//Explicitly write Core Data accessors
- (NSManagedObjectContext *) managedObjectContext {
    if (managedObjectContext != nil) {
        return managedObjectContext;
    }
    NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
    if (coordinator != nil) {
        managedObjectContext = [[NSManagedObjectContext alloc] init];
        [managedObjectContext setPersistentStoreCoordinator: coordinator];
    }

    return managedObjectContext;
}

- (NSManagedObjectModel *)managedObjectModel {
    if (managedObjectModel != nil) {
        return managedObjectModel;
    }
    managedObjectModel = [NSManagedObjectModel mergedModelFromBundles:nil];

    return managedObjectModel;
}

- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {
    if (persistentStoreCoordinator != nil) {
        return persistentStoreCoordinator;
    }
    NSURL *storeUrl = [NSURL fileURLWithPath: [[self applicationDocumentsDirectory]
                                               stringByAppendingPathComponent: @"<Project Name>.sqlite"]];
    NSError *error = nil;
    persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc]
                                  initWithManagedObjectModel:[self managedObjectModel]];
    if(![persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType
                                                 configuration:nil URL:storeUrl options:nil error:&error]) {
        /*Error for store creation should be handled in here*/
    }

    return persistentStoreCoordinator;
}

- (NSString *)applicationDocumentsDirectory {
    return [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject];
}


@end

这里是我的视图控制器代码

here is my view controller code

NSManagedObjectContext *context = [self managedObjectContext];
NSManagedObject *cardSet = [NSEntityDescription insertNewObjectForEntityForName:@"CardSet" inManagedObjectContext:context];
[cardSet setValue:@"Set 1" forKey:@"cardSetName"];

推荐答案

Xcode你看到他们通过AppDelegate中的ManagedObjectContext像这样：

If you look at the Master-Detail Application template in Xcode you see they pass the ManagedObjectContext in the AppDelegate like this:

#import "AppDelegate.h"

#import "MasterViewController.h"

@implementation AppDelegate

@synthesize managedObjectContext = _managedObjectContext;
@synthesize managedObjectModel = _managedObjectModel;
@synthesize persistentStoreCoordinator = _persistentStoreCoordinator;

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // Override point for customization after application launch.
    UINavigationController *navigationController = (UINavigationController *)self.window.rootViewController;
    MasterViewController *controller = (MasterViewController *)navigationController.topViewController;
    NSLog(@"navigationController viewControllers: %@",[navigationController viewControllers]);
    NSLog(@"navigationController.topViewController: %@",navigationController.topViewController);

    controller.managedObjectContext = self.managedObjectContext;
    return YES;
}

如果您在应用程序前需要一个TabBarViewController，代码如下所示：

If you need a TabBarViewController in front of your app the code looks like this:

#import "AppDelegate.h"

#import "MasterViewController.h"

@implementation AppDelegate

@synthesize managedObjectContext = _managedObjectContext;
@synthesize managedObjectModel = _managedObjectModel;
@synthesize persistentStoreCoordinator = _persistentStoreCoordinator;

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // Override point for customization after application launch.
    UITabBarController *tabController = (UITabBarController *)self.window.rootViewController;

    UINavigationController *navigationController = (UINavigationController *)[[tabController viewControllers] objectAtIndex:0];
    MasterViewController *controller = (MasterViewController *)[[navigationController viewControllers] objectAtIndex:0];
    controller.managedObjectContext = self.managedObjectContext;

    navigationController = (UINavigationController *)[[tabController viewControllers] objectAtIndex:1];
    controller = (MasterViewController *)[[navigationController viewControllers] objectAtIndex:0];
    controller.managedObjectContext = self.managedObjectContext;

    navigationController = (UINavigationController *)[[tabController viewControllers] objectAtIndex:2];
    controller = (MasterViewController *)[[navigationController viewControllers] objectAtIndex:0];
    controller.managedObjectContext = self.managedObjectContext;

    return YES;
}

这篇关于managedObjectContext为nil的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！