如何将管理对象上下文分配给iOS中的应用程序委托？ [英] How to assign a managed object context to the application delegate in iOS?

问题描述

我用一个实体创建了一个iOS项目和一个托管对象模型（ Model.xcdatamodeld ）。在应用程序：didFinishLaunchingWithOptions：我想测试是否存在托管对象模型，但我得到 nil 。在创建托管对象模型后，我必须做一些特殊的事情来创建托管对象上下文吗？

I created an iOS project and a managed object model (Model.xcdatamodeld) with one entity. In application:didFinishLaunchingWithOptions: I want to test if the managed object model exists, however I am getting nil. Do I have to do something special to create the managed object context after I create the managed object model?

对我可能做错了什么建议？

Any suggestions on what I may be doing wrong?

这是接口

@interface AppDelegate : UIResponder <UIApplicationDelegate>
{

}

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

@end

这是实施文件

#import "AppDelegate.h"
#import "RootViewController.h"

@implementation AppDelegate

@synthesize window = _window;
@synthesize managedObjectContext = _managedObjectContext;
@synthesize managedObjectModel = _managedObjectModel;
@synthesize persistentStoreCoordinator = _persistentStoreCoordinator;
@synthesize navigationController = _navigationController;


- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    if (![self managedObjectContext]) 
    {
        NSLog(@"Does not exist");
    }

    return YES;
}

推荐答案

旧Xcode生成适当的代码CoreData本身。发布方式如下：

Old Xcode generated appropriate code for CoreData by itself. Posting it below:

#pragma mark - Core Data stack

/**
 Returns the managed object context for the application.
 If the context doesn't already exist, it is created and bound to the persistent store coordinator for the application.
 */
- (NSManagedObjectContext *)managedObjectContext
{
    if (__managedObjectContext != nil)
    {
        return __managedObjectContext;
    }

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

/**
 Returns the managed object model for the application.
 If the model doesn't already exist, it is created from the application's model.
 */
- (NSManagedObjectModel *)managedObjectModel
{
    if (__managedObjectModel != nil)
    {
        return __managedObjectModel;
    }
    NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"iDocs" withExtension:@"momd"];
    __managedObjectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];    
    return __managedObjectModel;
}

/**
 Returns the persistent store coordinator for the application.
 If the coordinator doesn't already exist, it is created and the application's store added to it.
 */
- (NSPersistentStoreCoordinator *)persistentStoreCoordinator
{
    if (__persistentStoreCoordinator != nil)
    {
        return __persistentStoreCoordinator;
    }

    NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"iDocs.sqlite"];

    NSError *error = nil;
    __persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];
    if (![__persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:nil error:&error])
    {
        /*
         Replace this implementation with code to handle the error appropriately.

         abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development. If it is not possible to recover from the error, display an alert panel that instructs the user to quit the application by pressing the Home button.

         Typical reasons for an error here include:
         * The persistent store is not accessible;
         * The schema for the persistent store is incompatible with current managed object model.
         Check the error message to determine what the actual problem was.


         If the persistent store is not accessible, there is typically something wrong with the file path. Often, a file URL is pointing into the application's resources directory instead of a writeable directory.

         If you encounter schema incompatibility errors during development, you can reduce their frequency by:
         * Simply deleting the existing store:
         [[NSFileManager defaultManager] removeItemAtURL:storeURL error:nil]

         * Performing automatic lightweight migration by passing the following dictionary as the options parameter: 
         [NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithBool:YES], NSMigratePersistentStoresAutomaticallyOption, [NSNumber numberWithBool:YES], NSInferMappingModelAutomaticallyOption, nil];

         Lightweight migration will only work for a limited set of schema changes; consult "Core Data Model Versioning and Data Migration Programming Guide" for details.

         */
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }    

    return __persistentStoreCoordinator;
}

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