Where to Put Code Configuring Persistent Store for Migration

问题

I have a Mac App already created and distributed on the App Store to many customers, and I need to add an attribute to the Core Data Model. I read the documentation provided by Apple, available here.

However, it gives this block of code to enable automatic migration:

NSError *error;
NSPersistentStoreCoordinator *psc = <#The coordinator#>;
NSURL *storeURL = <#The URL of a persistent store#>;
NSDictionary *optionsDictionary =
    [NSDictionary dictionaryWithObject:[NSNumber numberWithBool:YES]
                    forKey:NSMigratePersistentStoresAutomaticallyOption];

NSPersistentStore *store = [psc addPersistentStoreWithType:<#Store type#>
                                configuration:<#Configuration or nil#>
                                URL:storeURL
                                options:optionsDictionary
                                error:&error];

.. And I have no idea where to put that. Someone mentioned (in another thread) that it goes into a PersistentStoreCoordinator, however, I simply used the default Cocoa App Template with "Use Core Data for Storage" enabled. I had to create my own AppDelegate and never saw anything about a PersistentStoreCoordinator (and still don't. I've tried creating a new app just to check). Any help here? I'm new to Cocoa but my app works perfectly fine without a PersistentStoreCoordinator, which is why I haven't implemented one yet.. I do have an AppDelegate I created, but when I put this code in there it throws many errors. HELP :/

EDIT (for nick): Here is the new code:

And my header file:

回答1:

Here you go: Core Data relevant header and methods.

Application Delegate Header - Core Data relevant parts

@interface CoreDataFirstStepsAppDelegate : NSObject <UIApplicationDelegate> 
{    
     // ...
@private
    NSManagedObjectContext *managedObjectContext_;
    NSManagedObjectModel *managedObjectModel_;
    NSPersistentStoreCoordinator *persistentStoreCoordinator_;
}

// ...

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

Application Delegate Implementation - Core Data relevant parts

/**
 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_;
    }
    NSString *modelPath = [[NSBundle mainBundle] pathForResource:@"CoreDataFirstSteps" ofType:@"momd"];
    NSURL *modelURL = [NSURL fileURLWithPath:modelPath];
    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:@"CoreDataFirstSteps.sqlite"];

    NSError *error = nil;
    persistentStoreCoordinator_ = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];
    if (![persistentStoreCoordinator_ addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:nil error:&error]) {

        NSError *error;
        NSURL *storeURL = storeURL;
        NSPersistentStoreCoordinator *psc = persistentStoreCoordinator_;
        NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:
                                 [NSNumber numberWithBool:YES], NSMigratePersistentStoresAutomaticallyOption,
                                 [NSNumber numberWithBool:YES], NSInferMappingModelAutomaticallyOption, nil];

        if (![psc addPersistentStoreWithType:NSSQLiteStoreType
                               configuration:nil
                                         URL:storeURL
                                     options:options 
                                       error:&error]) 
        {
            NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
            abort();
        }       
    }    

    return persistentStoreCoordinator_;
}

/**
 Returns the URL to the application's Documents directory.
 */
- (NSURL *)applicationDocumentsDirectory {
    return [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];
}

来源：https://stackoverflow.com/questions/5525349/where-to-put-code-configuring-persistent-store-for-migration