How to Add Core Data to an existing Utility Applic

Question

I´ve created an utility Application which is nearly complete, but now I´m on a point were I really have to persist Data.

Since XCode provides only Core Data Templates in an Navigation or Window Based App, is there an easy way to add Core Data to my Application? I´ve never worked with Core Data and just have to persist messages with 460 Characters and a contact Name tp it as a history of send Messages.

Or should I start with a new Window Based Application incl. Core Data and try to build the Utility / Flipside Part by hand?

Can someone suggest me the best practice for my situation?

Answer 1

Create a new project in XCode, using the templates provided - find one that has a box to check for using Core Data for storage.

That gives you an xcdatamodel file, and some code/class variables in the application delegate that you can copy from that project into your current one.

I also highly recommend the Apple tutorial mentioned by nall.

If you do decide to just use SQLLite directly instead, strongly consider using FMDB, which simplifies SQL code. It's one file you add to a project.

Answer 2

As I am also trying to understand the twighlightzone of core data as well, I figured out the following steps to migrate a 'normal' project to core data (by comparing an empty application project with an empty application project with core data)

Step 1: Add CoreData.framework

a) In "Project Target Summary" under "Linked Frameworks and Libraries" add the CoreData.framework with the + button
b) Select File / New / File and in the section 'Core Data' add a new 'Data Model' (and call it i.e. XXXXXXX (for the naming see 3.b)
c) In the file APPLIKATION-Prefix.pch (where APPLICATION is your project name) add the

   #import <CoreData/CoreData.h>

under the other two include directives

Step 2: In the AppDelegate.h add the following properties / method declarations:

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

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

Step 3: AppDelegate.m

a) Syntesize the properties:

@synthesize managedObjectContext = __managedObjectContext;
@synthesize managedObjectModel = __managedObjectModel;
@synthesize persistentStoreCoordinator = __persistentStoreCoordinator;

b) at the end of the module add the following lines:

Important: In the Methode managedObjectModel and persistentStoreCoordinator you have to replace the XXXXXXX withe the name of your personal .xcdatamodeld file

- (void)saveContext
{
    NSError *error = nil;
    NSManagedObjectContext *managedObjectContext = self.managedObjectContext;
    if (managedObjectContext != nil)
    {
        if ([managedObjectContext hasChanges] & ![managedObjectContext save:&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. 
             */
            NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
            abort();
        } 
    }
}

#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:@"XXXXXXX" 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:@"XXXXXXX.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. 

         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;
}

#pragma mark - Application's Documents directory

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

Answer 3

You'll need to add the CoreData framework to your target, create a data model, and instantiate the NSManagedObjectModel, NSPersistentStoreCoordinator, and NSManagedObjectContext objects.

Adding Core Data to an existing application is discussed briefly in this Apple document (search for "existing application")

You should also view the Apple tutorial to get a feel for what is involved.

You can always consider just using SQLite as well.