Creating the Core Data Stack

This chapter shows you how to create and configure the Core Data stack, from the managed object context to the underlying persistent store.

Overview

The managed object context is responsible for managing a graph of managed objects. The task of managing the persistent stores falls to the persistent store coordinator. Its job is to mediate between the managed object context or contexts and the persistent store or stores. It presents a façade to the contexts, representing a collection of stores as a single virtual store. In this example, the coordinator manages just a single store.

To add a store, you use the NSPersistentStoreCoordinator method addPersistentStoreWithType:configuration:URL:options:error:. This returns an object representing the new store (or nil if it cannot be created). You must specify both the store‚Äôs location in the file system and its type (this example does not make use of model configurations). In this example it is an XML store‚Äîbecause its reasonably human-readable form facilitates testing. The file name extension is not .xml. You should avoid using generic file extensions‚Äîconsider what would happen if all applications used the same extension‚Ķ

The managedObjectContext Function

The managedObjectContext function returns a fully configured managed object context. It also creates and configures the remainder of the Core Data stack if necessary.

Create the Context Instance

The first step is to determine whether the managed object context instance already exists. If it does, simply return it, if it doesn’t, create it and then configure the remainder of the stack.

1. At the top of the main source file, before main add a declaration for the function NSManagedObjectContext *managedObjectContext().

2. In the main source file, implement the managedObjectContext function. Declare a static variable for the context. If the variable is not nil return it immediately. If it is nil, create a new context, then return it as the function result.

   NSManagedObjectContext *managedObjectContext()

   {

   static NSManagedObjectContext *moc = nil;

   if (moc != nil) {

   return moc;

   }

   moc = [[NSManagedObjectContext alloc] init];

   // implementation continues...

   return moc;

   }

Set up the Persistent Store Coordinator and Store

The second main step is to create the persistent store coordinator and configure the persistent store. You also need to tell the context that it should use this new coordinator.

1. Create a persistent store coordinator, then set the coordinator for the context.

   NSPersistentStoreCoordinator *coordinator =

   [[NSPersistentStoreCoordinator alloc]

                   initWithManagedObjectModel:managedObjectModel()];

   [moc setPersistentStoreCoordinator: coordinator];

2. Create a new persistent store of the appropriate type. If for some reason the store cannot be created, log an appropriate warning.

   NSString *STORE_TYPE = NSXMLStoreType;

   NSString *STORE_FILENAME = @"CDCLI.cdcli";

   NSError *error = nil;

   NSURL *url = [applicationLogDirectory() URLByAppendingPathComponent:STORE_FILENAME];

   NSPersistentStore *newStore = [coordinator addPersistentStoreWithType:STORE_TYPE

   configuration:nil

   URL:url

   options:nil

   error:&error];

   if (newStore == nil) {

   NSLog(@"Store Configuration Failure\n%@",

   ([error localizedDescription] != nil) ?

   [error localizedDescription] : @"Unknown Error");

   }

Instantiate a Managed Object Context

So that you can test the implementation thus far, instantiate the managed object context.

1. In the main function, after the line in which the description of the managed object model is logged, declare a variable of type NSManagedObjectContext and assign its value to the result of invoking the managedObjectContext function.

   NSManagedObjectContext *moc = managedObjectContext();

Build and Test

Build and run the utility. It should compile without errors, although you should get a warning that the variable moc is unused in the main function. When you run the utility, the managedObjectContext function should not log any errors.

Complete Listing

The complete listing of the managedObjectContext function is shown in Listing 5-1.

Listing 5-1  Complete listing of the managedObjectContext function

NSManagedObjectContext *managedObjectContext() {

    static NSManagedObjectContext *moc = nil;

    if (moc != nil) {

        return moc;

    }

    moc = [[NSManagedObjectContext alloc] init];

    NSPersistentStoreCoordinator *coordinator =

        [[NSPersistentStoreCoordinator alloc]

                initWithManagedObjectModel: managedObjectModel()];

    [moc setPersistentStoreCoordinator: coordinator];

    NSString *STORE_TYPE = NSXMLStoreType;

    NSString *STORE_FILENAME = @"CDCLI.cdcli";

    NSError *error = nil;

    NSURL *url = [applicationLogDirectory() URLByAppendingPathComponent:STORE_FILENAME];

    NSPersistentStore *newStore = [coordinator addPersistentStoreWithType:STORE_TYPE

                                            configuration:nil

                                                      URL:url

                                                  options:nil

                                                    error:&error];

    if (newStore == nil) {

        NSLog(@"Store Configuration Failure\n%@",

                ([error localizedDescription] != nil) ?

                [error localizedDescription] : @"Unknown Error");

    }

    return moc;

}