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A persistent store coordinator coordinates one or more persistent stores and associates them with a managed object model While the managed object model defines what gets persisted, the persistent store takes care of the low-level details of how and where the data is actually saved Although advanced applications might have more than one persistent store, this chapter limits consideration to single-store applications Core Data can persist data in several different ways Store types you might use to persist data include NSSQLiteStoreType, NSBinaryStoreType, or NSInMemoryStoreType For most purposes, you ll want to use a SQLite database for the Persistent Store and that is what Xcode created in the persistentStoreCoordinator accessor in DogBreedsAppDelegate
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NSURL *storeUrl = [NSURL fileURLWithPath: [[self applicationDocumentsDirectory] stringByAppendingPathComponent: @"DogBreedssqlite"]];
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After obtaining the URL to the store, it creates an NSPersistentStoreCoordinator instance using the managed object model instance The persistent store contains the data, while the model defines how to interpret that data
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persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel: [self managedObjectModel]];
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NSError *error = nil; [persistentStoreCoordinator addPersistentStoreWithType: NSSQLiteStoreType configuration: nil URL: storeUrl options:nil error:&error]
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NSManagedObjectContext
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The NSManagedObjectContext represents an application s managed object context, or the object instances (NSManagedObject classes) that your application is manipulating Managed objects are fetched from the persistent store into the NSManagedObjectContext, and it is where you modify the objects Apple describes the context as a big scratch-pad because no manipulations to a context are persisted until code explicitly tells the context to persist the changes Xcode added a managedObjectContext property to DogBreedsAppDelegate and an accessor that creates the context on first access Obtain an application s managed context by allocating and initializing a new NSManagedObjectContext instance You then set its persistent store coordinator
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managedObjectContext = [[NSManagedObjectContext alloc] init]; [managedObjectContext setPersistentStoreCoordinator: coordinator];
NSManagedObject
The NSManagedObjectContext manages NSManagedObject instances NSManagedObjects are not entities, but rather, created from entities An application obtains data from the persistent store and uses the entities in the model to create the NSManagedObjects placed in the context Consider NSEntityDescriptions as the classes and NSManagedObjects as the objects The previous Try This task created entities in the xcdatamodel file Although the NSManagedObjectModel uses these entities, the NSManagedObjectContext does not; it manages NSManagedObjects Our AKCGroup and Breed classes are therefore subclasses of NSManagedObject
NSFetchedResultsController
As a navigation-based application, the DogBreeds application will use a hierarchy of UITableViews to display our AKC groups and breeds Displaying objects from Core Data in UITableViews is such a common task that iOS provides support for easily connecting fetched results to a table view The NSFetchedResultsController object and NSFetchedResultsControllerDelegate protocol make it relatively easy to retrieve a set of objects from Core Data and modify the set as the user adds, deletes, and updates them via a table view
NSFetchRequest
The NSFetchRequest class is how you query a persistent object store for its data It uses an NSEntityDescription to know which entity to fetch Listing 17-3 illustrates creation of an NSFetchRequest and an NSEntityDescription, and assigns the description to the request The NSManagedObjectContext then executes the request and returns the matching objects in an NSArray
iOS SDK Programming: A Beginner s Guide
Listing 17-3
NSFetchRequest example
NSFetchRequest * myRequest = [[NSFetchRequest alloc] init]; NSEntityDescription * entDesc = [NSEntityDescription entityForName:@"AKCGroup" inManagedObjectContext:myContext]; [myRequest setEntity:entDesc]; NSError * error; NSArray * fetchResults = [selfmanagedObjectContext executeFetchRequest:myRequest error:&error]; if(fetchResults == nil) { NSLog(@"an error occurred"); [error release]; }
Notice Listing 17-3 selects all the AKCGroups in myContext Unless you know there will always be a reasonable number of results, you will want to limit the results returned One way you could do this is through the NSFetchRequest s fetchLimit property This property limits the objects returned by a fetch request However, this property does not distinguish which objects to exclude Often, you will want to limit results to only objects meeting certain criteria For instance, you might want all Breeds in a particular AKCGroup The way you limit results based upon given criteria is through the NSPredicate class
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