vb.net print barcode labels Figure 9.6 Creating a managed object class in Objective-C

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Figure 9.6 Creating a managed object class
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Data: advanced techniques
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After clicking Next, you see a screen asking you for the path. Accept the default path, and click Next. On the following screen, you see a table of all your Core Data entities with check boxes by them. Select the check boxes by the entities for which you wish to create classes, and click Finish. When you ve completed this process, you should see .h and .m files added to your project for each entity in your Core Data model. You may now use these class files like any other class in your project. You ll see a little later how they re used to interface with your database. The last thing you must do to prepare your application is to add the Core Data framework to your project. To do this, right-click the Frameworks folder, and select Add > Existing Frameworks. Then, select coredata.framework from the list. Now you re ready to start writing the code to initialize your Core Data model.
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Initializing the Core Data objects
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As with SQLite, Core Data requires quite a bit of setup before you can get it up and running. Fortunately, the code for doing this is standard and is roughly the same in most situations. First, you must declare the objects needed by Core Data. As you did with SQLite, you declare them in your application delegate. This lets you send the context to only the classes that need to work with it. The objects you need to declare are model, context, and persistent store. Listing 9.8 shows this code.
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Listing 9.8 Declaring the Core Data objects
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#import <CoreData/CoreData.h> @interface CDJournalAppDelegate : NSObject <UIApplicationDelegate> { NSManagedObjectModel *managedObjectModel; NSManagedObjectContext *managedObjectContext; NSPersistentStoreCoordinator *persistentStoreCoordinator; UIWindow *window; UINavigationController *navigationController; } @property (nonatomic, retain, readonly) NSManagedObjectModel *managedObjectModel; @property (nonatomic, retain, readonly) NSManagedObjectContext *managedObjectContext; @property (nonatomic, retain, readonly) NSPersistentStoreCoordinator *persistentStoreCoordiantor; @property (nonatomic, retain) IBOutlet UIWindow *window; @property (nonatomic, retain) IBOutlet UINavigationController *navigationController; @end
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Now that the properties have been declared, they must be initialized. After they re initialized, only the managed object context will be used to interface with the data store. You must add a few methods to your delegate method to initialize all of these
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An introduction to Core Data
properties. They re pretty standard and can be implemented the same way in all your applications. We ll walk you through each of these methods. The first method is the getter for the persistentStoreCoordinator. It s where you ll be loading and initializing the database used by your Core Data application. Listing 9.9 shows the code for this method.
Listing 9.9 Setter methods for Core Data objects
- (NSPersistentStoreCoordinator *)persistentStoreCoordinator { if (persistentStoreCoordinator != nil) { Resolves path return persistentStoreCoordinator; to database } NSString *docs = [NSSearchPathForDirectoriesInDomains( NSDocumentDirectory, NSUserDomainMask, YES) lastObject]; NSURL *storeUrl = [NSURL fileURLWithPath: [docs stringByAppendingPathComponent: @"CDJournal.sqlite NSError *error = nil; persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]]; Initializes store
coordinator if (![persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeUrl options:nil error:&error]) { NSLog(@"Unresolved error %@, %@", error, [error userInfo]); abort(); } return persistentStoreCoordinator;
This is a fairly standard getter method. You first check to see if the store coordinator has already been initialized. If so, you return it. This is the case on every call following the first one to this method. Next, you resolve the path to the database used by your application B. As noted before, Core Data is built on top of SQLite. The name of the SQLite database you need to link to is the same as that of your Xcdatamodel file. In this case, it s CDJournal.sqlite. Finally, you initialize the persistent store coordinator with this path and the managed object model C. In the event that an error occurs, the abort methods tell the application to fail and generate an error report. The last line returns a reference to the persistentStoreCoordinator object. The next methods you ll implement are the setters for the managedObjectContext and managedObjectModel properties. The code in listing 9.10 shows how these methods are implemented.
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