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Core Data
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[selfmanagedObjectContext deleteObject:(NSManagedObject *)setObject]; } else NSLog(@"found an orange, leaving"); } NSError *error2; if(![selfmanagedObjectContext save:&error2]) { NSLog(@"Unresolved error %@, %@", error2, [error2 userInfo]); [error2 release]; } [sortDescriptors release]; [sortDescriptorRadius release]; [request release]; } - (IBAction) deleteFruitStand: (id) sender { NSFetchRequest * request =[[NSFetchRequest alloc] init]; NSEntityDescription * entity = [NSEntityDescription entityForName: @"FruitStand" inManagedObjectContext:selfmanagedObjectContext]; [request setEntity:entity]; NSLog(@"deleting the fruit stand"); NSError *error; NSArray * fetchResults = [selfmanagedObjectContext executeFetchRequest:request error:&error]; if(fetchResults == nil) { NSLog(@"Unresolved error %@, %@", error, [error userInfo]); [error release]; } NSEnumerator * enumerator = [fetchResults objectEnumerator]; id setObject; while( (setObject = [enumerator nextObject]) != nil) { NSLog(@"deleting"); [selfmanagedObjectContext deleteObject: (NSManagedObject *)setObject]; } NSError *error2; if(![selfmanagedObjectContext save:&error2]) { NSLog(@"Unresolved error %@, %@", error2, [error2 userInfo]); [error2 release]; } } - (void)dealloc { [managedObjectContext release]; [super dealloc]; } @end
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iPhone SDK Programming: A Beginner s Guide
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Consider the application s behavior Upon invoking the initFruitStand method in FruitStandViewController, the application creates a new FruitStand, two crates, two apples, and two oranges It assigns an apple and orange to each crate At the method s end, it persists the changes to the managedObjectContext by calling its save method Multiple clicks result in multiple FruitStand instances being created Although Core Data keeps the different instances separate internally, to make the example easier to view in the Debugger Console, you added a random number to the fruit stand s name Invoking the listFruitStandContents method creates an NSFetchRequest that selects all fruit stands from the persistent data store The persistent store returns the fruit stands as an NSSet, and so the method enumerates through each fruit stand, selecting the stand s crates The method then enumerates through each crate, listing each crate s contents Each crate s contents are also an NSSet, so the method enumerates through the fruit also Listing 17-11 is the Debugger Console s logging after invoking the initFruitStand method twice and then invoking the listFruitStandContents method
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Listing 17-11
The listFruitStandContents method s logging
FruitStand[530:20b] ****** FRUIT STAND ********** FruitStand[530:20b] stand name: James's Grand
2009-04-15 07:38:20350 2009-04-15 07:38:20351 Fruit Stand 1681692 2009-04-15 07:38:20351 2009-04-15 07:38:20352 2009-04-15 07:38:20352 2009-04-15 07:38:20353 2009-04-15 07:38:20353 2009-04-15 07:38:20353 2009-04-15 07:38:20355
FruitStand[530:20b] FruitStand[530:20b] FruitStand[530:20b] FruitStand[530:20b] FruitStand[530:20b] FruitStand[530:20b] FruitStand[530:20b]
-------Crate-----------orange with radius: 4200000 apple with radius: 1900000 -------Crate-----------apple with radius: 2400000 orange with radius: 3300000 ********************
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iPhone SDK Programming: A Beginner s Guide
2009-04-15 07:38:20356 2009-04-15 07:38:20356 Fruit Stand 1714636 2009-04-15 07:38:20357 2009-04-15 07:38:20357 2009-04-15 07:38:20357 2009-04-15 07:38:20358 2009-04-15 07:38:20358 2009-04-15 07:38:20358 2009-04-15 07:38:20359 FruitStand[530:20b] ****** FRUIT STAND ********** FruitStand[530:20b] stand name: James's Grand
FruitStand[530:20b] FruitStand[530:20b] FruitStand[530:20b] FruitStand[530:20b] FruitStand[530:20b] FruitStand[530:20b] FruitStand[530:20b]
-------Crate-----------apple with radius: 1900000 orange with radius: 4200000 -------Crate-----------orange with radius: 3300000 apple with radius: 2400000 ********************
Notice in the example that you save immediately upon changing the managed object context Sometimes this might not be the desired behavior You can delay persisting changes to the context until the application terminates There are also many life-cycle management
Figure 17-13
The application running in the iPhone Simulator
17:
Core Data
methods in NSManagedObjectContext that you might use for instance, detectConflicts ForObject, refreshObject, processPendingChanges, insertedObjects, updatedObjects, and deletedObjects For more information refer to the NSManagedObjectContext Class Reference The listApple method illustrates using a simple predicate Upon invocation, the method creates an NSFetchRequest for an apple It also creates an NSPredicate telling the request to only fetch apples with a radius greater than 21
NSPredicate * predicate = [NSPredicate predicateWithFormat: @"radius > %@", [NSNumber numberWithFloat:(float)21]];
After fetching the results as an NSSet, it loops through each id, casting it as an apple and then printing its radius The removeApple method illustrates deleting managed objects Select the object(s) you are interested in, delete them, and, if you wish, save the changes to the managed context immediately
[selfmanagedObjectContext deleteObject:(NSManagedObject *)setObject];
Listing 17-12 illustrates the logging after invoking initFruitStand followed by invoking removeApple Listing 17-12 Debugger Console logging after invoking initFruitStand and removeApple methods
2009-04-15 07:39:41153 2009-04-15 07:39:41153 2009-04-15 07:39:41154 2009-04-15 07:39:41154 2009-04-15 07:39:43385 2009-04-15 07:39:43385 Fruit Stand 1957747 2009-04-15 07:39:43386 2009-04-15 07:39:43386 2009-04-15 07:39:43387 2009-04-15 07:39:43388 2009-04-15 07:39:43388 FruitStand[530:20b] FruitStand[530:20b] FruitStand[530:20b] FruitStand[530:20b] FruitStand[530:20b] FruitStand[530:20b] found an orange, leaving found an orange, leaving removing an apple removing an apple ****** FRUIT STAND ********** stand name: James's Grand
FruitStand[530:20b] FruitStand[530:20b] FruitStand[530:20b] FruitStand[530:20b] FruitStand[530:20b]
-------Crate-----------orange with radius: 3300000 -------Crate-----------orange with radius: 4200000 ********************
The deleteFruitStand method also illustrates deleting managed objects However, it also illustrates Cascade Delete When you delete a fruit stand, you delete all its crates Each crate deletes all its fruit When using Cascade Delete, you can delete much data in a short time, so be certain this is the desired behavior
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