Threading - Core Data - page 81

In the -mailRecipe method, we construct a PPRExportOperation instance, passing it
the recipe. We then give the PPRExportOperation a completion block. The comple-
tion block is used to handle the results of the export operation. Note that the
block receives back both an NSData and an NSError . If the operation was success-
ful, the NSData is populated. If the operation failed for any reason, the NSData
is nil , and the NSError is populated. In a production application, we would want
to respond to the error. For now, we have a developer-level logic check to
capture the error.
Building the PPRExportOperation
With the changes completed in the PPRDetailViewController , the next step is to
build the PPRExportOperation . The goal of the operation is to accept a single recipe
and turn it into a JSON structure. When the export is complete, the operation
will execute a completion block and pass the resulting data back to the caller.
This gives us the following header:
Baseline/PPRecipes/PPRExportOperation.h
#import "PPRRecipeMO.h"
typedef void (^ExportCompletionBlock)(NSData *jsonData, NSError *error);
@interface PPRExportOperation : NSOperation
@property (nonatomic, copy) ExportCompletionBlock completionBlock;
- (id)initWithRecipe:(PPRRecipeMO*)recipe;
@end
The initializer for our export operation needs to retain only the NSManagedObjectID
of the incoming NSManagedObject . Because the initWithRecipe method is being called
on the thread on which the incoming NSManagedObject was created, we can
access its methods. Once we are in our own -main method, we can no longer
access it. However, the NSManagedObjectID is thread-safe and can cross the
boundary. So, we grab it now and hold onto it in a property. We pass in the
entire NSManagedObject so that we can also grab a reference to the NSPersistentStore-
Coordinator without having to explicitly request it.
Baseline/PPRecipes/PPRExportOperation.m
- (id)initWithRecipe:(PPRRecipeMO*)recipe;
{
if (!(self = [super init])) return nil;
[self setIncomingRecipeID:[recipe objectID]];
NSManagedObjectContext *moc = [recipe managedObjectContext];
[self setPersistentStoreCoordinator:[moc persistentStoreCoordinator]];
return self;
}