Question

I’m working with a Navigation Controller based iOS app. There are multiple tableView screens that pull and save data from a Core Data persistent store. Most of the data for the different table views comes from NSFetchedResultsController instances or NSFetchRequests.

The app works as intended but I have been getting a few random crashes and glitches that seem to be related to Core Data. For example sometimes when I save the context the app will crash but not always. Another thing I’ve been seeing is the very first tableView doesn’t always update the reflect the data that was modified in it’s detail view.

Currently I’m passing around a single Managed Object Context that was created in the app delegate to each of the different view controllers by setting the context property of the view controller just before I push it onto the navigation stack.

This seems like a clunky, hacky way of getting the job done. Is there a better design pattern to use?

I noticed in one of the WWDC sessions using delegation but I’ve never used creating my own delegates before and haven’t been able to puzzle it out of the WWDC session.

Thanks.

=)

Answer 1

1)
Use a singleton for your CoreData setup (NSPesistentStoreCoordinator, NSManagedObjectModel & NSManagedObjectContext). You can use this singleton to execute the fetch requests you created in your Models and to add or delete Entities to your Context.

2)
Delegates are not that hard. Following is a sample:

@class SomeClass

@protocol SomeClassDelegate <NSObject> //Implements the NSObject protocol
- (void) someClassInstance:(SomeClass *)obj givesAStringObject:(NSString *)theString;
- (BOOL) someClassInstanceWantsToKnowABooleanValue:(SomeClass *)obj //Call to delegate to get a boolean value

@optional
- (NSString *) thisMethodIsOptional; 

@end

@interface SomeClass : NSObject {
   id<SomeClassDelegate> delegate;
   //Other instance variables omitted.
}
@property (assign) id<SomeClassDelegate> delegate;
@end

@implementation SomeClass
@synthesize delegate;
- (void) someMethodThatShouldNotifyTheDelegate {
   NSString *hello = @"Hello";
   if (self.delegate != nil && [self.delegate respondsToSelector:@selector(someClassInstance:givesAStringObject:)]) {
      [self.delegate someClassInstance:self givesAStringObject:hello];
   }
}
@end

Option 1 could be something like this, you will have to setup the variables in the init of the object (and implement the singleton ofcourse):

#import <Foundation/Foundation.h>
#import <CoreData/CoreData.h>

@interface CoreDataUtility : NSObject {
@private
    NSManagedObjectModel *managedObjectModel;
    NSManagedObjectContext *managedObjectContext;
    NSPersistentStoreCoordinator *persistentStoreCoordinator;
}

+ (CoreDataUtility *)sharedCoreDataUtility;
- (NSEntityDescription *) entityDesctiptionForName:(NSString *)name;
- (NSMutableArray *) executeRequest:(NSFetchRequest *)request;
- (id) getInsertedObjectForEntity:(NSString *)entity;
- (void) deleteAllObjects:(NSString *) entityName;
- (void) deleteManagedObject:(NSManagedObject *)object;
- (void) saveContext;

@end