core-data Core Data Stack Objective-C Example

Example

This is a simple but robust core-data set-up for iOS 10+. There are exactly two way to access core-data:

1. viewContext. The viewContext can only be used from the main thread, and only for reading.
2. strong enqueueCoreDataBlock. All writing should be done using enqueueCoreDataBlock. There is no need to save at the end it will automatically save. All writes are enqueued in an operationQueue so there can never be be write conflicts.

Make sure to NEVER use any managedObjects from context in another context. Also discard all objects that are created or fetched in enqueueCoreDataBlock as the context that backs them will be destroyed after the block is executed.

// CoreDataManager.h

@interface CoreDataManager : NSObject
@property (nonatomic, readonly) NSManagedObjectContext * viewContext;
- (void)enqueueCoreDataBlock:(void (^)(NSManagedObjectContext* context))block;
@end

// CoreDataManager.m

@implementation NSManagedObjectContext(SaveIfNeeded)
-(BOOL) saveIfNeeded{
    BOOL toReturn = YES;
    if ([self hasChanges]) {
        NSError *error;
        toReturn = [self save:&error];
        if (toReturn == NO || error)
        {
            //Here you should log to your analytics service
            NSLog(@"--- Failed to commit data\n error: %@", error);
        }
    }
    return toReturn;
}
@end
@interface CoreDataManager ()
@property (nonatomic, strong) NSPersistentContainer* persistentContainer;
@property (nonatomic, strong) NSOperationQueue* persistentContainerQueue;    
@end
@implementation CoreDataManager

- (id)init
{
    self = [super init]
    if (self)
    {
        self.persistentContainer = [[NSPersistentContainer alloc] initWithName:@"PROJECT_NAME_ALSO_NAME_OF_MODEL" managedObjectModel:managedObjectModel];
        [self.persistentContainer loadPersistentStoresWithCompletionHandler:^(NSPersistentStoreDescription * description, NSError * error) {              
        }];
        self.persistentContainer.viewContext.automaticallyMergesChangesFromParent = YES;
        _persistentContainerQueue = [[NSOperationQueue alloc] init];
        _persistentContainerQueue.maxConcurrentOperationCount = 1;
        _persistentContainerQueue.name = @"persistentContainerQueue";
        dispatch_queue_t queue = dispatch_queue_create("persistentContainerQueue.dispatchQueue", DISPATCH_QUEUE_SERIAL);
        _persistentContainerQueue.underlyingQueue = queue;            
    }
}

- (void)enqueueCoreDataBlock:(void (^)(NSManagedObjectContext* context))block{
    void (^blockCopy)(NSManagedObjectContext*) = [block copy];

    [self.persistentContainerQueue addOperation:[NSBlockOperation blockOperationWithBlock:^{
        NSManagedObjectContext* context =  self.persistentContainer.newBackgroundContext;
        [context performBlockAndWait:^{
            blockCopy(context);
            [context saveIfNeeded];
        }];
    }]];
}

-(NSManagedObjectContext*) viewContext{
    if (![NSThread mainThread]) {
        //here you should log to you analytics service. If you are in developer mode you should crash to force you to fix this
        NSLog(@"access context on wrong thread!!");
    }
    return self.persistentContainer.viewContext;
}