Prototype of an assertion that checks a NSManagedObject is being used on the queue belonging to its NSManagedObjectContext. This should obviously only be used during development.

gistfile1.m

/////////// Header

#import <CoreData/CoreData.h>

@interface NSManagedObject (FTCoreDataQueueContainmentAssertions)
+ (void)FTCDQCA_enableAssertions:(Class)klass;
@end

/////////// Implementation

#import <objc/runtime.h>

@implementation NSManagedObjectContext (FTCoreDataQueueContainmentAssertions)

// Problem is that using performBlockAndWait: can lead to deadlocks when used on the main queue.
//
//   __block dispatch_queue_t queue = nil;
//   [self performBlockAndWait:^{ queue = dispatch_get_current_queue(); }];
//
- (id)FTCDQCA_dispatchQueue;
{
  id dispatchQueue = object_getIvar(self, class_getInstanceVariable([NSManagedObjectContext class], "_dispatchQueue"));
  NSParameterAssert(dispatchQueue);
  return dispatchQueue;
}

- (void)FTCDQCA_validateOnCorrectQueue;
{
  NSAssert(dispatch_get_current_queue() == self.FTCDQCA_dispatchQueue, @"Accessing managed object from a different queue than its context uses: %@", self);
}

@end

@implementation NSManagedObject (FTCoreDataQueueContainmentAssertions)

+ (void)FTCDQCA_enableAssertions:(Class)klass;
{
  Method FTCDQCA_willAccessValueForKey = class_getInstanceMethod(klass, @selector(FTCDQCA_willAccessValueForKey:));
  class_addMethod(klass, @selector(willAccessValueForKey:), method_getImplementation(FTCDQCA_willAccessValueForKey), "v@:@");
  
  Method FTCDQCA_willChangeValueForKey = class_getInstanceMethod(klass, @selector(FTCDQCA_willChangeValueForKey:));
  class_addMethod(klass, @selector(willChangeValueForKey:), method_getImplementation(FTCDQCA_willChangeValueForKey), "v@:@");
}

- (void)FTCDQCA_willAccessValueForKey:(NSString *)key;
{
  [self.managedObjectContext FTCDQCA_validateOnCorrectQueue];
  [self FTCDQCA_willAccessValueForKey:key];
}

- (void)FTCDQCA_willChangeValueForKey:(NSString *)key;
{
  [self.managedObjectContext FTCDQCA_validateOnCorrectQueue];
  [super willChangeValueForKey:key];
}

@end

gistfile2.m

  // NotYourEntity is a NSManagedObject subclass.

  // Enable assertions:
  [NSManagedObject FTCDQCA_enableAssertions:[NotYourEntity class]];
  
  NotYourEntity *mainQueueEntity = (NotYourEntity *)[NSEntityDescription insertNewObjectForEntityForName:@"NotYourEntity"
                                                                         inManagedObjectContext:self.managedObjectContext];
  mainQueueEntity.anObjectValue = @"Correct";
  
  NSManagedObjectContext *backgroundContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
  backgroundContext.parentContext = self.managedObjectContext;
  
  __block NotYourEntity *backgroundQueueEntity = nil;
  
  [backgroundContext performBlockAndWait:^{
    // This will hit the assertion.
    mainQueueEntity.anObjectValue = @"YOLO";
    
    backgroundQueueEntity = (NotYourEntity *)[NSEntityDescription insertNewObjectForEntityForName:@"NotYourEntity" inManagedObjectContext:backgroundContext];
    backgroundQueueEntity.anObjectValue = @"Correct";
  }];

  // This will hit the assertion.  
  backgroundQueueEntity.anObjectValue = @"YOLO";

One thing I haven’t figured out yet is a nice way to enable it for all NSManagedObject subclasses. I’ll look at this later, but if you know from experience, please leave a comment!

Also, dispatch_get_current_queue() is obviously deprecated, but as this is for development use only, it’s still good for now.

One thing I haven’t figured out yet is a nice way to enable it for all NSManagedObject subclasses. I’ll look at this later, but if you know from experience, please leave a comment!

What do you mean? This would work for subclasses as long as they call super in the -willXXX methods.

What I mean is that I was not able to find a way to apply this to all NSManagedObject subclasses yet, possibly because iirc CD does some trickery with its subclasses.