Question

I am using multi-threading to get data, parse it, create objects and store them. And after this is all done, I want the window to be shown.
But now I have 2 issues:

• I have a deadlock
• My barrier does not act as a barrier.

I think the deadlock is because I am updating the managedObjectContext in several threads at once.

1. So I changed my managedObjectContext with the ConcurrencyType:

   __managedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
   

2. And created an importContext for the concurrency queue and assigned the parentContext:

   NSManagedObjectContext *importContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
   importContext.parentContext = self.managedObjectContext;
   

3. And put my operations in a performBlock for the importContext:

   [importContext performBlock:^{
   
     dispatch_async(backgroundQueue, ^{
     [myObject methodAWithContext:importContext];
     });
   
     dispatch_async(backgroundQueue, ^{
     [myObject methodBWithContext:importContext];
     });
   
     dispatch_async(backgroundQueue, ^{
     [myObject methodCWithContext:importContext];
     });
   
   
     dispatch_barrier_async(backgroundQueueM, ^{
     // create barrier to wait for the first 3 threads to be completed.
      dispatch_async(dispatch_get_main_queue(), ^{
   
       // Save the data from the importContext tot the main context on the main queue
       NSError *importError = nil;
       [importContext save:&importError];
       [importContext.parentContext performBlock:^{
          NSError *parentError = nil;
          [importContext.parentContext save:&parentError];
       }];
   
      [self.window makeKeyAndVisible];
   
      });
     });
   }];
   

Approach 1:
In each method, I select a subset of object, delete these and then create new objects and save this.
(I thought the delete was quicker than doing a fetch and check for the existence for every object to be created).
So:
In Method A I select all AObjects, delete them and create new AObjects.
In Method B I select all BObjects, delete them and create new BObjects.
In Method C I select all CObjects, delete them and create new CObjects.

But then I get an error “An NSManagedObjectContext cannot delete objects in other contexts”.

So approach 2:
I removed the delete. But now I get various different errors…..
And the barrier does not wait for the other threads to be executed.

Q1: What am I doing wrong?

Q2: how do I get the barrier to wait for the 3 threads to be completed

Q3: how can I delete / purge objects on various threads?

(I have read the Apple release notes and doc’s, but I can’t find this a clear explanation on the combination for multithreading and managedContext.)

Answer 1

You cannot call dispatch_async within performBlock. A managed object context of type NSPrivateQueueConcurrencyType has it’s own dispatch queue for executing the operations.

You try to do several operations in parallel by moving them to a different dispatch queue, but that is not possible.

If you really have to do multiple operations in parallel, you must create a private concurrency type MOC for each operation.

ADDED:

There are several ways to wait for all operations to complete:

• You could increment a counter at the end of each performBlock: and check if it’s value is (in your example) 3.
• You could create a semaphore (dispatch_semaphore_create) for each operation with initial value zero, wait for all the semaphores (dispatch_semaphore_wait) and signal the semaphore at the end of each performBlock.
• And I am sure that there are better/more elegant/more sophisticated ways to do this.

BUT: As I re-read your question, I see that you try to delay the

[self.window makeKeyAndVisible];

until all Core Data fetch operations have completed. This is not a good design, because the user will see nothing until your data import is done.

A better design is to show an initial view immediately, and update that view when the background operations have fetched data.