ios - Can only use -performBlock: on an NSManagedObjectContext that was created with a queue -

i want use multithreading core data. parse xml-file in nsmanageobjects. use code below , runtime error can use -performblock: on nsmanagedobjectcontext created queue. what's wrong?

//xmlparser  - (void)main {     dispatch_queue_t queueb = dispatch_queue_create("create books", null);     dispatch_async(queueb, ^{         // opening xml         // ...         nsmanagedobjectcontext* context = [[nsmanagedobjectcontext alloc] init];         [context setpersistentstorecoordinator:maincontext].persistentstorecoordinator];         [context performblock:^{              // ...              [self _parsenode:container_node appendto:books incontext:context];              // ...              nserror* error = nil;              [context save:&error];              [maincontext performblock:^{                  nserror* parenterror = nil;                  [maincontext save:&parenterror];              }];          }];          [context release];      });      dispatch_release(queueb); }  - (int)_parsenode:(axlnode*)inode appendto:(nsmutablearray*)ioarray incontext:(nsmanagedobjectcontext*)context {     // ...     [context executefetchrequest:request error:&error];     //... } 

performblock can used managed object context (moc) of nsmainqueueconcurrencytype or nsprivatequeueconcurrencytype. in case, should create context with

nsmanagedobjectcontext *context = [[nsmanagedobjectcontext alloc]                      initwithconcurrencytype:nsprivatequeueconcurrencytype]; 

and there no need create dispatch queue or use dispatch_async(). moc creates , manages own queues, , performblock ensures block executed on moc's queue.