GRTaskQueueNew.m

- (void)addTask:(GRTask*)task 
       importer:(Class<GRImporter>)Importer
      resources:(NSArray*)requiredResources
completionBlock:(GRImportCompletionBlock)complete
{
    NSDictionary* taskInfo = [task dictionaryRepresentation];
    GRImportCompletionBlock done = [complete copy];

    [self addOperationWithBlock:^{
        // ask plugin to generate an import block for this task
        GRImportOperationBlock import;
        import = [Importer newImportBlockWithInfo:taskInfo];

        // acquire resource locks
        [GRResource acquireResources:requiredResources];

        // execute import block generated by plugin
        NSError* error = nil;
        BOOL status = import(taskInfo, &error);

        // unlock resources
        [GRResource relinquishResources:requiredResources];

        // execute the completion block
        done(status, error);

        // clean up
        [complete release];
        [import release];
    }];
}

GRTaskQueueOld.m

- (void)addTask:(GRTask*)task 
       importer:(Class<GRImporter>)Importer
      resources:(NSArray*)requiredResources
   successBlock:(GRImportCompletionBlock)succ
   failureBlock:(GRImportCompletionBlock)fail
{
    // get execution block for the import task
    NSDictionary* taskInfo = [[task dictionaryRepresentation] copy];
    GRImportOperationBlock block;
    block = [Importer newImportBlockWithInfo:taskInfo
                                successBlock:succ
                                failureBlock:fail];

    // create NSBlockOperation wrapper
    NSBlockOperation* op = [NSBlockOperation blockOperationWithBlock:block];

    // set up dependencies
    // check task resource requirements
    NSMutableArray* savedDepArrays = [NSMutableArray array];
    for (NSString* resource in requiredResources) {
        NSMutableArray* depends = [resources objectForKey:resource];

        if (depends == nil) {
            depends = [NSMutableArray array];
            [resources setObject:depends forKey:resource];
        }
        else {
            for (NSBlockOperation* dep in depends)
                [op addDependency:dep];
        }

        [depends addObject:op];
        [savedDepArrays addObject:depends];
    }

    [op setCompletionBlock:^{
        Block_release(succ);
        Block_release(fail);
        Block_release(block);

        [taskInfo release];

        // remove operation from dependency arrays
        [savedDepArrays makeObjectsPerformSelector:@selector(removeObject:)
                                        withObject:op];
    }];

    [self addOperation:op];
}