Parallel dependent tasks

gistfile1.m

__block NSArray *databaseObjects;
__block NSArray *fileContents;

NSOperationQueue *backgroundQueue = [[NSOperationQueue alloc] init];
NSBlockOperation *operation1 = [NSBlockOperation blockOperationWithBlock:^{
    databaseObjects = [databaseClient fetchObjectsMatchingPredicate:predicate];
}];

NSBlockOperation *operation2 = [NSBlockOperation blockOperationWithBlock:^{
    NSMutableArray *filesInProgress = [NSMutableArray array];
    for (NSString *path in files) {
        [filesInProgress addObject:[NSData dataWithContentsOfFile:path]];
    }

    fileContents = [filesInProgress copy];
}];

NSBlockOperation *finish = [NSBlockOperation blockOperationWithBlock:^{
    [self finishProcessingDatabaseObjects:databaseObjects fileContents:fileContents];
    NSLog(@"Done processing");
}];

[finish addDependency:operation1];
[finish addDependency:operation2];
[backgroundQueue addOperation:operation1];
[backgroundQueue addOperation:operation2];
[backgroundQueue addOperation:finish];