gistfile1.m

@interface RetrievalOperation : NSOperation
+ (id)retrievalOperationWithBlock:(id (^)(void))block;
@property (readonly, strong, nonatomic) id result;
@end

@interface RetrievalOperation (/* Private */)
@property (readwrite, strong, nonatomic) id result;
@property (copy, nonatomic) id (^block)(void);
@end

@implementation RetrievalOperation

+ (id)retrievalOperationWithBlock:(id (^)(void))block
{
    RetrievalOperation *operation = [[self alloc] init];
	[operation setBlock:block];
	return operation;
}

- (void)main
{
	id result = [self block]();
	[self setResult:result];
}

@end

int main(int argc, const char **argv)
{
	@autoreleasepool {
		id databaseClient;	// some database client object
		NSArray *fileURLs;			// some file URLs from somewhere else
		NSPredicate *predicate;		// some predicate from somewhere else
		
		NSOperationQueue *backgroundQueue = [[NSOperationQueue alloc] init];
		
		RetrievalOperation *databaseRetrievalOperation = [RetrievalOperation retrievalOperationWithBlock:^ id (void) {
			return [databaseClient fetchObjectsMatchingPredicate:predicate];
		}];
		[backgroundQueue addOperation:databaseRetrievalOperation];
		
		NSMutableArray *fileRetrievalOperations = [NSMutableArray array];
		for (NSURL *fileURL in fileURLs) {
			RetrievalOperation *fileRetrievalOperation = [RetrievalOperation retrievalOperationWithBlock:^ id (void) {
				return [NSData dataWithContentsOfURL:fileURL];
			}];
			[fileRetrievalOperations addObject:fileRetrievalOperation];
			[backgroundQueue addOperation:fileRetrievalOperation];
		}
		
		NSOperation *completionOperation = [NSBlockOperation blockOperationWithBlock:^ {
			NSArray *databaseObjects = [databaseRetrievalOperation result];
			
			NSMutableArray *fileContents = [NSMutableArray array];
			for (RetrievalOperation *fileRetrievalOperation in fileRetrievalOperations) {
				[fileContents addObject:[fileRetrievalOperation result]];
			}
			
			[self finishProcessingDatabaseObjects:databaseObjects fileContents:fileContents];
		}];
		[completionOperation addDependency:databaseRetrievalOperation];
		for (NSOperation *fileRetrievalOperation in fileRetrievalOperations) {
			[completionOperation addDependency:fileRetrievalOperation];
		}
		[[NSOperationQueue mainQueue] addOperation:completionOperation];
	}
	
	return 0;
}