Have a block operation depend on a network operation (AFNetworking)

dependencies.m

// The problem: AFNetworking calls the operation's completion block asynchronously, so it breaks `-[NSOperation addDependency:]` order as the dependent is started *before* the dependency's completion block.

dispatch_semaphore_t sem = dispatch_semaphore_create(0);
__block NSOperation* blockOperation = nil;
void (^networkOperationHandler)(NSOperation *, id) = ^(NSOperation *operation, id __unused unused) {
    if (operation.cancelled)
    {
        NSLog(@"networkOperation: networkOperation cancelled!");
        [blockOperation cancel];
    }

    NSLog(@"networkOperation: networkOperation signalling blockOperation");
    dispatch_semaphore_signal(sem);
    NSLog(@"networkOperation: networkOperation finished");
};

NSMutableURLRequest *request = [_net.requestSerializer requestWithMethod:@"GET" URLString:@"http://www.google.com/" parameters:nil error:0];
NSOperation *networkOperation = [_net HTTPRequestOperationWithRequest:request
                                                              success:networkOperationHandler
                                                              failure:networkOperationHandler];
blockOperation = [NSBlockOperation blockOperationWithBlock:^{
    NSLog(@"blockOperation: started");
    if (sem)
    {
        NSLog(@"blockOperation: waiting for networkOperation");
        dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
        NSLog(@"blockOperation: networkOperation finished");
    }

    if (blockOperation.cancelled)
    {
        NSLog(@"blockOperation: blockOperation cancelled!");
        return;
    }

    NSLog(@"blockOperation: blockOperation finished");
}];

[blockOperation addDependency:networkOperation];
[_net.operationQueue addOperations:@[networkOperation, blockOperation] waitUntilFinished:NO];
return networkOperation;

...
// - blockOperation's block waits for the network operation to have called its completion handler
// - cancelling networkOperation properly cancels the chain:
[networkOperation cancel];