Can you use cross-queue dependencies for NSOperation objects?

Is it possible/legal to have cross-queue dependency operations?

Consider the following...

let operationA = NSBlockOperation(block: someBlock)
let operationB = NSBlockOperation(block: someOtherBlock)

let operationC = NSBlockOperation(block: finalBlock)
operationC.addDependency(operationA)
operationC.addDependency(operationB)

OperationQueue().addOperation(operationC)
OperationQueue().addOperation(operationA)
OperationQueue().addOperation(operationB)

I don't see anywhere in the docs that says this is a restricted usage, but I'm also not sure if such thing is possible/supported. (Note: I'm also not saying if this is good practice or not. Just trying to 'poke' the language to see what's possible as food for thought.)

Yes

"The other neat thing about dependencies is that they are not limited by operation queues. Now, what do I mean by this? If you have two operation queues in your application, operations in the first queue can be dependent on the operations in the second queue."

https://developer.apple.com/videos/play/wwdc2015/226/

Here is an example you can try:

NSOperationQueue *queue1 = [[NSOperationQueue alloc] init];

NSBlockOperation *a1 = [[NSBlockOperation alloc] init];

NSBlockOperation *a2 = [[NSBlockOperation alloc] init];

[a1 addExecutionBlock:^{
    NSLog(@"a1");
    
    NSOperationQueue *queue2 = [[NSOperationQueue alloc] init];
    
    NSBlockOperation *b1 = [[NSBlockOperation alloc] init];
    
    NSBlockOperation *b2 = [[NSBlockOperation alloc] init];
    
    [b1 addExecutionBlock:^{
        NSLog(@"b1");
    }];
    
    [b2 addExecutionBlock:^{
        NSLog(@"b2");
    }];

    [a2 addDependency:b2]; // makes this inner queue finish first
    
    [b2 addDependency:b1];
    
    [queue2 addOperations:@[b1, b2] waitUntilFinished:NO];
    
}];

[a2 addExecutionBlock:^{
    NSLog(@"a2");
}];

[a2 addDependency:a1];

[queue1 addOperations:@[a1, a2] waitUntilFinished:NO];

Outputs:

2017-01-08 00:27:03.019497 OpTest[26141:36238589] a1
2017-01-08 00:27:03.019621 OpTest[26141:36238621] b1
2017-01-08 00:27:03.019721 OpTest[26141:36238589] b2
2017-01-08 00:27:03.019773 OpTest[26141:36238621] a2

As you can see the [a2 addDependency:b2] line allows a sub-queue of operations to finish within the main queue.