Detect when UIViewController no longer presents a view controller

I would like to observe the view controller that the view controller presents to determine when it stops presenting it.

I was thinking about trying KVO, which I'm not very familiar with:

[observedVC addObserver:self forKeyPath:NSStringFromSelector(@selector(presentedViewController)) options:0 context:NULL];

      

        
        
        
      

    

And then:

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    NSLog(@"Change: %@", keyPath);
}

      

        
        
        
      

    

But I don't see what caused it. Is there a better way to do this? I'm working on an SDK, so I don't necessarily have control over the observable VC.