How does UIView nextResponder know what the UIViewController is?

Question

Just as a matter of curiosity, how does the nextResponder method implementation of the UIView class know who is the UIViewController that manages the view? The UIResponder documentation states this, and I can see it work, but I don't quite understand it. To the best of my knowledge, a UIView does not maintain a reference to it's controller, so what's happening behind the scenes? Or am I just missing something obvious?

I'm still very new to Objective-C and iPhone development so I apologize if this is something obvious, but I am quite curious.

Thanks!

Answer 1

The responder chain is separate from the view hierarchy. The responder chain might look like this:

First Responder > View Hierarchy > Window > Window Delegate > etc...

However, objects can insert themselves into the responder chain and that's what UIViewController does. From the docs:

Because view controllers are tightly bound to the views they manage, they are also part of the responder chain used to handle events. View controllers are themselves descendants of the UIResponder class and are inserted into the responder chain between the managed view and its superview.

In Big Cocoa, this is accomplished using the -setNextResponder: method. That method isn't public in Cocoa Touch, but nevertheless that's what UIViewController appears to do.

Answer 2

If you look at UIView.h, you can see a member variable named _viewDelegate that is of type UIViewController*, this is probably where the view controller reference is stored when the viewcontroller's view property is set, and where it knows to look when you call nextResponder.