I recently had to debug a complex view hierarchy and find out why touch events were not handled by some UIViewController in the responder chain.

Well, this small UIResponder extension turned out to be of great help :

public extension UIResponder {
  func printResponderChain() {
    print(self)
    next?.printResponderChain()
  }
}

For example, by calling printResponderChain() on a UIView, it recursively prints the whole related chain, so you can figure out where things get weird.