def on_pollfd_added &block
      @on_pollfd_added = proc do |fd, events, _|
        pollfd = Pollfd.new fd, events
        block.call pollfd
      end
      Call.libusb_set_pollfd_notifiers @ctx, @on_pollfd_added, @on_pollfd_removed, nil
    end
def on_pollfd_added &block
      @on_pollfd_added = proc do |fd, events, _|
        pollfd = Pollfd.new fd, events
        block.call pollfd
      end
      Call.libusb_set_pollfd_notifiers @ctx, @on_pollfd_added, @on_pollfd_removed, nil
    end
Register a notification block for file descriptor additions.

 This block will be invoked for every new file descriptor that
 libusb uses as an event source.

 Note that file descriptors may have been added even before you register these
 notifiers (e.g. at {Context#initialize} time).

 @yieldparam [Pollfd] pollfd  The added file descriptor is yielded to the block