def on(event_name, &block)
      hooks[event_name.to_sym] ||= []
      hooks[event_name.to_sym] << block
    end
def on(event_name, &block)
      hooks[event_name.to_sym] ||= []
      hooks[event_name.to_sym] << block
    end
Public: Registers a hook to this manifest. Hooks get fired when some
 lifecycle events happen, like the creation of a component or its
 destruction.

 event_name - A String or Symbol with the event name.
 &block     - The block to run when the hook gets triggered.

 Returns nothing.