def once(event, &block)
      add_listener(event) do |*args, &callback|
        block.call(*args, &callback)
        :delete
      end
    end
def once(event, &block)
      add_listener(event) do |*args, &callback|
        block.call(*args, &callback)
        :delete
      end
    end
Subscribe to next event (at most once) for specified type.

 @param event [Symbol]
 @param block [Proc] callback function