class Net::IMAP

def idle(timeout = nil, &response_handler)

[RFC2177[https://tools.ietf.org/html/rfc2177]].
The server's capabilities must include +IDLE+

===== Capabilities

Related: #idle_done, #noop, #check

end
end
...
imap.idle(60) do |res|
loop do

checks the connection for each 60 seconds.
+timeout+ can be used for keep-alive. For example, the following code
If +timeout+ is given, this method returns after +timeout+ seconds passed.

Use #idle_done to leave IDLE.

responses from the server during the IDLE.
that waits for notifications of new or expunged messages. Yields
{[IMAP4rev2 §6.3.13]}[https://www.rfc-editor.org/rfc/rfc9051#section-6.3.13]
Sends an {IDLE command [RFC2177 §3]}[https://www.rfc-editor.org/rfc/rfc6851#section-3] 
def idle(timeout = nil, &response_handler)
  raise LocalJumpError, "no block given" unless response_handler
  response = nil
  synchronize do
    tag = Thread.current[:net_imap_tag] = generate_tag
    put_string("#{tag} IDLE#{CRLF}")
    begin
      add_response_handler(&response_handler)
      @idle_done_cond = new_cond
      @idle_done_cond.wait(timeout)
      @idle_done_cond = nil
      if @receiver_thread_terminating
        raise @exception || Net::IMAP::Error.new("connection closed")
      end
    ensure
      unless @receiver_thread_terminating
        remove_response_handler(response_handler)
        put_string("DONE#{CRLF}")
        response = get_tagged_response(tag, "IDLE", @idle_response_timeout)
      end
    end
  end
  return response
end