AsyncTcpConnection触发不了onMessage

$connection_tcp = new AsyncTcpConnection('tcp://127.0.0.1:8888');
$connection_tcp->send("hello");
echo '111111';
$connection_tcp->onMessage = function($connection_tcp, $result)
{
    echo '222222';
    echo $result;
    $connection_tcp->close();

};
$connection_tcp->connect();
echo '333333';