父进程如何知道子进程全部结束(或者是：如何判断所有异步结束)

$task_connection->onMessage = function($task_connection, $result){
        if (!isset($task_connection->times)) $task_connection->times = 0;
        if (++$task_connection->times >= 15) {
            echo "任务完成";
        }
};