How to use rosbag to take a snapshot of topics ?

edit

Hello everyone,

I am trying to record a snapshot of topics when I used a button of my joypad. A naive approach consist to specify a topic trigger for rosbag record.

I found something look good in the source code (github.com/ros/ros_comm/blob/groovy-devel/tools/rosbag/src/recorder.cpp) :

if (options_.snapshot)
{
    record_thread = boost::thread(boost::bind(&Recorder::doRecordSnapshotter, this));

    // Subscribe to the snapshot trigger
    trigger_sub = nh.subscribe<std_msgs::Empty>("snapshot_trigger", 100, boost::bind(&Recorder::snapshotTrigger, this, _1));
}

but I don't understand how to use this option !

Maybe someone can help me or suggest another way to capture topics when I want ?

Thanks

best regards